# Closed-loop control of a directed energy deposition process for repair applications

Stéphane Touzé, Carolina Navarro Valero, Guillaume Rückert, Jean-Yves Hascoët

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Stéphane Touzé, Carolina Navarro Valero, Guillaume Rückert, Jean-Yves Hascoët. Closed-loop control of a directed energy deposition process for repair applications. Progress in Additive Manufacturing, 2024, 10.1007/s40964-024-00867-8. hal-05095091

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Submitted on 3 Jun 2025

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Distributed under a Creative Commons Attribution 4.0 International License
# Closed-loop control of a directed energy deposition process for repair applications

Stéphane Touzé<fnref n="1" />,<fnref n="3" />☐, Carolina Navarro Valero<fnref n="1" />,<fnref n="3" />, Guillaume Rückert<fnref n="2" />,<fnref n="3" />, Jean-Yves Hascoët<fnref n="1" />,<fnref n="3" />

Received: 15 July 2024 / Accepted: 7 November 2024

© The Author(s) 2024

## Abstract

Additive manufacturing processes of the directed energy deposition family such as laser metal deposition or wire arc additive manufacturing offer great control and improved metallurgical properties over traditional processes such as casting. They are very promising for many industrial applications such as the fabrication of intricate parts with high-performance alloys as well as repair applications. However, these processes are generally very sensitive to process conditions, yet are most often operated in open loop. In this study, a closed-loop controller with a suitable error criteria based on 3D scanning data in-between layers is implemented. It is applied to various repair samples made of 316L stainless steel and Inconel 625 materials. The results show that the controller can compensate for suboptimal operating points and in-process deviations while maintaining satisfactory metallurgical and mechanical properties.

**Keywords** Additive manufacturing · Directed energy deposition · Laser metal deposition · Closed-loop control · Repair · Inconel 625 · 316L · Monitoring · 3D scanning · Process monitoring · Process control

## 1 Introduction

Surface defects are common occurrences with industrial parts. They can be generated during fabrication, even with mature and tightly controlled manufacturing processes, or during their service life through unavoidable wear and tear. Instead of being scraped, the defective part may instead be efficiently repaired, either through conventional welding processes or with a DED (directed energy deposition) process such as WAAM (wire arc additive manufacturing) and LMD (laser metal deposition) [10–14]. The latter option can induce smaller and thus stronger microstructures in the repaired area with a smaller heat affected zone, thus minimizing the impact of the repair on the microstructure and geometry of the surrounding part. The LMD process may even be necessary when repairing low weldability alloys as

it induces rapid cooling due to its laser-based heat source, which can prevent hot cracking during repairs [12].

A repair process can save time, effort, material and energy provided that the digital thread and the overall repair process are properly set up along with a suitable machine tool. The LMD machine should indeed host a milling spindle for surface milling as it allows performing subtractive operations before and after the additive operation for surface repair within the same workspace without unclamping the part, thereby keeping the same reference frame throughout the whole process.

The repair process begins with the detection of a surface defect by any number of means, from visual inspection to dye penetrant testing to X-ray or ultrasound analysis for subsurface yet shallow defects. The defect area can be marked and the part set up in the work area of the hybrid LMD machine. After properly setting the origin, the surface or shallow defect is machined out through surface milling, typically using a hemispherical milling cutter with a diameter suited to the part and defect geometry. The resulting cavity is then refilled by the LMD process, typically using the same material as the base part for homogeneous repairs. The LMD process parameters and toolpaths must be optimized so that the deposit is well penetrated and shows no defects such as lack of material, porosity or cracks. The deposit should

<footnote>Stéphane Touzé
stephane.touze@ec-nantes.fr</footnote>

<footnote n="1">¹ Nantes Université, Ecole Centrale Nantes, CNRS, GeM, UMR 6183, F-CP44000 Nantes, France</footnote>

<footnote n="2">² Naval Group Research, Technocampus Ocean, 5 Rue de L'Halbrane, 44340 Bouguenais, France</footnote>

<footnote n="3">³ Centrale Nantes-Naval Group, Joint Laboratory of Marine Technology (JLMT), Nantes, France</footnote>
<figure><img src="image_2.png" /><figcaption>Fig. 1 SEM micrograph of IN625 (left) and 316L (right) powder</figcaption></figure>

<figure><img src="image_3.png" /></figure>

slightly extend above the surface of the part and overextend to the side so that it forms a cap over the cavity that can be machined out to tightly match the part surface while ensure its proper filling near the edges.

Repair applications are a practical use case where a closed-loop controller can ensure that the deposit matches the anticipated height on the first try while limiting tedi- ous process optimization. Unlike fabrication applications on a bare substrate, repair operations affect the underly- ing part, thus multiple trials should be avoided. However, additive manufacturing (AM) processes are generally very sensitive to layer height as a small discrepancy between the pre-programmed and experimental layer heights can rapidly accumulate and induce an excessive deviation between the optimal and effective working distance. This is especially true for DED processes such as LMD and WAAM. Such deviations with LMD produce defocusing issues with the laser beam and powder stream that eventually lead to defects such as lack of fusion and, ultimately, to the complete failure of the deposition process. The WAAM process is a bit more robust in working distance variations but critical condi- tions such as arc instabilities will be triggered beyond some threshold. Thus, when performing DED of metallic mate- rials in open loop, the pre-programmed and experimental layer heights must match precisely as there is no correc- tion during fabrication. The more layers are deposited, the closer these heights should match in order to remain within the operating window during the whole fabrication time. As the LMD process allows for fine layer heights (i.e., down to about 0.1 mm), calibrating the layer height with sufficient precision so as to avoid significant errors throughout an entire fabrication or repair operation is challenging, and the required precision increases with the number of deposited layers.

