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 250^HL 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.

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.

Conclusions