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Lattice structure tensile specimen manufactured with laser melting (LM) process out of the material H13. Show image information
Industry partners of the DMRC Show image information
Industry partners of the DMRC Show image information
Quality control during a Laser Sinter (LS) build job by a researcher of the DMRC Show image information
Fused Deposition Modeling (FDM) process during the manufacture of an Ultem 9085 part Show image information
Additive manufactured reaction wheel bracket for telecomunication satellites Show image information
Employees of the DMRC working with the "freeformer" from Arburg Show image information
Tactile measurement of a SLM part with a Coordinatemeasuring machine (CMM) Show image information
Powder particles are used as raw material for laser-based additive manufacturing Show image information

Lattice structure tensile specimen manufactured with laser melting (LM) process out of the material H13.

Industry partners of the DMRC

Industry partners of the DMRC

Quality control during a Laser Sinter (LS) build job by a researcher of the DMRC

Fused Deposition Modeling (FDM) process during the manufacture of an Ultem 9085 part

Additive manufactured reaction wheel bracket for telecomunication satellites

Employees of the DMRC working with the "freeformer" from Arburg

Tactile measurement of a SLM part with a Coordinatemeasuring machine (CMM)

Powder particles are used as raw material for laser-based additive manufacturing

High Temperature Processing of Metallic SLM Powders

The aim of this project is to reduce the high residual stresses and the shrinking of the material caused by the high cooling rate during the building process, which leads to crack formation. In this project, a heated building platform helped to reduce the temperature gradient, which leads to certain microstructural changes that made these materials processable with selective laser melting.

Objectives
In most industrial applications, the lightweight design and the development of individual, functional customized products have been and still are of major interest for material development. Therefore, materials such as aluminum-, titanium-, nickel-based alloys, as well as steels came into focus due to their specific, aligned density and mechanical characteristics. In view of these aspects, high strength aluminum alloys, e.g., EN AW 7075, have not been considered yet for additive manufacturing due to their inferior processability. Aluminum is only given as an example to emphasize the need for research and to discern the effects of a heated building platform concerning the resulting mechanical and microstructural properties. With regards to the future microstructural and mechanical tests, a tool steel (H13, X40CrMoV5-1) was chosen since this material is a commonly applied steel in industrial applications. Moreover, this steel is difficult to process via SLM without further processing modifications. The second analyzed material was the aluminum alloy AA7075. Due to the occurring hot cracks, a heated platform up to 400°C represents a promising solution to avoid these defects.

Results
In the first period of this project, cuboidal blocks were built on a small building platform at different temperatures (RT; 100°C; 200°C; 300°C; 400°C). Here, two criteria for a stable production and analyzing of samples were considered. The first criterion is the residual stress, which increases during the manufacturing for H13 . The main reason for the residual stress is the high cooling rate encountered during the process. The rapid cooling leads to a fast phase transformation from gamma iron into alpha iron (martensite). The second criterion is the geometry of the sample to ensure that the influence of each parameter set of the building process can be analyzed. For all of these samples, hot cracking has been detected. The pre-heated samples with a temperature of 100°C and 200°C show the largest and thickest cracks. The cracks in the other samples (300°C, 400°C) are have been very thin and a lower porosity. The 300°C samples had more cracks compared to the 400°C sample. The influence of the pre-heating of the platform was not successful to prevent hot cracking. The size of the melting pool had a higher impact on the crack formation.

Tensile tests of pre-heated H13 samples built with a layer thickness of 50 μm were compared with near-net-shape samples from a previous project. These samples were built with the SLM 250HL with a layer thickness of 30 μm. Figure 1 shows that the pre-heated specimens do not reach the values known from the literature, and, additionally, do not meet the tensile strength values of the near-net-shape sample, which are comparable to values obtained for the conventional processed material.

Figure 1: Quasi-static properties of H13 samples

The microstructure of the 100°C and 200°C samples are shown in Figure 2. On the top, the IPF image shows a typical martensite structure for this material after a fast quenching. The phase image (bottom) depicts the information concerning the amount of retained austenite (green), which had been detected in prior investigations as well. During the tensile test, the retained austenite transforms into martensite, which leads to a hardening effect, thus employing a higher mechanical strength. The strain values collected in the tensile tests showed very little evidence of changes between the variable build platform temperatures. At a preheating temperature of 400 °C, based on a quasi-iso-thermal conditions, a martensitic to bainitic microstructure developed.

Figure 2: Electron Backscatter Diffraction (EBSD) mapping depicting a martensitic microstructure by means of Inverse Pole Figures (IPFs) and the respective phase-mappings in which martensite is displayed in red and austenite is displayed in green

Conclusions
The approach to increase the pre-heating temperature above the Ms-temperature of H13 does not avoid hot cracking of the material. Based on results obtained from the near-net-shape specimens built with a smaller layer thickness, it is assumed that a higher energy density will result in a reduction or removal of hot cracking. Therefore, it is proposed that, in order to increase the energy density, the melt pool area should be reduced via a reduction of the hatch distance. Based on the energy density relationship, this should alleviate the hot cracking problem.

Further project information
Project statusSuccessfully finished
Project duration12 months
Funding100 % DMRC industry partner
Research leaderProf. Dr.-Ing. Mirko Schaper (LWK)
Research assistantAlexander Taube, M. Sc.
PartnerDMRC Industry Partner
Contact
Phone:
+49 5251 60-3855
Fax:
+49 5251 60-3854
Office:
E5.112

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