Moreover, the experimental layer height may not be suf- ficiently constant in DED processing. It may initially match the pre-programmed height precisely but can diverge during

deposition. In the case of powder-based DED processes such as LMD, the material to be deposited is brought in as a stream of fine powder that can be unstable depending on the type of powder dispenser, the quality of the powder and environmental conditions such as ambient humidity and temperature. In addition, the instantaneous powder flowrate measurement is not available in most LMD equipment due to a lack of reliable sensors, especially at smaller flowrates, so that only average values can be measured before deposi- tion. During WAAM, the wire feed speed may vary due to wearing of the feeding system or non-uniform wire quality in terms of surface or diameter, which can only be partly compensated by the synergetic curve of the welding genera- tor through a power variation.

Operating in a closed loop with layer height correction would solve these issues as it makes DED processes such as LMD more stable and robust regarding all the possible param- eter inaccuracies and instabilities, thus giving more flexibility in setting the initial operating point. It is therefore an important step towards the industrialization of this process, whether in repair or in fabrication scenario. An added benefit of closed- loop control comes from the necessary in-situ sensor meas- urements as they can be used for traceability purposes. Such data are very valuable for the quality control of industrial parts fabricated and repaired by AM processes, which is paramount for their industrialization and widespread use.

Some early work on closed-loop control of the LMD pro- cess focused on melt pool size and surface roughness as well as total height control to minimize post-processing [1, 2]. A

<table><caption>Table 1 Powder characteristics</caption><thead><tr><th>Powder material</th><th>IN625</th><th>316L</th></tr></thead><tbody><tr><td>Supplier</td><td>Höganäs</td><td>Höganäs</td></tr><tr><td>Granulometry (µm)</td><td>45–125</td><td>45–90</td></tr><tr><td>Apparent density (g/cm3)</td><td>4,94</td><td>4,17</td></tr><tr><td>Hall flowability (s/50 g)</td><td>14,79</td><td>18,10</td></tr></tbody></table>

<figure><img src="image_4.png" /></figure>
<table><caption>**Table 2** IN625 chemical composition</caption><thead><tr><th>Element</th><th>Ni (balance)</th><th>Cr</th><th>Mo</th><th>Nb</th><th>Fe</th><th>Si</th><th>Mn</th><th>Al</th><th>Ti</th><th>C</th><th>O</th><th>N</th></tr></thead><tbody><tr><td>IN625 (%wt.)</td><td>63,43</td><td>21,4</td><td>9,1</td><td>3,33</td><td>1,5</td><td>0,43</td><td>0,41</td><td>0,09</td><td>0,12</td><td>0,03</td><td>0,05</td><td>0,11</td></tr></tbody></table>

<table><caption>**Table 3** 316L chemical composition</caption><thead><tr><th>Element</th><th>Fe (Balance)</th><th>Cr</th><th>Ni</th><th>Mo</th><th>Mn</th><th>Si</th><th>C</th></tr></thead><tbody><tr><td>316L (%wt.)</td><td>66,49</td><td>16,5</td><td>12,4</td><td>2,5</td><td>1,5</td><td>0,6</td><td>0,012</td></tr></tbody></table>

layer-to-layer height controller with a model-based approach with iterative learning control was applied to adapt layer height through powder flowrate using a laser displacement sensor [3, 4]. Another control scheme based on iterative learning control and a step-height controller was applied to a robotic laser-wire process for wire feed speed and robot tool center point (TCP) adjustment based on a laser line scanning [5]. A contact-tip-to-work-distance control scheme was implemented for the WAAM process based on the acoustic signature of the arc [6]. Structured light scanning was used from a fixed point on a robotic LMD cell for layer height control during part fabrication, either by deleting or repeating layers [7] or by reslicing the CAD model at the measured height with the corrected layer height value over a subsequent group of layers and keeping the process parameters constant [8].

The present work introduces a discrete time, model-based adaptive controller with a two-step correction of the layer height followed by process parameters so that the target deposit height is achieved with the original number of layers. This approach enhances the robustness of the LMD process against suboptimal initial operating points and in-process instabilities while avoiding a reslicing step on external software. It also differs from more common control schemes where the layer height is set to be fixed and is controlled by varying process parameters, which requires a reslicing due to the change in total number of layers.

The second section of this paper describes the materials used, the experimental setup for the closed-loop LMD experiments and the control scheme. The third section discusses the results of experiments on IN625 and 316L repairs both with and without closed-loop control.

# 2 Materials and methods

## 2.1 Powder materials

Two powder materials, Inconel 625 45–125 µm and stainless steel 316L 45–90 µm, were used for homogeneous repair experiments with and without closed-loop control.

The morphology of the powders was assessed via scanning electron microscope (SEM) imaging to ensure of their proper sphericity and lack of satellites (Fig. 1). Their apparent density and Hall flowability was measured before testing (Table 1). Their chemical compositions (Tables 2, 3) and granulometry (Fig. 2) taken from material certificate data from the supplier are also given.

The SEM micrographs (Fig. 1) show that IN625 powder particles are larger than the 316L powder particles, in accordance with the sieve analysis (Fig. 2). Both of these powders have an adequate sphericity with a punctual presence of satellites and ovoid particles. The general quality and properties of these powders are satisfactory and could be used for the LMD repair experiments as they showed no flowability issues.

## 2.2 Laser metal deposition machine

The LMD machine of the rapid manufacturing platform (RMP) of the Ecole Centrale de Nantes (ECN) comprises a HURON 5-axis machine, in which are mounted two LMD deposition heads based on patents from IREPA LASER [9] that are connected to a 2 kW fiber laser with a wavelength of 1070 nm. The laser spot is 0.5 mm in diameter with a top-hat power density distribution. The nominal working distance of the LMD head is 5 mm. The machine also contains a

<figure><img src="image_5.png" /><figcaption>**Fig. 2** Sieve analysis from supplier data</figcaption></figure>
milling spindle of 4.6 kW for surface milling operations at
up to 30,000 rpm.

## 2.3 3D scanner

A structured light 3D scanner is rigidly mounted on the LMD head carriage to retain a fixed position and orientation with respect to the LMD head (Fig. 3). The maximum repeatability along its z-axis is 4.7 µm, and the resolution along the horizontal axes used for the experiments is 0.1 mm. The scanner operates by taking snapshots of the deposited surface and is brought to a fixed location above the deposition area between deposited layers before triggering the scan. This fixed position is determined based on the working distance of the scanner along the vertical axis and the deposit location within the LMD reference frame for the horizontal axes. For small-scale repairs, the position of the 3D scanner within the LMD reference frame can remain fixed throughout the deposition. For larger scale repairs, it can be set based on the coordinates of the last deposited layer so that the scanner keeps operating at the proper working distance with its field of view remaining focused on the region of interest.

