Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
1

Development of Additive Manufacturing Technology
toward Practical Utilization

TAKASHI ISHIDE*1

YASUYUKI FUJIYA*2

KAZUNORI KUGA*2

MASAYA HATANAKA*3

TAKAFUMI SHINOGI*3

RYUICHI NARITA*4

The development of additive manufacturing technology and consideration for its application
has widely been promoted as a method capable of manufacturing parts with complicated shapes ina short period of time. Various types of additive manufacturing equipment has been developed andsold, but in order to satisfy requirements for quality, cost and manufacturing period, and to receive
the benefits of its application, it is necessary even for the equipment user to develop a wide rangeof technologies. Mitsubishi Heavy Industries, Ltd. (MHI) has also been developing various
technologies such as powder management technology, optimum support design technology, theoptimization of manufacturing conditions, quality monitoring, inner surface polishing technology,
etc., and promoting the application of additive manufacturing technologies to our products.

|1. Introduction
Powder bed additive manufacturing (AM) technology originated from Japan in 1980. As far
as metals go, there was a method to mold an object using powder mixed with bronze powder
developed by Fraunhofer-Gesellschaft, but this additive manufacturing was the first completed
technology for performing molding using a single metal powder. After that, AM equipment was
sold by various manufacturers. Recently, various types of equipment have been sold, such as an
EB-laser hybrid type aiming for larger size and higher speed1), a binder jet type2), a metal platingtype that forms planes3), equipment for melting a wire by resistive heating or directly spraying
metal droplets to perform high-speed molding4), etc. Under such circumstances, it is often thoughtthat AM equipment can be used simply and immediately after purchase. However, it is necessary tosolve various problems such as the specifications and cost reduction of metal powder, theoptimization of manufacturing conditions and support conditions, quality in-process monitoring,etc. This paper presents the technology development that we are working on in order to solve suchproblems.

|2. Powder management technology
Metal additive manufacturing generally uses a powder raw material. The powder bed systems
that are generally being considered for use in this application lay metal powder supplied from apowder tank to a uniform thickness using a squeegee blade, and then applies a heat source such as alaser or an electron beam to the necessary point and melts and solidifies the powder. By repeating
this, a three-dimensional shape is formed.
Generally, gas atomized powder (Figure 1), which consists of spherical particles, is used formetal additive manufacturing. There is a problem, however, that the influence of the powder
characteristics such as the particle size and the particle shape on molding quality could not beclarified. As a measure to solve this problem, we implemented molding tests using various powder
characteristics as parameters to clarify the influence of such characteristics, and formulated powder
*1
*2
*3
*4

Director, Global Research & Innovation Center, Research & Innovation Center
Chief Staff Manager, Manufacturing Technology Research Department, Research & Innovation Center
Manufacturing Technology Research Department, Research & Innovation Center
Mitsubishi Heavy Industries Europe. Ltd. Senior Manager

Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)2

management standards for obtaining stable molding quality.
The characteristics required for powders used for metal additive manufacturing are mainly(1) that the powder can be laid uniformly and stably, (2) that the molded object made by solidifying
the melted powder is accurate, (3) and that the molded object has the necessary mechanicalproperties. With regard to these requirements, we evaluated the degree of influence of factors suchas particle size distribution, set the management items for each of them, and set the management
scope separately for cases with different behaviors depending on the material. Figure 2 presents asituation in which powder with a difference in fluidity is laid as an example of the evaluation of theinfluence of the powder characteristics. Based on these experimental data, we understood thephenomena, set powder management standards for obtaining stable quality and have put them into
practice.

