TECHNICAL REPORT

3D Additive Manufacturing for Alloy Tool Steel
M. TANO Y. MISADA T. NAGAHAMA T. MIZOGUCHI T. MITSUI

H. HOSHINO

Laser beam powder bed fusion (PBF-LB) for alloy tool steel is developed to fabricate casting dies for automobile
parts by additive manufacturing. Internal defects such as pores, micro cracks and carbide, which deterioratemechanical properties, have been improved by the developed process parameters, reduction of crack susceptibility
index, and heat treatment. As a result, the workpieces manufactured using PBF-LB achieved a tensile strength andCharpy impact value equivalent to the wrought samples.
Key Words: additive manufacturing, laser beam powder bed fusion, alloy tool steel

1. Introduction

2. Assessing the Current Status of PBF-LB

Recently, 3D additive manufacturing (AM) technology
has been drawing growing attention as an innovative
production technology 1) . Laser Beam Powder Bed
Fusion (PBF-LB), one of the major methods in metal
additive manufacturing, is capable of fabricating internal
structures and complex shapes that cannot be achievedby conventional methods2). One example of application
is for the casting molds for automobile parts, where
cooling paths with complex shapes inside are installed
for optimizing the cooling conditions based on the shape,
and this has enabled improved product dimensional
accuracy and longer mold lifespans compared to ordinary
molds3)-5). The PBF-LB method, on the other hand, uses
a laser to melt and solidify metal powder, and if thefabrication conditions and powder characteristics are notappropriate, defects will arise inside the built sample andits mechanical properties will be degraded. In particular,materials with a high carbon content are prone to
cracking and are difficult to form. Therefore, when molds
are processed by the PBF-LB method, maraging steel
with high toughness is used, but it is inferior to the alloy
tool steel used for ordinary casting molds in terms of
performance characteristics such as thermal conductivity
and thermal fatigue (heat check).
For this reason, this study aims to establish afoundation for 3D additive manufacturing technology of
the SKD61 alloy tool steel that is used for casting molds.
This paper presents the results of our investigation on thecurrent status of PBF-LB for alloy tool steels, the issues
that need to be addressed to improve the fabrication
quality, the measures that were considered, and the resultsof our verification of the effects.

2. 1 Forming Method
PBF-LB, one of the major AM technologies, is usedprimarily for fabricating metal products. Figure 1 showsa schematic diagram of the PBF-LB process. The powder
bed is formed by spreading metal powder on a baseplate using a recoater. In this process, the powder bed
is repeatedly irradiated with a laser to melt and solidify
the powder, and the layers are formed one by one. In
this way, it is possible to fabricate internal structures andcomplex shapes by stacking and forming the shape of the
product one layer at a time.

2. 2 Fabrication Experiment
The fabrication machine used was an SLM280 HL
manufactured by SLM Solutions (maximum fabrication
dimensions: 280mm × 280mm × 350mm). SKD61
powder (grain sizeu10 to 45 µm) was used as the metal
powder. Table 1 shows the chemical composition.
Figure 2 shows the major fabrication parameters for the
PBF-LB process. In the PBF-LB process, it has beenreported that there is a correlation between the energy
density E (J/mm3), which is a measure that encompasses
these fabrication parameters, and the quality of the

Laser
Recoater

Galvanoscanner
Powder bed

Laser scanPowderBase-plate
Building platform

Fig. 1 Schematic of PBF-LB

JTEKT ENGINEERING JOURNAL English Edition No. 1018E (2022)

35

3D Additive Manufacturing for Alloy Tool Steelbuilt sample6). The energy density is expressed by thefollowing equation where P is the laser power (W), v isthe scan speed (mm/s), s is the hatching pitch (mm), and t
is the layer thickness (mm).

E =P /vst⑴

In this study, we also evaluate the fabrication resultsfrom the relationship between the energy density andthe relative density, which is an evaluation index for thebuilt sample’s quality. Table 2 shows the fabrication
conditions. 16 specimens (15mm × 15mm × 30mm)with different fabrication parameters were fabricated ona base-plate, and the relative density to the theoreticaldensity of 7.78 g/cm3 was measured using Archimedes’
principle. To reduce thermal stress by mitigating thetemperature gradient in the built sample, the powder
bed is preheated by a heater placed under the base-plate,
and the temperature was set to 200℃ for the fabrication
conducted in this report.