## 2.4 Process control loop

The process control loop, represented in Fig. 4, is driven by a control program running continuously on a computer connected to both the numerical controller (NC) of the LMD machine and the 3D scanner. In parallel, the G-code program executing on the NC during the deposition provides the nominal toolpaths and process parameters as well as motions and pauses for 3D scanning between layers. Essentially, the control program edits this NC program in-between layers based on 3D scanning data and subsequent data transfer, data processing and correction calculation for the pre-programmed layer height and process parameters. The duration of the inter-layer time delay thus depends on several factors and is on the order of 1 min with the current hardware setup, software implementation and control loop configuration. A powder flowrate correction involves an additional 20 s delay due to the response time of the powder flowrate whereas the travel speed correction is instantaneous. The duration of the inter-layer time delay generally affects the thermal history of AM deposits and thus the stress, deformation and underlying microstructure. Let us note that laser-based AM processes such as LMD provide more localized heating and faster cooling than arc-based AM processes such as WAAM so that inter-layer time delay is generally less of a consideration for small-scale deposits. These inter-layer or interpass time and temperature aspects are thus ignored in the current study but could warrant further investigation.

<figure><img src="image_6.png" /><figcaption>Fig. 3 Experimental setup in LMD machine of the RMP platform</figcaption></figure>

Before initiating the closed-loop repair process, an initial NC program is generated in G-code in a standard manner with CAM (computer-aided manufacturing) software. This NC program is then fed into the control program, which registers the process parameters and toolpaths for each layer.

The control program also reads a configuration file containing the controller parameters (see Closed-loop correction in Sect. 3) as well as calibration data for the transformation of the 3D scanner coordinates into the LMD coordinates.

After the initialization step, the control program is executed on the control PC concurrently with the NC program. After a layer or a group of layers is deposited, the NC program moves the LMD head to the 3D scan position where the top surface of the deposit is measured. The 3D scanner is then triggered automatically by the control program, and the raw scan data are sent to the control PC for processing.

The number of layers deposited between each scan and control step depends on the precision of the 3D scanner and the overall measurement process compared to the deposited layer heights. If single layers have a height too small compared to the measurement precision, the scanning step and subsequent correction can be applied after the deposition of multiple layers with the calculations being adjusted accordingly.

The scan data are pre-processed by the control program, first by applying a threshold to remove potential outliers created by measurement artifacts, which are mostly caused by the reflective nature of most metals and the concave geometry of the cavities to be repaired [10, 11]. The 3D data point coordinates are then transformed from the scanner reference frame into the LMD head reference frame (i.e., the

<figure><img src="image_7.png" /></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 4** Flowchart of the LMD control loop (top), experimental setup in LMD machine (bottom)</figcaption></figure>

deposition reference frame) so that they can be compared to
toolpaths in the G-code program.

After the transformation, the region of interest (ROI)
where the layer is deposited can be extracted from the trans-
formed 3D scan data by analyzing the extents of the toolpath
of the current layer in the horizontal plane, i.e., along the
X and Y axes.

For the vertical Z-axis, a calibration step is initially per-
formed to compensate for inaccuracies in defining the trans-
formation between the scanner and LMD head reference
frame. This is done by extracting scan data in the region
right outside of the ROI, i.e., the near-by existing part for
repairs. The Z-axis transform is then set so that the average
scan height corresponds to the expected height of the part
in the G-code program (typically at z=0).

To determine the corresponding deposit height based on
the pre-processed data, two methods can be adopted depend-
ing on the quality of the deposit and scan data. In a first
approach, the experimental deposit height is taken as the
average of the z-coordinates of the ROI, and the standard
deviation is used as an indication of the reliability of this
average value in representing the actual deposit height.

In a second approach, a plane is fitted on the ROI through
a RANSAC algorithm [10, 11]. After a plane is fitted on the
ROI, the average height of the plane over the ROI can be cal-
culated to represent the current experimental deposit height.
A robust segmentation algorithm such as RANSAC is use-
ful either when the deposited layer is excessively uneven,
when the layer orientation is not sufficiently horizontal or
when there are many artifacts in the scan data. An uneven
deposit may occur due to unstable deposition conditions,
for instance because of low powder flowability [15]. Layer
misorientation with respect to the XY horizontal plane may
be due a misalignment of the 3D scanner with respect to the

LMD reference frame. Measurement artifacts can be caused
by reflections and concavity of the deposition region.

Once the control program has processed the surface data
and consequently corrected the NC program as needed based
on the measurement (see next section on Process Control
Scheme), the deposition is resumed until the end condition is
triggered, which is when the measured height of the deposit
has reached the target height of the initial NC program. The
target height generally coincides with the part surface to be
repaired (typically at z=0) plus extra layers for finishing
operations such as surfacing and polishing.

**2.5 Process control scheme**

The general approach of the closed-loop control scheme of
the present study is to correct the pre-programmed layer
height and corresponding process parameters based on 3D
scan data acquired in-between layers with the goal of reach-
ing the target deposit height while retaining the initial num-
ber of layers in order to avoid a reslicing step.

The controller can be described as a discrete-time, model-
based adaptive controller with two steps, including a layer
height correction step similar to a PI (Proportional-Integral)
correction and a model-based process parameter correction
step with variable set point (i.e., layer height). This control
approach differs from other control objectives and schemes
where, for example, the layer height is controlled to match
a target set point by changing process parameters as the
total number of layers will change. Correcting both the pro-
grammed layer height and the process parameters ensures
that the target total height is achieved with the original num-
ber of layers, thereby avoiding the complications of adding
or subtracting layers. Adjusting the layer height accounts for
accumulated errors and number of layers left while adjusting
the process parameters ensures that each deposited layer physically matches the programmed layer height. This two-step correction makes the system more robust against both suboptimal initial operating points and in-process instabilities, ultimately leading to more consistent build quality and more precise dimensional accuracy.