Figure 1

Gas Atomized Powder

Figure 3 Changes in powder oxygen
concentration during recycling

Figure 2

Effect of fluidity on powder layability

In the case of the powder bed system, the powder surrounding the molded object remains as
surplus powder after molding, so it is possible to efficiently manage the use of the powder byrecovering and reusing it, and this leads to a reduction in manufacturing cost. However, the state of
the powder changes due to the thermal effect, etc., during molding. Therefore, to maintain stablemolding quality, it is necessary to understand the changes to the powder caused by repeated use andset the usage limit. Figure 3 gives the changes in powder oxygen concentration during repeatedmolding. The oxygen concentration of the powder is increased by repetitive molding, so we set theupper limit of the oxygen concentration for each material to ensure the mechanical properties of themolded product and manage the reused powder.
As described above, by setting the powder management standards and the use limit of therecycled powder, it became possible to stabilize the molding quality and reduce the powder cost.

|3. Optimum support design technology
Metal additive manufacturing simultaneously molds not only the intended object, but also asupport member at the site where thermal deformation occurs during lamination, mainly for the
suppression of thermal strain. The arrangement and shape of the support member are very
important. When a support member has a highly-rigid shape and the area to be connected with themodeled object is wide, the effect of suppressing the thermal strain is high, but it takes a very long

Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
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time to remove the support member in post-processing. Meanwhile, if a support member is moldedin a space inside the molded product which is difficult to access with a removal tool, it cannot be
removed in post-processing. For this reason, it is necessary to be able to form a support memberthat is small in quantity, has low rigidity and within the target shape tolerance while considering
the workability in the process of removing the support member.
Therefore, we constructed an optimum support design system with the product posture as avariable. Figure 4 illustrates the process of this system. In order to make this system morepractical, we evaluated the ease of tool access for support removal (Figure 5 (a)), rather thansimply minimizing the support amount. In general, when the angle of the modeling surface withrespect to the orthogonal stacking direction exceeds a certain value, support is unnecessary (Figure5.2). For this reason, the determination of whether a support is necessary is a piecewise
discontinuous function with respect to the product posture (angle). The optimal solution searchalgorithm uses GA (genetic algorithm) to prevent falling into a local solution. However, since thenumber of calculations for GA is enormous, the calculation including modeling takes much time.
Therefore, a response phase was created, and optimization analysis was performed. At this time,
the accuracy of the response surface was confirmed by confirmation calculation and the optimal
solution was sought again after updating the response surface. As a result of repeating suchconfirmation, the prediction accuracy was improved and we could obtain a better solution.
Figure 6 presents the results of trial calculation targeting a certain mechanical component.
By building an optimum support design system suitable for the characteristics of metal additive
manufacturing based on these results, a molding posture that minimizes the support amount can bederived while eliminating dependency on individual skill.

Figure 4

Optimum support design system process

Figure 6

Figure 5

Evaluation of supportability and support necessity

Trial calculation results

Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)4|4. Process condition optimization technology
The process conditions suggested by the equipment manufacturer are those for the material
supplied from the equipment manufacturer. Therefore, it is unknown whether the optimum process
condition settings are set. In addition, when molding materials and shapes that have not beenmolded so far, it is necessary to ask the equipment manufacturer to consider the process conditions,so the selection of the optimum process conditions requires a long period of time and enormous
cost, which leads to a bottleneck. Therefore, we standardized the condition selection flow to obtain
the appropriate process conditions for applying additive manufacturing to a new material.
Specifically, to obtain a laminated model with a high density without leaving an un-melted
layer or porosity, the process condition settings for selecting the overlapping condition and thelamination pattern of each bead in stages and narrowing down the process conditions has beenstandardized in consideration of the stability and cross-sectional shape of the bead formed by laser
melting. Table 1 depicts the process condition selection flow. In the case of a powder bed fusion
apparatus, the basic process parameters of process are laser power, scanning speed and beadoverlapping amount (hatch interval). Therefore, first the laser power and scanning speed range
where stable beads can be obtained with one bead are selected, followed by an appropriate hatch
interval within the range of conditions to select the single layer process condition range (STEP 1).
Next, while narrowing down the execution condition range by changing the laser power, thescanning speed, the pattern, overlap margin and rotation angle of the basic cell at a predeterminedlamination thickness and hatch interval, the density is evaluated using a cubic block, and thecondition range with a high density is narrowed down (STEP 2).
Table 1