To evaluate the mechanical properties of the built
samples, a tensile test (JIS Z 2241 compliant, specimen
dimensions u7mm × 64mm) of a specimen processed
under conditions where a relative density of 99.5% was
obtained and a Charpy impact test (JIS Z 2242 compliant,specimen dimensions 12mm × 12mm × 56mm) wereconducted. The hardness of the as-built specimen was
55 HRC, and so the specimen was heat-treated at 600℃
for 4 hours (air-cooled) two times to bring it to 47 HRC,
which is the hardness required for casting molds (42 HRC
to 52 HRC). Figure 4 shows the results of the mechanicalproperty measurements. The tensile strength and Charpy
impact values of the specimens were found to be reduced
by 80% and 20%, respectively, compared to the wrought
sample. Since it was thought that defects inside thebuilt sample may be a factor in the reduced mechanicalproperties, we observed cross-sections of the built sample
and found micropores, microcracks, and precipitation ofcarbides as shown in Fig. 5.

2. 3 Experiment Results

Table 1 Chemical composition (wt%)

100

Relative density, %Figure 3 shows the relationship between the energy
density and the relative density. This shows that, as
the energy density increased, the relative density also
increased, but once the energy density exceeded an
appropriate level, the relative density decreased. Thus,
high relative densities were obtained in the appropriate
energy density range, with a maximum value of 99.5%. In
the range where the energy density is not appropriate, we
think that the relative density is low because the powder
remains unmelted during laser irradiation and spatters,
resulting in subpar melting, which left pores inside thebuilt sample.

98
96
94
92
90

0

50
100
150
Energy density, J/mm3

Fig. 3 Relationship between energy density and relative
density
25

Mn

0.41

1.09

0.42

P

S

0.012 0.006

Cr

Mo

V

5.02

1.25

1.10

Hatching pitch: sScan speed: v
Laser power: P

Charpy impact value, J/cm2

Si

Tensile strength, MPa

2 000

C

1 500

1 000

500

Layer thickness: t
0

Fig. 2 Process parameters

Table 2 Fabrication conditions

36

Laser power

155 to 480 W

Scan speed

460 to 1 430mm/s

Hatching pitch

0.08 to 0.21mm

Layer thicknes

0.05mm200

Wrought Built
sample samplea) Tensile strength

20
15
10
5
0

Wrought Built
sample sampleb) Charpy impact value

Fig. 4 Measurement result of mechanical properties

JTEKT ENGINEERING JOURNAL English Edition No. 1018E (2022)

3D Additive Manufacturing for Alloy Tool Steel

200 Pm

500 Pma) Poresb) Cracks

10 Pmc) Segregation of carbide

Fig. 5 Optical micrograph of specimens

3. Improving Fabrication Quality
Micropores, microcracks, and precipitation of carbides
were cited as factors reducing the mechanical propertiesof the built samples. To achieve the same mechanicalproperties as those of the wrought sample, we investigatedmeasures to reduce internal defects and improve thefabrication quality.

3. 1 Porosity Reduction
The shape of the pores suggests that they are causedby poor fusion during laser irradiation, and so weinvestigated how to suppress the spatter that causes this.
Figure 6 shows the result of the thermal fluid analysis.
The center of the molten pool is particularly hot and theevaporation pressure increases, which is thought to cause
spattering. Therefore, the laser power must be reduced tosuppress the over-heat input. However, the relative density
is expected to decrease because the fabrication quality iscorrelated with the energy density. Therefore, noting thatthe laser power has a significant effect on the depth of themolten pool, the layer thickness was reduced to 0.03mm
by taking into account the powder grain size. As a result,
the relative density was improved to 99.8% using thismethod, compared with the 99.5% obtained under theconventional fabrication conditions.

3. 2 Suppression of Cracking
The cracks are thought to be caused by the openingof the thin liquid phase section remaining between thesolid phases due to thermal stress during the solidification
process of the molten pool. Therefore, in order to reduce
the amount of low-melting-point compounds remaining
in the liquid phase section, a high thermal conductivity

material for molds (manufactured by Daido Steel, referred
to as “high thermal conductivity material” below) was
selected because it has a low high-temperature cracking
susceptibility coefficient K , which is used as an index
of cracking susceptibility in the field of welding, and is
expected to suppress the occurrence of cracking. The
high-temperature cracking susceptibility coefficient K
is calculated from the content of each element by thefollowing equation7).
Si＋ Ni/10025
× 100
3Mn ＋ Cr ＋ Mo ＋ V

C × S ＋P＋

K＝⑵

Table 3 shows the chemical composition. Due tothe low composition of carbon and silicon, the hightemperature crack susceptibility coefficient was reduced
from the 2.9 of SKD61 to 0.7. While the conventional
number of cracks was 26 per mm2 in Fig. 5 (b), no cracks
were found within the observed range, indicating thatcracks could be significantly suppressed (Fig. 7).

