3DP für MIM
Contact
Small batch production of complex metallic components using binder-based 3D printing technology
Duration
04/2018-07/2020
Problem
Metal injection molding (MIM) is an established production process for small metallic components with high geometry requirements. During molding, threads, undercuts, bores or splines can be realized. In the subsequent sintering process, the green body is consolidated into a fine-grained component, which has a high surface quality as well as dimensional accuracy in combination with good mechanical properties.
Each new component geometry requires its own injection mold, which is why MIM production only becomes economical from high volumes of around 5000 parts. The additive manufacturing process of powder bed-based 3D printing technology (3DP) has numerous process analogies to MIM. Compared to other additive manufacturing processes, low process costs for pre-production and low-volume production via 3DP will be possible in the future. In addition, carbide-rich materials can be processed, which would lead to tool wear with MIM. A systematic comparison between 3DP and MIM in terms of achievable manufacturing tolerances, surface quality and mechanical properties is not available.
Objective
The project aimed to prove that MIM components can be manufactured from typical MIM powders using 3DP. The quality of the components was evaluated on the basis of comparison criteria defined at the outset from MIM standards or characteristic values of MIM benchmark components. In addition, the aim was to test how well hard materials from milled powders can be processed using 3DP. To meet the set criteria, powder and printing process parameters had to be adapted to achieve a reproducible and high green density. In addition, suitable debinding, sintering and heat treatment processes had to be developed. The anisotropic shrinkage of the 3DP parts was to be compensated by sintering simulation in order to achieve high dimensional stability.
Procedure
The two materials proposed in the project proposal were confirmed: the precipitation-hardening stainless steel X5CrNiCuNb17-4-4 (17-4PH, 1.4548) and the hard material ferro-titanite in the specification Nikro128® (DEW brand). Comparison criteria were jointly defined on the basis of which a comparison of the properties of 3DP and MIM was to be carried out. Using systematic density analyses of cubes or tensile specimens as a function of build space position, depowderability, sintering shrinkage and build space orientation, parameters for the additive printing process, debinding and sintering were determined. In addition, parameters for the simulation of anisotropic sintering were determined. An existing simulation model was extended, supplemented with the model data and validated. Specimens were tested in the HIP post-compacted and heat-treated (HIP+WB) as well as heat-treated only (WB) condition, depending on the direction of construction, to determine roughness, hardness, strength, ductility, temperature-dependent toughness as well as fatigue strength.
The research and development work was carried out by the two research institutes in intensive exchange with each other and with the companies involved in the PA. Thanks to the intensive discussions at the regular project meetings and the two-week telephone calls, it was possible to work extensively on the original project idea, despite the geographical distance; additional, unforeseen questions could also be dealt with in the necessary depth.
Summary of the results
The material Nikro128® consists of an iron base with about 40%-vol titanium carbides. Conventionally, powders generated via reaction sintering and milling are processed into simple components by pressing, sintering and hard machining. The same starting powders, see Figure 1 (left) were used for the 3DP process. Due to the high specific surface area and poor flowability, parameter optimization was lengthy and difficult. A high binder saturation of 160% and in-house developed binders were finally necessary to guarantee a successful de-powdering of the green bodies after the printing process, see Figure 1 (center). During sintering, a very narrow process window was observed: 1390 °C was ideal, significantly lower densities were obtained at lower temperatures, and above 1395 °C a liquid phase occurred, which, among other things, destroyed dimensional stability. At ideal sintering parameters, a relative density of 98% was achieved; after HIP compaction, 100%. Component dimensions in the green and sintered states, determined on cube specimens from several build jobs, are shown in Figure 1 (right). The aim of the study was to compensate for the sintering shrinkage, which depends on the direction of construction, and to produce cubic specimens of high dimensional stability.
Figure 1: Characterization results of the Nikro128®: SE image of the starting powder (left), successfully de-pulverized cube (center), representation of the anisotropic and position-dependent sintering shrinkage (right).
In addition to cubes, rods and spheres were produced and heat treated. A 3-point bending strength of 1490 ± 60 MPa in the x-direction, 1430 ± 80 MPa in the y-direction and 1100 ± 50 MPa in the z-direction was achieved, and a hardness of 56 ± 1 HRC, independent of the direction of construction. Conventionally produced bars achieved 1200 MPa flexural strength with a hardness of 63 HRC. Presumably, the carbon distribution was altered by the binder and sintering, resulting in reduced carbide hardness and matrix embrittlement. Balls with diameters of 2 mm and 5 mm could be produced with a deviation from the mean circle diameter of only +4/-1%. However, due to the rough surface, the best convexity was only 83%. The final fabrication parameters for Nikro128® can be found in Table 1.