3D process condition selection flow

[STEP 1] Narrowing down the manufacturing conditions[STEP 2] Selecting theby one layer manufacturing (set for each layer thickness)process condition
1. Bead stability
2. Bead cross3. Bead overlapping 4. Evaluation of filling
evaluation (1section shape
shape evaluation
factorbead)
evaluation (1(Single-layer(Multilayer multipass
bead)
multipass beadlaminated bead: Cube
block)
stacking)
Proper penetration Appropriate bead Based on the lamination
Stable beadshape conditions overlapping
proper condition range
forming
are selectedof one layer, the properconditions areconditions are- Penetration
pattern to homogenize
selected
selecteddepth (d0)modeling quality are- Overlap bead
1.5t≦d0≦2t
selectedroughness- Pattern of basic cell
- Overlap(unit filling area)
penetration(stripe, chess)
depth (d1)
- Basic cell overlapping
margin- Basic cell rotation
angle (θ)[STEP 3] Selecting the conditions
according to the part shape
5. Limit shape and surface condition
evaluation (various-shapelaminating)Appropriate heat input conditions
are selected according to the shapeof the part under the conditions
within the proper condition range
where the filling rate of Step 2 issecured- Horizontal hole (upper limit withno support, moldable lower limit)- Moldable slit lower limit- Lowest plate thickness that can bemolded- Limit angle

Finally, evaluation of laminations in various sizes was performed in the condition range
where the high-density ratio is secured, and conditions are set according to the shape and surface
roughness requirement (STEP 3).
Based on the standard setting flow, the proper process condition range was selected using
standard materials (SUS316 and Inconel 718) which are widely applied to molding. As a result, it
was confirmed that a process condition range with a density ratio of 99.9% or more could beobtained, so process conditions for applying a new material can be optimized at a low cost and in ashort period of time.

Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
5|5. Quality monitoring technology
Metal additive manufacturing takes several dozen hours for the completion of lamination
molding. However, currently there are no methods or criteria for stopping the molding when afailure occurs. Therefore, if the molded object fails the quality inspection after molding operation,
significant rework will be required. As such, for the purpose of the prevention of rework andsecuring traceability, we developed a monitoring technology targeting the state of the powder
laying and the surface condition after laser irradiation as the molding situation.
A fringe projection method was adopted to ascertain the surface condition. Figure 7
illustrates the equipment configuration. Because there is an optical system for laser irradiation in
the center of the additive manufacturing equipment, a projector is installed in the corner inside theadditive manufacturing equipment to reflect the laser with a mirror and change the direction to
generate a striped pattern on the modeling surface. In this system, two cameras used for thestabilization of the measurement are arranged to be accommodated within the SLM device.

Figure 7 Application of fringe projection method to 3D molding

Figure 8 gives an example of measurement by the fringe projection method. In themeasurement, two kinds of fringe patterns as can be seen in the upper part of Figure 8 are projectedon the target surface. By processing these data, A stereoscopic view as shown at the bottom left iscreated. Then contour representation as presented in the bottom right is used for checking theconcavities and convexities.

Figure 8 Measurement example of fringe projection method

To confirm the measurement accuracy of the fringe projection method, comparison with ameasurement result obtained by a laser displacement gauge (LK-G System) was implemented forverification. The Z axis of the device was mechanically moved up and down by 10 μm and 100 μm,

Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)6

and the height change of the modeling surface was measured by the two. As a result, it was verified
that the measurement of the fringe projection method was comparable to the measurement of thelaser displacement gauge. Table 2 is a comparison of the measurement results of the 1 mm stepped
flat plate obtained by the fringe projection method and by a three-dimensional measuring
instrument. It was confirmed that the measurement of the fringe projection method was comparable
to the measurement of the three-dimensional measuring instrument. Based on the above two
verification results, it was confirmed that the fringe projection method can implement
highly-precise measurement.
Table 2 Comparison of measurement results of fringe
projection method and three-dimensional measuring
instrument
Fringe projection
Three-dimensional measuring instrument