Basing the correction on the layer height error rather than the overall deposit height allows for an error criteria that is independent of deposit geometry and is more related to the process itself. This facilitates the setting of the user-defined error threshold for triggering the correction and defining maximum and minimum limits for the corrected layer height.

If despite the control loop, the target height is not reached after all the pre-programmed layers have been deposited, the final layer is repeated as many times as necessary. This is generally acceptable in a repair context where the geometry of the final layers does not drastically change. If the initial operating point is decently chosen and the process reasonably stable, the number of repeated layers should remain limited in practice, if needed at all. Conversely, if the target height is reached before all the pre-programmed layers have been deposited, the deposition process is terminated.

The correction is carried out in two steps, where the programmed layer height is corrected followed by a change in the process parameters (i.e., powder flowrate or travel speed) that reflects the newly programmed layer height.

For the first step, the updated layer height written into the NC program is obtained by compensating for the error between the pre-programmed and measured layer height, assuming this error would remain constant throughout the rest of the deposition. The error is thereby redistributed over the remaining layers. The layer height error $δz$ is calculated based on the last deposit height $z_{current}$ obtained from measurements, the target deposit height $z_{max}$, the number of remaining layers $N_{layers} - N_{current}$ and the currently programmed layer height $dz$ (see Eq. 1):

$$ \delta z = dz - \frac{z_{max} - z_{current}}{N_{layers} - N_{current}} \quad (1) $$

The updated layer height $dz'$ is then calculated based on this layer height error $δz$ (see Eq. 2):

$$ dz' = dz - \delta z = \frac{z_{max} - z_{current}}{N_{layers} - N_{current}} \quad (2) $$

The layer height correction is based on the cumulative error distributed over the remaining layers so as to deposit the expected initial number of layers to avoid reslicing, thus inducing a variable set point (i.e., layer height) for the model-based correction of the process parameters.

When the error $δz$ is sufficiently small (i.e., below the user-defined threshold), the currently programmed layer height $dz$ adequately matches the experimental layer height,

which allows to reach the target deposit height with the number of layers that remain in the initial NC program.

When the error $δz$ exceeds the user-defined threshold, the updated layer height $dz'$ compensates for the deviation and allows to reach the target height in the same number of layers despite the accumulated error, assuming that the error would have otherwise remained constant throughout the deposition. In practice, the correction will occur several times until the controller converges to a programmed layer height that corresponds to the experimental layer height.

The user-defined threshold for layer height error has a lower bound given by the measurement accuracy (about ± 5 µm) and the positioning accuracy of the machine (about ± 10 µm). A higher bound can be obtained by considering the maximum allowable deviation on working distance over the number of layers to be deposited, which in most cases should be known beforehand using the proposed controller. For an LMD deposit of 20 layers and a maximum deviation of 1 mm for a nominal working distance of 5 mm, this yields an upper bound of 0.05 mm. In our current set of experiments, minimum and maximum layer height error thresholds are thus respectively set around 0.015 mm and 0.05 mm.

In a second step, the controller can adjust the process parameters to effectively correct the actual layer height being deposited in an effort to match it with the updated layer height $dz'$ in the NC program. This is particularly needed to correct larger errors caused for example by in-process instabilities in the powder mass flowrate.

Process parameters such as travel speed $V$ and powder flowrate $\dot{m}$ can be directly linked to the layer height $dz$ through Eq. 3, assuming a simplified bead shape and melt pool dynamics and using the law of mass conservation [12]:

$$ dz = \frac{\beta \dot{m}}{\rho V w_l} \quad (3) $$

As the catchment efficiency $β$, layer width $w_l$ and material density $ρ$ are often not precisely known, they are wrapped up into a variable $C$ that is assumed to remain constant near the operating point (Eq. 4). The value of $C$ can be evaluated based on the initial operating point and the measured powder mass flowrate:

$$ dz = C \frac{\dot{m}}{V} \quad (4) $$

For instance, the nominal operating point identified experimentally for 316L is 2.1 g/min at 1200 mm/min for a layer height of 0.165 mm (see Table 4). The resulting value is $C = 94.3 \text{ mm}^2/\text{g}$, where the associated laser power is 210 W for a top-hat laser spot with a 0.5 mm diameter.

The value of $C$ depends on catchment efficiency, density and layer width. Density varies with temperature but mostly
during a phase change so that it can reasonably be considered constant from layer to layer for a given material composition despite some heat accumulation. Catchment efficiency and layer width will however vary to some extent with the operating point. Thus, when corrections lead to a new stable operating point where layer height error remains below the threshold, the value of C could be updated based on the corresponding set of process parameters and layer height. However, it is kept constant in the current implementation.

Considering Eq. 4, two control strategies have been implemented in order to adjust the height of the deposited layers: a first strategy with variable travel speed V and constant powder flowrate $\dot{m}$, and conversely, a second strategy with variable powder flowrate $\dot{m}$ and constant travel speed V. Other process variables such as laser power remain constant.

At each control step, the error on layer height deviation $\delta z$ can be compensated by setting an updated programmed layer height $dz'$ according to Eq. 2, and adjusting the process parameter according to Eq. 4. Using the first strategy, an updated travel speed $V'$ is calculated using the updated layer height $dz'$ as well as the constant C and the mass flowrate $\dot{m}$ of the initial operating point (i.e., $V' = C \frac{\dot{m}}{dz'}$). With the second strategy, the powder mass flowrate is updated according to $\dot{m}' = \frac{Vdz'}{C}$.

Minimum and maximum limits are set by the user in the configuration file, either in absolute or in relative terms with respect to the initial value, so as to restrict the layer heights and process parameters corrections to acceptable ranges, which depend on the LMD machine configuration. Moreover, the rate of change of process parameter or layer height can also be limited to avoid drastic changes in deposition conditions between layers, thereby favoring a more gradual correction towards a more optimal operating point and thus limiting excessive layer-to-layer differences in solidification conditions. These limits of course affect the rate of convergence of the process controller towards a more optimal operating point.

