Mitsubishi Heavy Industries Technical Review Vol. 56 No. 2 (July 2019)1

Rapid Prototyping of Turbocharger Turbine Housings
through Powder Bed Fusion Additive Manufacturing

Atsushi Takita*1

Keisuke Shinzawa*1

Yuzuru Ogura*2

To leverage the rapidly-increasing demand for turbochargers into an increase in the numberof orders, it is important to shorten the development cycle and reduce the time required before
presenting a prototype to the customer. The interface shapes of the compressor cover and turbine
housing, both of which are parts of a turbocharger, are specific to each car type. Moreover, as
these parts are manufactured by molding, their production lead time is long. In an attempt toeliminate the need for molds and shorten the lead time, the applicability of metal additive
manufacturing to compressor cover and turbine housing production was evaluated. In its
application, the metal additive manufacturing element technologies introduced in Vol. 55, No. 2 of
the Mitsubishi Heavy Industries Technical Review were employed. Handling the issues particular
to compressor cover and turbine housing prototyping including the optimization of build conditions
suitable for conventional casting materials, mechanical property evaluation of the built parts,
evaluation of the internal flow path geometry and evaluation of performance as a turbocharger,
Mitsubishi Heavy Industries Engine & Turbocharger, Ltd. (MHIET) has succeeded in reducing thelead time required for turbocharger prototyping to one-third or less.

|1. Introduction
With the potential of leading to better fuel economy and reducing exhaust emissions,turbochargers are in increasing demand. To increase the number of orders, we have taken on thechallenge of responding promptly to the needs of our customers by reducing the lead time requiredfor development. In prototyping a turbocharger, producing compressor cover and turbine housing
prototypes is a time-consuming process because the interface shapes vary depending on the car
type and the production processes involve molding. Therefore, reducing the time required for
prototyping these parts has been regarded as critical.
In metal additive manufacturing, parts can be made directly from 3D models without molds
being created, so its application to the prototyping of parts that are otherwise produced by moldingcan be expected to reduce the time required before delivery. Therefore, we prototyped acompressor cover and turbine housing through metal additive manufacturing to establish a processwith a shorter production time (if molded, it would normally take about one month).
For the application of metal additive manufacturing, we considered factors such as thematerials to be used, part sizes and operability of 3D printers, and decided to adopt laser powder
bed fusion 3D printing. The issues to be addressed when using metal additive manufacturing toprototype compressor covers and turbine housings are: (1) the selection of a suitable powder formof the casting material, (2) Optimization of additive manufacturing conditions for the powder
selected in (1), and (3) the evaluation of the internal flow path geometry of the prototyped parts.
This report mainly presents the results of turbine housing prototypes. Figure 1 shows flow chartsfor prototyping.

*1

Production Department, Production Division, Mitsubishi Heavy Industries Engine & Turbocharger, Ltd.

*2

Research Manager, Service Engineering Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 2 (July 2019)2

Figure 1

Flow charts for prototyping by molding and metal additive manufacturing|2. Selection of the powder raw material
As the casting materials used to produce turbocharger parts are not available commercially ina powder form that is suitable for metal additive manufacturing, these materials have to bepowdered from scratch. In powder bed fusion metal additive manufacturing, the powder raw
material needs to be laid uniformly (several dozen micrometers in thickness), thus requiringsuperior fluidity. Spherical, gas-atomized powders of 10-45 μm in diameter are generally used as araw material. Therefore, we formulated a powdered raw material that has the same composition as
the casting material, which was followed by the particle size classification. Based on the results of(1) morphological observation by electron microscopy, (2) laser diffraction particle size
distribution measurement and (3) fluidity evaluation according to the time required to flow throughan orifice, four types of powdered metals were selected. Table 1 shows these four types of powder
raw materials in terms of electron microscopic images, particle size distribution and the timerequired for 50g of powder to flow through an orifice of 3.15 mm in diameter. The electron
microscopic images indicate that sample powders of all types have a spherical morphology.
However, the particle size distribution measurement results show that powders A and B containparticles with a diameter of >70 μm and <10 μm respectively, so both were excluded. Finally,
based on the fluidity evaluation results, powder D was selected as the powder raw material for use
in metal additive manufacturing because of its superior fluidity.