Table 1: Overview of the adjusted/tuned fabrication parameters, as sintering base best results were obtained with Al2O3 plates coated with a Y2O3-ethanol mixture.
|
17-4 PH |
Nikro128® |
|
|
3DP |
|
|
|
Sintering |
|
|
|
HIP |
T HIP=1150°C, p HIP=100MPa und t Halt =3h. |
T HIP>1000°C, p HIP>100 MPa und t Halte>1h. |
|
WB |
H900 Heat treatment according to ASTM A564
|
|
At 17-4PH, a spherical, fine (< 20µm) powder was used, see Figure 2 (left). After parameter studies, reproducible manufacturing parameters could be obtained. Alternating porous areas and dense areas perpendicular to the build direction could be identified in CT analyses of green bodies as well as in micrographs of sintered samples. This was particularly evident in micrographs nearly parallel to the powder bed plane (see Figure 2 right). A local porosity of up to 15% was determined in these poorly compacted areas, whereas the porosity in the denser areas was about 3%. The global porosity was about 5%. The specimens were successfully densified by HIP (Figure 2, bottom center).
Figure 2: Characterization results of the 17-4PH: SE image of the starting powder (left), light microscope image of a transverse section through tensile specimens before and after HIP (center), etched microsection.
Even with optimized production parameters, the scatter of the green density as a function of the position in the installation space and consequently the scatter of the sintering shrinkage of 13.8-16.5% (x,y-direction) and 17.3-20.4% (z-direction) was relatively large. A system change from Innovent to Innovent+ reduced the scatter as a result of ultrasonic-assisted powder spreading. This also implies a scattering of the mechanical properties. In addition, with the exception of hardness, these were strongly dependent on the build direction. Tensile/impact and axial vibration specimens in x-direction showed the best mechanical properties, y-direction still similar and z-direction the worst. An overview of the determined properties, averaged over all build directions, is shown in Table 2. With the exception of roughness, all MIM reference values are achieved in the heat-treated condition. Nevertheless, there are a few outliers which are clearly below the MIM specification, but which are not noticeable when the mean value and standard deviation are given. HIP significantly increases strength, but only slightly increases ductility. Toughness at room temperature is only slightly increased by HIP. However, since room temperature toughness is in the transition region from brittle to ductile behavior, this averaged value is not very meaningful. HIP nearly doubles the toughness at high position: from 56 J to 107 J for X-direction, from 46 J to 94 J in Y-direction and from 33 J to 70 J in Z-direction. The transition temperature starts at -50 °C to 0 °C, depending on the design direction, and ends in the range between 50 °C and 150 °C.
Table 2: Comparison of the obtained characteristic values of 3DP specimens with MIM reference values (*MPIF min/typical according to H900 WB, **MIM component self-measured, ***doi.org/10.1179/174329006X89317 ) at RT.
|
Parameter |
Unit |
3DP-samples |
MIM reference |
||
|
sintered |
WB |
HIP + WB |
|||
|
HV10 |
[-] |
260 |
360 |
-/325* |
|
|
Roughness Ra [µm] |
[µm] |
3,32 ± 0,32 |
0,97 ± 0,20** |
||
|
Roughness Rz [µm] |
[µm] |
16,09 ± 1,35 |
4,83 ± 1,09** |
||
|
Yield strength Rp0.2 |
[MPa] |
1078 ± 24 |
1191 ± 47 |
970/1090* |
|
|
Tensile strength Rm |
[MPa] |
1191 ± 26 |
1303 ± 23 |
1070/1190* |
|
|
Uniform elongation Agt |
[%] |
2,6 ± 0,4 |
3,3 ± 0,3 |
- |
|
|
Elongation at break At |
[%] |
2-12 |
4/6* |
||
|
standardized toughness αK |
[J/cm2] |
53 ± 25 |
69 ± 50 |
28 ± 16***; 70* |
|
Reliable evaluation of the axial vibration tests was hampered by a high degree of scatter. Overall, however, a high level of fatigue strength and fatigue strength above 300 MPa is obtained compared to LPBF and MIM (Figure 3). On the fracture surfaces, tubular pores are identified as fracture initiators in the WB condition. In specimens in the WB+HIP condition, no pores or other conspicuous features recognizable as microstructural defects were observed at the positions from which the fracture lines extend.
Figure 3: left: Comparison of the direction-dependent Wöhler curves based on the line for 50% failure probability at R=-1 and references (*EMPA: Introduction to metal injection molding, **DOI: 10.1111/ffe.13200); right: exemplary fracture surfaces in the WB (top) and HIP+WB (bottom) states.
Using the scaling factors determined, several demonstrators "jaws" (Fig. 4 left) and "rollers" were manufactured and measured. In direct comparison with the MIM parts, length deviations (distances a-d) of up to 6% resulted, with an average deviation of 1%. The comparison of the (unscaled) target geometry with the actual 3D-measured geometry, shown in a false-color image in Figure 4 on the right (CloudCompare) shows, for example, a slight warpage on the distance "a". This is due to friction and hindered shrinkage between the sintering base and the sintered material. The developed model for anisotropic sinter simulation can predict this distortion.
Figure 4: Demonstrator component "jaws" made of 17-4PH (left) and comparison with the target geometry (right).
The IGF project 19733 N of the Forschungsvereinigung Forschungsgesellschaft Stahlverformung e.V. (Research Association for Steel Forming) was funded by the German Federal Ministry for Economic Affairs and Energy via the AiF within the framework of the program for the promotion of joint industrial research (IGF) on the basis of a resolution of the German Bundestag. The long version of the final report can be requested from FSV, Goldene Pforte 1, 58093 Hagen, Germany.
Funding information
Grant number: 19733 N