The controller can thus be adapted to compensate for the errors in a more or less aggressive manner depending on the expected errors such as a suboptimal initial operating point or in-process instabilities (e.g., mass flowrate variations at the powder dispenser due to unexpected changes in powder flowability). For example, if calibration tests for setting process parameters and corresponding layer height

are limited but based on reasonable values from previous experiments in similar conditions, the initial systematic error on the operating point could be small to moderate, and this type of error will accumulate from the beginning of the deposition. If the LMD process is very stable, the limits on parameter value change and rate of change limits could be set to conservative, small values. Conversely, if the initial calibration is done very precisely but the powder has such low flowability that the powder flowrate can quickly change and lead to a sudden large error on layer height, larger limits could be employed to rapidly correct the powder flow rate. While some heuristics could help identify the type of error depending on when and how fast they occur and grow, the control scheme can operate without discriminating between the various sources of error.

## 2.6 Experimental design

A series of experiments were carried out in order to identify suitable nominal parameters for open loop homogeneous repairs of machined cavities on a substrate. Two types of materials were tested: 316L powder on a 316L stainless steel substrate, and IN625 powder on IN625 substrate (see Sect. 2.1).

A first phase of the experiments consisted in depositing single bead walls (Fig. 5) in order to adjust nominal parameters such as laser power, travel speed, powder flowrate and layer height. These thin walls were deposited with a raster toolpath strategy where the N + 1 layer is deposited starting from the end of the Nth layer.

In a second phase, blocks with a square base of 10 mm were fabricated (Fig. 5) in order to set the horizontal overlap between beads and make the necessary adjustments to the vertical increment, which generally changes slightly from the case of single bead walls. It also served to evaluate the penetration of the blocks inside the substrate with the chosen parameters (Fig. 7).

The third phase involved the repair of spherical cavity in order to evaluate the applicability of the process parameters previously established for blocks to the case of cavities, for which further parameter adjustments of layer height are necessary. The parameters were validated by considering the geometry and integrity of the deposit as well as its penetration inside the substrate.

<table><caption>**Table 4** Nominal LMD parameters for spherical cavity repairs in 316L and IN625</caption><thead><tr><th>Powder</th><th>Powder flowrate (g/min)</th><th>Power (W)</th><th>Travel speed (mm/min)</th><th>Layer height (mm)</th><th>Interbead distance within infill (mm)</th><th>Interbead distance between infill and contour (mm)</th></tr></thead><tbody><tr><td>IN625</td><td>2,4</td><td>210</td><td>1000</td><td>0,23</td><td>0,25</td><td>0,2</td></tr><tr><td>316L</td><td>2,1</td><td>210</td><td>1200</td><td>0,165</td><td>0,25</td><td>0,2</td></tr></tbody></table>
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 5 Thin walls (top) and blocks (bottom) in IN625</figcaption></figure>

Table 4 presents the deposition strategy and the nominal parameters selected for the cavity repairs. The deposition strategy combines 2 types of trajectories for each layer, i.e., a contour followed by a raster infill with 90° rotation between each layer.

In order to guarantee a good filling of the edges of the cavity near the part surface where the cavity walls are vertical, three additional layers were added at the end of the deposit (Fig. 6). These extra layers were widened compared to the last layers so that their periphery is shifted by + 1 mm with respect to the cavity boundary to allow surfacing during post-processing. The quality of the cavity repairs using the nominal parameters in open loop was evaluated through a metallographic analysis of the samples using 2D optical imaging, scanning electron microscopy (SEM) and Vickers hardness testing. The samples were prepared for metallographic analysis by cutting through the middle section of the deposit, polishing to mirror finish and etching with the Adler reagent to reveal the microstructure. The absence of macroscopic defects such as porosities, macrocracks and lack of material was validated by 2D optical imaging, and the penetration on the substrate was measured. The microstructure was observed by SEM at the substrate/deposit interface and throughout the deposit to detect potential microscopic defects such

as microcracks. The mechanical strength was evaluated through Vickers hardness tests using filiation of 10 points.

After the validation of the nominal parameters in open loop, several experiments were performed using closed-loop control. In those tests, two approaches were adopted for setting the initial conditions. In the first approach, the parameters used for open loop tests were set as the initial parameters to test the capability of the control loop to compensate for small errors, considering that the open loop operating point is reasonably close to optimal conditions. In the second approach, the initial parameters were purposefully deviated from the nominal operating point identified during the open loop experiments by a significant margin (i.e., between 15 and 25%). This allows to test the correction of larger errors that may occur with either a very suboptimal operating point or large deviations in process parameters such as powder flowrate during the deposition. The objective is to test whether the deposit can recover and converge to a reasonable value.

These tests were performed with 316L and IN625 materials on two types of cavity geometries (sphere and groove) to reflect various repair cases. The cavities were machined on coupons made with a flat substrate as well as on a demonstration part in IN625 that represents a Y-junction with cylindrical pipe geometries.

Initial conditions for two cavity repair tests with closed-loop control are presented in Table 5. These two examples, one for each of the two materials 316L and IN625, show the behavior of the control loop using the adaptive speed (316L sample SS for “Steel Speed”) and adaptive powder flowrate (IN625 sample FI for “Flowrate Inconel”) control strategies. For both tests, the initial value of the process parameter was significantly modified compared to the nominal value from open loop tests. Note that the variation in layer height between samples SS and FI of identical geometry induces a different number of layers between these samples that is in line with the nominal values identified for these materials. These two samples thus provide different cases for testing the controller over a different number of layers and with different materials.

Table 6 indicates the values for the error threshold beyond which the layer height is corrected, as well as the limits in values and rate of change of parameters expressed relatively to their nominal value (Fig. 7).