|3. Optimization of additive manufacturing conditions
In metal additive manufacturing, the powder raw material is rapidly cooled down after being
fused by laser. This process raises concerns about the formation of cracks in the casting material.
Proper layer-by-layer building of a part using a powder raw material necessitates appropriately
controlling the depth of the melt pool produced by laser beam irradiation. To determine the rangeof the irradiation conditions involving no cracking, a laser used in metal additive manufacturing (aYb fiber laser, operating at a wavelength of 1060 nm with a beam diameter of 0.1 mm) was first
applied to a metal plate surface. Next, the process of constructing a bed of the powder raw material
followed by laser beam irradiation was repeated, and the range of irradiation conditions for thelayer-by-layer building process was narrowed down. A cubic block was then manufactured under
several selected conditions. Based on the filling ratio and shape of the cubic block, the appropriate
additive manufacturing conditions were finally determined. The suitability of the built blocks as aturbine housing was examined according to the required tensile strength, 0.2% proof stress, andelongation.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 2 (July 2019)3Table 1 Comparison of powder raw materials in terms of particle morphology, size
distribution and fluidity
Powder A

Powder B

Powder C

Powder D

10.0 s

11.0 s11.3 s10.8 sElectronmicroscopic
image

Particle size
distribution

Time required for 50g ofpowder to flow through aφ 3.15 orifice (s)3.1

Irradiation testing on a metal plate surface

To determine the range of laser-beam irradiation conditions involving no cracking (in termsof laser power and scan speed), a laser beam was applied to a plate surface with combinations offour different levels of laser power and scan speed. The irradiation marks werestereo-microscopically observed to examine the presence/absence of cracks and the bead width.
Figure 2 shows the results, which were plotted with the x-axis representing the laser power and they-axis the scan speed. Under the conditions of high laser power and low scan speed, cracking
occurs. On the other hand, when the laser power is low and the scan speed is high, no cracking
occurs but fusion defects become a concern because the fusion area is much smaller than the layer
thickness. Considering that the intermediate range was suitable as the laser-beam irradiation
conditions, we selected seven different test conditions for the additive manufacturing testing(indicated by (1) to (7) in Figure 2).

Figure 2

Surface texture after laser beam irradiation of the casting material

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 2 (July 2019)4

3.2

Additive manufacturing testing

In metal additive manufacturing, the raw material (powder) is used to construct a bed ofseveral dozen micrometers thick, and the building process of a part progresses by applying a laser
beam to the bed, allowing the powder to be fused with the substrate. In this test, we repeated theprocess of constructing a bed of the powder followed by laser beam irradiation 10 times, before
evaluating the quality of the built part. Repetition of laser beam irradiation was conducted undereach of the seven different test conditions that were selected based on the sheet-surface irradiation
test results mentioned earlier. In addition to building a wall, fabrication of a 10-mm-squared slab bylaser hatching was also performed. The hatching distance was set based on the bead widths
resulting from the laser beam irradiation of a sheet surface.
The typical morphologies of the walls and slabs built in the tests are shown in Figure 3.
Under condition (1), the powder raw material was not fused with the substrate, making it
impossible to build a wall or slab. Regarding condition (7), the wall was fabricated but black sooty
powder was formed while the slab was being built. After these two test conditions were excluded,
the remaining five conditions (2) to (6), were selected for further examination.