Measurement result
1.032 mm +/- 17.8 µm (1σ)
1.037 mm +/- 9.7 µm (1σ)

Figure 9 presents the shape measurement results during additive manufacturing by the fringe
projection method. After powder laying, streaks (a-1) and foreign substances (a-2) which areconsidered to be caused by looseness of the rubber of the recoater and uneven pressing wereconfirmed. After laser irradiation, areas where the powder laying was not uniform (b-1) anddeformation (b-2) were confirmed.

Figure 9 Shape measurement results during additive manufacturing obtained by fringe
projection method|6. Inner surface polishing technology
Problems common to metal additive manufacturing products include the coarse surface
roughness. Especially for the fluid flow path, etc., the surface roughness greatly affects productperformance, so the establishment of polishing technology for inner surfaces with complicatedshapes that tools cannot reach is required.
An inner surface polishing method was examined for turbocharger parts as an example ofinner surfaces with complicated shapes. Figure 10 is the schematic shape of the compressor cover.
Since it is difficult to access the inner surface of the compressor cover with a polishing tool,candidate polishing methods include viscoelastic abrasive grain flow polishing, centrifugal barrel
polishing, chemical polishing, etc. For this time, centrifugal barrel polishing was selected from theperspective of the polishing efficiency and the prevention of shape loss in a narrow space, and thepolishing performance was evaluated.

Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)7

The problem of centrifugal barrel polishing is to make the polishing media collide efficiently
over the inner surface with a complicated shape. As a solution to this problem, a method toencapsulate the polishing media on the inner surface of the workpiece was adopted. Figure 11 is aschematic view of the centrifugal barrel polishing of the polishing medium encapsulation system.
From the perspective of the mobility on the inner surface of the workpiece, the shape, size,
material, encapsulation amount, etc., according to the inner surface shape are important. Therefore,
after determining these polishing conditions in the preliminary element test, the polishing test of
the compressor cover was carried out.

Figure 10 Outline ofshape of compressor
cover, which is aturbocharger part

Figure 11 Centrifugal barrel polishing with polishing media
encapsulation method

Figure 12 depicts the surface roughness of the inner surface of the compressor cover before
and after polishing. The surface roughness before polishing did not satisfy the required value overthe entire inner surface. The surface roughness after polishing satisfied the required value over theentire inner surface from a relatively large space to a narrow space. From these results, it was foundthat the polishing medium encapsulated in an appropriate amount was moved over the entire area inthe workpiece due to the rotation and revolution motion of the workpiece, generating polishingforce. It was confirmed that this method is effective as a surface roughness improving method forthe inner surface with a complicated shape.

Figure 12

Surface roughness of inner surface of compressor cover before and after polishing

Currently, based on these results, the applicability to various turbocharger parts is being
continuously evaluated. We plan to verify each kind of material in each of the main sizes andevaluate the surface roughness and the product performance improvement effect to put thistechnology into practical use.

Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)8|7. Conclusion
In order to receive the benefits of the application of additive manufacturing technology,
which is being developed as a method capable of manufacturing parts with complicated shapes in ashort period of time, it is necessary even for the equipment user to develop a wide range oftechnologies. This paper introduced powder management technology, optimum support design
technology, the optimization of manufacturing conditions, quality monitoring and inner surface
polishing technology as an approach to these technological developments at the Mitsubishi Heavy
Industries, Ltd. Research & Innovation Center. Some of the latest manufacturing technologies
developed have already been applied to the manufacture of our products after verification of theireffectiveness. We will continue to improve the Quality, Cost and Delivery of our products andpromote technological development in order to meet customer needs.

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3D natives.com
https://www.3dnatives.com/en/fabric8labs-interview-050220184/
3D Printing Media Networkhttps://www.3dprintingmedia.network/additive-manufacturing/am-technologies/what-is-nanoparticle-jetting/