<figure><img src="image_11.png" /><figcaption>Fig. 6 316L/316L spherical cavity repairs with nominal parameters</figcaption></figure>

<figure><img src="image_12.png" /></figure>
<table><caption>**Table 5** Process parameters for closed-loop tests</caption><thead><tr><th>Sample</th><th>Material</th><th>Geometry</th><th>Laser power (%)</th><th>Nominal travel speed (mm/min)</th><th>Initial travel Speed (mm/min)</th><th>Nominal powder flowrate (%)</th><th>Initial powder flowrate (%)</th><th>Layer height (mm)</th><th>Overall target height (mm)</th></tr></thead><tbody><tr><td>FI</td><td>IN625</td><td>Groove</td><td>16</td><td>1000</td><td>1000</td><td>41.2</td><td>35</td><td>0.23</td><td>0.14</td></tr><tr><td>SS</td><td>316L</td><td>Groove</td><td>16</td><td>1200</td><td>900</td><td>36</td><td>36</td><td>0.165</td><td>0.29</td></tr></tbody></table>

<table><caption>**Table 6** Controller configuration</caption><thead><tr><th>Sample</th><th>Layer height error threshold (mm)</th><th>Parameters max/ min limit (%)</th><th>Parameter rate variation limit (%)</th></tr></thead><tbody><tr><td>FI</td><td>0.05</td><td>+/-75</td><td>+/-20</td></tr><tr><td>SS</td><td>0.025</td><td>+/-75</td><td>+/-20</td></tr></tbody></table>

# 3 Results and discussion

## 3.1 Nominal parameters

The macrographs (Fig. 6) and micrographs (Fig. 8) of samples deposited in open loop with nominal parameters dense deposit without major pores, cracks or lack of material. The geometry of the deposit conforms properly to the cavity and the top surfaces of the repairs are relatively flat. The penetration of the deposit into the substrate is also satisfactory according to the optical microscope analyses as it is reasonably deep and homogeneous throughout the interface.

The average penetration depth measured on the three spherical cavities is 230 µm, with a maximum penetration located at the bottom of the cavity. The maximum

penetration measured is 360 µm. These measurements are similar to the results obtained for the blocks deposited with the same parameters, where the average penetration is about 250 µm (cf. Fig. 7). The adhesion at the deposit/substrate interface seems satisfactory, as confirmed by hardness testing.

Hardness filiations were carried out with a Vickers macroindenter on the middle section of the spherical cavities as seen on Fig. 9 for a selected 316L sample called sample NP (for “Nominal Parameter”). The hardness results show that the hardness is lowest in the substrate (see Fig. 10). The differences in hardness measured between the deposit and the substrate are significant in terms of the mean hardness values in each zone and the corresponding standard deviations, so that the differences in the means between the two zones amount to several standard deviations.

The hardness measured at the interface indicates intermediate hardness levels between the deposit and the substrate for IN625 and hardness levels similar to those of the deposit for 316L. There was, therefore, no drop in hardness at the interface, demonstrating that the deposit is satisfactorily fused to the substrate.

These results for open loop deposits serve as a reference for comparison with the deposits performed with closed-loop control, which are detailed in the following section.

<figure><img src="image_13.png" /><figcaption>**Fig. 7** Penetration on 316L/316L block</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 8 Micrographic image of the center of the repaired hemispherical cavity</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 9 Vickers hardness filiations (left) and results of one of the indentations (right) on 316L NP sample</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Fig. 10 Results of the Vickers hardness filiation for sample NP (316L)</figcaption></figure>

<table><caption>Vickers hardness HV0.5<br>Sample NP (316L)</caption><thead><tr><th>Localisation</th><th>Average</th><th>Standard deviation</th></tr></thead><tbody><tr><td>Deposit</td><td>230,67</td><td>6,66</td></tr><tr><td>Interface</td><td>244,25</td><td>4,19</td></tr><tr><td>Substrate</td><td>197,67</td><td>6,51</td></tr></tbody></table>

<figure><img src="image_17.png" /></figure>
## 3.2 Closed-loop correction

Figure 11 illustrates deposits obtained using closed-loop control. Figure 12 shows the graphical results of the experiments for sample FI (left column) and sample SS (right column). The top graphs show the variation of process parameters (power, travel speed, powder flowrate) as well as the nominal value for open loop conditions. The middle graphs show the evolution of the layer height error ($δz$ i.e., “dz_error” in the figures for clarity) with the associated user-defined threshold (“dz_error_threshold” in the figures) as well as the changes in programmed layer height ($dz'$, i.e., “dz_corrected” in the figures for clarity) induced by the

control loop. The bottom graphs show the programmed and experimental deposit height and the corresponding error.

In order to test the adaptive powder flowrate control strategy (see sample FI in left column of Fig. 11, IN625 cylindrical cavity with length of 50 mm, depth of 4 mm and diameter of 8 mm), the initial flowrate for sample FI is purposefully reduced to 35% when the nominal flowrate is at 41% or approximately 2.4 g/min. As shown in Fig. 12, the flow correction starts after the 3rd layer when the error on the layer height exceeds the error threshold. The flowrate then increases to around 42% and remains stable during 4 layers until the layer height again falls below the error threshold after layer 7, so the flowrate is significantly increased by the controller at layer 8 then slightly reduced

<figure><img src="image_18.png" /><figcaption>Fig. 11 Samples of groove cavity repairs—sample FI (left) and sample SS (right)</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>Fig. 12 Experimental results for samples FI (left) and SS (right)</figcaption></figure>
at layer 9 as the error returns below the threshold. Several small corrections of the layer height and flowrate take place over the next layers until the final layers are deposited at a final value of 46.7% of the maximum flowrate. The average throughput over the 18 layers is 42.6%, which is relatively close to the nominal value. The final height measured by the scanner is 0.52 mm, which is higher than the programmed final height of 0.14 mm.