Figure 3

Additive manufacturing testing results (building-up of 10 layers)

A wall can be built but a black sooty substance is attached to the slab: condition (7)A wall and slab with metallic glow are produced: condition (6)No wall or slab is fabricated: condition (1)3.3Block fabrication

Under each of the five test conditions with different combinations of laser power and scanspeed, a 10 mm cube was additively manufactured to evaluate the filling ratio. By processing thecross-sectional optical microscopic images of fabricated blocks, the void areas were estimated toobtain the filling ratios. The results are given in Figure 4. With low laser power (conditions (2) to(4)), the filling ratio tended to be low because of the remaining pores. On the other hand, conditions(5) and (6) indicated a filling ratio of nearly 100% in the built blocks, along with absence of tinycracks. For the evaluation of mechanical properties, therefore, we decided to build blocks underthese two test conditions.

Figure 4

Relationship between the filling ratio of the built block and the build conditions

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 2 (July 2019)5

To prepare test pieces for the mechanical property evaluation, a block with dimensions of 30mm × 30 mm × 10 mm was built under either condition (5) or (6). Electrical discharging machining
was used to cut out a very small plate test piece for tensile testing in such a way that the tensile
direction can be parallel to either the build direction or the layer plane. With the tensile testingmachine, a strain-stress curve was obtained to estimate the tensile strength, 0.2% proof stress andelongation. Figure 5 shows the resulting mechanical properties. The test pieces of both build
conditions yielded larger tensile strength, 0.2% proof stress and elongation when force was appliedin the direction parallel to the layer plane rather than in the build direction. However, it has been
demonstrated that all the results satisfied the mechanical properties required for the turbine
housing.

Figure 5

Mechanical properties of the blocks built under conditions (5) and (6)|4. Geometric evaluation
As the internal flow path geometry of the turbine housing or compressor cover determines
the performance of turbocharger, the geometric evaluation is of importance. To realize a shorter
lead time for prototyping, it is also necessary to shorten the inspection process. We therefore
assessed the geometry of the internal flow path by high-power microfocus X-ray computed
tomography (CT), with which non-destructive measurement becomes possible within a short periodof time. One of our goals in particular was to evaluate the applicability of X-ray CT through theevaluation of the turbine housing internal path geometry. This is because, being ferrous and having
a low x-ray transmission, the turbine housing is more difficult to assess than an aluminum-alloy
compressor cover.
In the evaluation, a molded turbine housing was first used to measure the internal geometryby X-ray CT and the error in the internal flow path dimensions was estimated by comparing withthe 3D model used for additive manufacturing. The geometry of the metal additively-manufactured
turbine housing was then measured and compared with the 3D model. As shown in Figure 6, it has
been demonstrated that the error in the dimensions of the metal additively-manufactured turbine
housing was as small as that of the molded turbine housing.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 2 (July 2019)
6Figure 6 Internal geometry measurement of the metal additively-manufactured turbine
housing using X-ray CT

Result: Color-coded based on the degree of deviation from the 3D model reference contours, illustrated in thecross section parallel to the turbine axis.

|5. Conclusion
For the application of metal additive manufacturing to turbine housing prototyping, weselected the powder raw material, extracted the build conditions involving no cracking or theformation of pores in parts being built, assessed the mechanical properties of the built parts anddemonstrated that metal additive manufacturing can produce parts as good as molded ones. It has
also been demonstrated that the geometry of the metal additively-manufactured turbine housing canbe assessed by microfocus X-ray CT, along with the error in the dimensions being as small as thatof the molded turbine housing. As seen in Figure 7, the application of metal additive
manufacturing can substantially reduce the lead time required for prototyping to one-third or less,when compared with the conventional molding method. We will further improve the metal additive
manufacturing process to enable standardized quality prototypes to be produced at all our basesaround the world, as well as reduce the transportation time by digitally transmitting the data for
prototyping and building a prototype in proximity to the customer (“digital transportation”).
Through these efforts, we will contribute to product development in accordance with thecustomer’s needs by providing prototypes more promptly, and further reducing the development
time.

Figure 7 Comparison of the molding and metal additive manufacturing processes in terms of
the lead time required for prototyping a turbine housing
The standard waiting time is included in each stage (both machining and inspection are omitted because thesestages are common in the two processes).