Selective Laser Melting of Aluminum, Hastelloy X, Tool Steel and Cobalt-Chrome: Compositional Modification and use of Base Plate Preheating

Selective laser melting (SLM) is an emerging additive manufacturing production technique for metals, offering a perfect solution for production of small quantity series and complex part geometries. Examples are production of spare parts or custom made biomedical objects.

One of the main drawback at this point is the limited number of materials that are processable using SLM. Specific knowledge that is acquired within SLM of one material can not be automatically transferred to SLM of another material, which complicates the material development highly.

In this thesis, four materials are investigated. For two types of materials, high strength aluminum alloys and Hastelloy X, difficulties are encountered during the SLM process. Crack formation in the parts prohibits their industrial usage. These problems are tackled within the frame of this thesis.

Furthermore, two materials, namely H13 tool steel and CoCr, are examined that do not suffer from major problems. The in depth analysis performed here, reveals however some surprising results.

The high strength aluminum alloys suffer from major cracking issues when processed through SLM. Two strategies are applied separately in order to prevent crack formation.

In the first strategy, the chemical composition of the material is varied slightly. More precisely, small percentages of grain refining elements are added to the alloy in order to obtain a more fine microstructure that is less susceptible to crack formation. In this way, crack free parts are obtained with high hardness after heat treatment of 195 µHv.

The second strategy applies preheating during the SLM process. In this way, thermal gradients during the process are reduced. Furthermore, precipitation formation, and therefore embrittlement of the material, is prohibited during the SLM process because of the relatively high processing temperature of 400°C. It was found that the scanning strategy applied has a high influence on the thermal gradients and therefore on the resulting microstructure of the SLM part. Combination of a scanning strategy consisting of short scan vectors and application of preheating leads to a microstructure consisting of nearly equiaxed grains and a significant reduction of cracks. Complete crack free parts were, however, not obtained so far.

The nickel alloy Hastelloy X also suffers from crack formation after SLM. It was found that the amount and the size of the cracks is highly dependent on the laser parameter combination used. Tuning of the laser parameters combined with application of preheating resulted in minimal crack formation. Mechanical analysis consisting of tensile testing and hardness confirmed good mechanical properties. It can be concluded that the residual crack formation only had minimal impact on the strength of the material.

Although no problems arise during SLM of H13 tool steel without preheating, production of this steel with application of powder bed preheating shows interesting results. A preheating temperature of 400°C is situated above the martensite formation temperature of this steel. When produced without preheating, the resulting microstructure is a highly tempered martensitic structure. The martensitic phase in each newly deposited layer is namely tempered to a large extend during the melting of consequent layers. During production with preheating, on the other hand, the material stays in its austenitic phase during SLM. After production, during the cooling down phase to room temperature, a bainitic phase is formed throughout the entire part. This bainitic phase is harder and stronger compared to the tempered martensitic phase. The increased hardness and strength of the as-built part when preheating is applied, could cancel the need for post-process heat treatment of the SLM parts.

The last material that was investigated is the medical CoCr alloy. The importance to apply a post-process heat treatment was shown in order to achieve satisfying mechanical properties. After heat treatment, the microstructure is coarsened which softens the material and makes it more ductile in order to improve fatigue characteristics of this alloy. Furthermore, the benefit of residual compressive stresses at the outer surface of the parts is demonstrated with regards to fatigue properties.