In order to test the adaptive speed control strategy (see sample SS in right column of Fig. 11, cylindrical cavity with length of 50 mm, depth of 4 mm and diameter of 8 mm), the initial speed for sample SS is purposefully reduced by 25% to 900 mm/min compared to the nominal speed of 1200 mm/min used for 316L material. As shown in Fig. 12, the speed correction starts after the 9th layer when the layer height error exceeds the error threshold. At this point, the speed is increased to reduce the layer height, which effectively corrects it for layer 10. The speed remains the same until layer 14 where the error again gradually reached the upper threshold value so the speed is further increased slightly beyond the nominal speed (shown in dashed yellow line). The error again falls near zero but the same behavior repeats where the error grows until layer 18 where it once again crosses the upper threshold. The speed then increases until it reaches a final and maximum value of 1575 mm/min, which occurs due to the maximum limit of parameter change that is set at +75% of the initial value of 900 mm/min (see Table 6). The error, therefore, grows in the final layers as the speed reached its maximum allowed value and cannot be further increased, which could be necessary for metallurgical reasons for example. Interestingly, the overall height error still converges towards zero thanks to previous corrections so the controller can still compensate for the overall height deviation despite hitting the speed limit set by the user. The average speed over the 26 layers is 1201 mm/min, which is almost equal to the nominal speed. The final height measured by the scanner is 0.54 mm, which is slightly higher than the programmed final height of 0.29 mm. Note that the layer height error increases beyond the positive threshold on

the last layers because the maximum allowed speed of 1575 mm/min is reached due to the limit at +75% of the initial value. The control loop does not correct beyond this point, but the error is positive and, therefore, allows the desired height to be reached.

Thus, the control loop achieves a satisfactory cavity repair and final deposition height by means of an inter-layer correction that makes the powder flowrate or travel speed converge to an average value almost equal to the nominal value. Whether the initial value for speed or flowrate is higher or lower than the nominal value, a similar correction process would unfold. If the speed is set higher than required for instance instead of lower for sample SS, the layer height error would quickly fall below the lower threshold value and the speed would be reduced by the controller until the error falls within the threshold or it reaches a limit value set by the user.

Macroscopic observations (see Figs. 13, 14) of the middle section grooves show no material gaps, macrocracks or macroporosities on the deposit even when varying the powder flowrate (FI) or the speed (SS) over several layers. In addition, there are no major discontinuities (i.e., missing material, cracks, etc.) at the deposit/substrate interface, suggesting satisfactory adhesion.

It should be noted that while the geometry of these grooves (8 mm diameter, 4 mm depth, and 50 mm length) is purposefully unfavorable to complete filling using 3-axis trajectories (2.5D) since the depth is equal to the radius and thus the walls are vertical at the top of the cavity, the repair was nevertheless performed satisfactorily. Indeed, there are no discontinuities on the vertical walls and the footprint of the deposited beads can be identified all along the interface, indicating that the repair has correctly penetrated the substrate. This shows that the control loop can compensate for considerable variations from the nominal operating point while maintaining acceptable metallurgical quality.

Scanning electron microscope micrographs (see Figs. 15, 16) of the grooves middle section reveal the difference in microstructure size and morphology between the substrate and the deposit, which is finer in the deposit due to the rapid

<figure><img src="image_20.png" /><figcaption>**Fig. 13** Macrograph of sample FI (IN625)</figcaption></figure>

<figure><img src="image_21.png" /></figure>
solidification caused by the laser source. These micrographs do not reveal any major microscopic defects (hot cracking, microporosity, lack of material, etc.). The integrity of the deposited material is thus satisfactory. The shape of the deposit microstructure is generally cellular with columnar structures near the refusion zones between layers, which is typical of AM deposits by LMD. The direction of the local thermal gradient is indicated by the orientation of these columnar structures.

Analogously to the results obtained with the nominal parameters (see Fig. 10), the Vickers hardness filiations carried out on the middle section of the two grooves (IN625/ IN625 and 316L/316L) show in both cases that the hardness is significantly lower in the substrate in terms of the mean

hardness values in each zone and the corresponding standard deviations.

There is no drop in hardness at the interface, the average hardness values obtained are between the deposit and the substrate measures for IN625 and similar to those of the deposit for 316L (see Figs. 17, 18).

The various metallurgical characterizations (optical microscopy, SEM observations and hardness testing) show that the mechanical properties of the deposit and the interface are superior to the substrate. This confirms the material integrity and overall quality of the LMD repair when process parameters are deviated from their nominal point.

Comparing results from Fig. 10 (sample NP: 316L hemispherical cavity without control) and Fig. 18 (sample SS: 316L cylindrical cavity with control), the hardness

<figure><img src="image_22.png" /><figcaption>**Fig. 15** Micrographs of sample FI (IN625)—left (15.1), middle bottom (15.2), right (15.3)</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>**Fig. 16** Micrographs of sample SS (316L)—left (16.1), middle bottom (16.2), right (16.3)</figcaption></figure>
values in all regions are comparable. This suggests that the quality of the deposited material under closed-loop control remains similar to the open loop case in terms of hardness. The macroscopic and microscopic observations also do not show major differences between the two cases. Although these two samples are from cavities of different geometry, it shows that the control loop helps provide better geometric accuracy at little cost to deposit quality in the experiments performed, provided suitable thresholds and limits are applied.

## 3.3 Application

Various repairs were performed on a cylindrical Y-junction in IN625 in order to vary the type of cavity geometries to be repaired and to test the closed-loop control on real parts.

An example of each of the two different control strategies tested (i.e., adaptive speed and adaptive powder flowrate) are presented in Table 7 along with controller configuration in Table 8.

The cavities are cylindrical grooves machined through the periphery of the cylindrical wall of the Y-junction (see Fig. 19). Sample SI and FI2 have a maximum cavity depth of 4 mm for a diameter of 12 mm. The toolpaths were generated on a volume that was slightly larger in length and depth than the theoretical volume in order to ensure a deposit height

<figure><img src="image_24.png" /><figcaption>**Fig. 17** Results of the Vickers hardness filiation—sample FI (IN625)</figcaption></figure>

<table><caption>**Fig. 18** Results of the Vickers hardness filiation—sample SS (316L)</caption><caption>Vickers hardness HV0.5<br>Sample FI (IN625)</caption><thead><tr><th>Localisation</th><th>Average</th><th>Standard deviation</th></tr></thead><tbody><tr><td>Deposit</td><td>414,00</td><td>25,45</td></tr><tr><td>Interface</td><td>365,25</td><td>33,72</td></tr><tr><td>Substrate</td><td>267,50</td><td>13,60</td></tr></tbody></table>