3. 3 Suppression of Carbide Segregation
Because the composition of the high thermal
conductivity material is different from that of SKD61, weinvestigated the tempering temperature where a hardness
in the mold usage range can be obtained. Figure 8 shows
the relationship between the tempering temperature andthe hardness. A hardness within the mold usage range
was obtained below 600℃. Figure 9 shows the result ofcross-sectional observation after tempering at 550℃. Thisconfirms that the carbide precipitation observed in Fig. 5(c) was reduced.

3. 4 Evaluation of Mechanical Properties
To improve the fabrication quality, a combinationof the suppression of over-heat input, reduction of thecrack susceptibility coefficient, and the optimization of
the tempering temperature were used in the fabrication
experiments, and the mechanical properties were
evaluated. Since the hardness of each specimen after
tempering at 550℃ was 50 HRC, it was compared with theTable 3 Chemical composition (wt%)

°C
3 100
2 400

C

Si

Mn

0.32

0.06

0.37

P

S

0.008 0.007

Cr

Mo

V

5.21

1.22

0.40

1 700
1 000
300

200 PmFig. 6 Result of thermal fluid analysis

JTEKT ENGINEERING JOURNAL English Edition No. 1018E (2022)

Fig. 7 Optical micrograph of specimens

373D Additive Manufacturing for Alloy Tool Steel

40
30
20
10
600

800

Tempering temperature, °C

Charpy impact value, J/cm2

Tensile strength, MPa

Hardness, HRC

50

0
400

50

2 000

60

1 500
1 000
500
0

Fig. 8 Relationship between tempering temperature andhardnessWrought Built
sample sample(Catalog values)a) Tensile strength

40
30
20
10
0

Wrought Built
sample sample(Catalog values)b) Charpy impact value

Fig. 10 Measurement result of mechanical properties

References
10 Pm

1)

Fig. 9 Optical micrograph of specimens

2)

catalog value of SKD61 wrought sample with equivalenthardness8). Figure 10 shows the measurement results ofthe mechanical properties. Due to the improvements in the
fabrication quality, it was confirmed that tensile strengthand Charpy impact value equivalent to or higher thanthose of the SKD61 wrought sample could be obtained. In
particular, we believe that the significant improvement in
the Charpy impact value compared to the wrought sample
is due to the finer metallurgical structure caused by rapid
cooling solidification, which is a characteristic of the
PBF-LB process.

3)

4)

5)

4. Conclusion
To realize future production technology innovations,w e h ave e s t a b l i s h e d a f o u n d a t i o n f o r a d d i t ive
manufacturing technology for alloy tool steel by the
PBF-LB process. To resolve the issue of degradation of
mechanical properties due to internal defects caused byheat treatment and melting and solidification during laser
irradiation, measures to improve the fabrication quality
were investigated, and mechanical properties equivalent
to those of wrought samples were obtained.
Looking forward, we will evaluate prototype molds
and develop optimal design technology for utilizing PBFLB, and we aim to bring about production technology
innovations through practical applications of thistechnology.

38

6)

7)

8)

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no Shomondai -Gyoukoware ni tsuite-, Journal of theJapan Welding Society, Vol. 44, No. 7 (1975) 546-551 (inJapanese).
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JTEKT ENGINEERING JOURNAL English Edition No. 1018E (2022)

3D Additive Manufacturing for Alloy Tool Steel

M. TANO*

Y. MISADA** T. NAGAHAMA***

T. MIZOGUCHI**** T. MITSUI**** H. HOSHINO******

Innovative Processing R&D Dept., Research &Development Division, Dr. Eng.
**
Material R&D Dept., Research & Development
Division*** BR Marketing Dept., Sales & Marketing Division**** Innovative Processing R&D Dept., Research &Development Division***** Materials Innovation Engineering Dept., Production
Engineering Division

JTEKT ENGINEERING JOURNAL English Edition No. 1018E (2022)

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