<figure><img src="image_25.png" /></figure>

<table><caption>Vickers hardness HV0.5<br>Sample SS (316L)</caption><thead><tr><th>Localisation</th><th>Average</th><th>Standard deviation</th></tr></thead><tbody><tr><td>Deposit</td><td>242,60</td><td>11,19</td></tr><tr><td>Interface</td><td>246,67</td><td>3,51</td></tr><tr><td>Substrate</td><td>198,00</td><td>6,98</td></tr></tbody></table>
<table><caption>**Table 7** Process parameters</caption><thead><tr><th>Sample</th><th>Material</th><th>Geometry</th><th>Laser power (%)</th><th>Nominal travel speed (mm/min)</th><th>Initial travel speed (mm/min)</th><th>Nominal powder flowrate (%)</th><th>Initial powder flowrate (%)</th><th>Layer height (mm)</th><th>Overall target height (mm)</th></tr></thead><tbody><tr><td>FI2</td><td>IN625</td><td>Groove</td><td>16</td><td>1000</td><td>1000</td><td>42</td><td>50</td><td>0.23</td><td>0.83</td></tr><tr><td>SI</td><td>IN625</td><td>Groove</td><td>16</td><td>1000</td><td>800</td><td>42</td><td>42</td><td>0.23</td><td>0.83</td></tr></tbody></table>

<table><caption>**Table 8** Closed-loop configuration</caption><thead><tr><th>Sample</th><th>Layer height error threshold (mm)</th><th>Parameters max/ min limit (%)</th><th>Parameter rate variation limit (%)</th></tr></thead><tbody><tr><td>FI2</td><td>0.025</td><td>+/-90</td><td>+/-50</td></tr><tr><td>SI</td><td>0.025</td><td>+/-75</td><td>+/-50</td></tr></tbody></table>

bigger than that of the substrate, and thus allow for possible finishing in the case of industrial applications.

For the FI deposit, the initial powder flowrate was increased to 50%, whereas for the SI deposit, the initial travel speed was reduced by 20% to 800 mm/min (see Table 7).

The powder flowrate correction starts after the 8th layer when the layer height error exceeds the error threshold. The mass flowrate decreases as expected until it reaches a final value of 30.7%. The average powder flowrate over the 21 layers is 43.7%, which is close to the theoretical value.

The travel speed correction starts after the 7th layer when the layer height error exceeds its threshold. The speed increases as expected until it reaches a maximum value of 1343 mm/min. The average speed over the 21 layers is 997 mm/min, representing a deviation of around 0.3% from the nominal value.

The final height measured by the scanner is 0.88 mm for the FI deposit and 0.85 mm for the SI deposit, which is very close to the programmed final height in both cases, i.e., 0.83 mm.

Thus, the control loop enables a suitable deposit height close to the target height to be obtained by correcting the powder flowrate or travel speed when the values differ from the identified nominal operating point (Fig. 20).

One of the extremities of two of the repairs carried out on the demonstrator was finished by grinding in order to assess the quality of the LMD repair. Figure 21 shows that the grounded ends of the repairs as pointed by the blue arrows do not reveal any lack of material or other defect. There is no visible difference after finishing between the repair area and the rest of the part for these two grooves.

## 4 Conclusion

The closed-loop controller proposed in this study allows for the online correction of DED processes such as LMD by adjusting the layer height and process parameters such as travel speed and powder flowrate based on in-between layers 3D scan of the deposit. This improves the build quality and dimensional precision of such AM processes compared to the usual open loop operation.

The controller objective is based on reaching the target height with the original number of layers. It is a discrete time, model-based adaptive controller that performs a two-step correction where the layer height and process parameters are adjusted between layers. This allows to avoid a reslicing step by aiming at retaining the initial number of layers, and thus permits a faster and fully automated control loop. Such a control scheme also lowers the sensitivity of DED processes to both suboptimal initial operating points and unexpected in-process variations.

This approach can be leveraged for other DED processes such as WAAM, where the controlled process parameters can differ (e.g., wire feed speed instead of powder flowrate) but the control scheme would remain essentially the same. Adjustments would have to be made to error threshold and parameter variation limits as the process specifications and capabilities differ. Notably, WAAM typically operates on a larger scale than LMD. However, these processes both belong to the family of DED processes and thus share similar principles so that the current controller could easily be used for WAAM.

Moreover, the 3D scanning data collected throughout the deposit give more traceability to the deposition process, which benefits quality control procedures after deposition and facilitates the industrial certification of the LMD process for repair applications.

The metallurgical analysis and hardness tests performed on various 316L and IN625 samples confirm that the deposit

<figure><img src="image_26.png" /><figcaption>**Fig. 19** Samples of IN625 repairs—sample SI (left) and sample FI2 (right)</figcaption></figure>
<figure><img src="image_27.png" /><figcaption>**Fig. 20** Experimental results for samples FI (left) and SI (right)</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>**Fig. 21** Repairs appearance after a finishing operation (blue arrows show the initial extremity of each repair before grinding)</figcaption></figure>

quality remains satisfactory under closed-loop control. Even the least favorable samples in terms of repair geometry were adequately refilled, without any major defect such as cracks, lack of fusion or lack of penetration.

Therefore, operating AM processes such as LMD or WAAM and other processes with such a closed-loop controller significantly improve the applicability of AM processes for industrial applications by increasing their robustness, in particular for repair applications.

Future work would consist in refining the control scheme by improving the physics model and formally studying its stability, convergence and performance. Moreover, experiments could be conducted on other materials and DED processes such as WAAM. The repair application could be extended to fabrication applications, which will widen the applicability of the controller to more complex deposits at the cost of added complexity in the algorithm implementation, for example when extracting the region of interest from the 3D scan. Finally, it would be useful to establish more

<figure><img src="image_29.png" /></figure>
comprehensive guidelines for setting user-defined thresholds
and limits based on empirical data and machine capabilities.

**Funding** Open access funding provided by Centrale Nantes.

Declarations

**Conflict of interest** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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