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New generation of selective Laser Melting (SLM) machines are evolving towards higher power lasers as well as multi laser systems in order to increase the productivity. The increase in laser power and the modification of the laser power distribution leads to microstructural and mechanical property variations that are still not well understood. This work aims at better understanding the interaction of a 1 kW top-hat power distribution laser on a well know material, 316 L stainless steel. The influence of texture and microstructure on relative density and crack density, when varying scan rotation, was evaluated. The high power (HP) laser and low power (LP) laser were compared with respect to microstructure and mechanical properties. HP leads to an increase in morphological and crystallographic texture together with a coarsening of the cell structure in contrast to the more random and finer cells found in LP processed material. Hot isostatic pressing was applied as a post-process treatment in order to close remaining pores and cracks. This helped in achieving higher elongations for LP and HP processed materials, while competitive mechanical properties to the 316 L material specifications were obtained in both cases. ispartof: Additive Manufacturing vol:23 pages:402-410 status: Published online more...

As laser sintering is increasingly being used for the production of actual end-use parts, there is considerable interest in developing materials that would enable new applications for this technique. Considering their properties and current applications, elastomeric polymers such as thermoplastic polyurethanes (TPU) have a very high potential in this regard. This study investigates the material properties that are involved in TPU sintering through the analysis of four distinct TPU grades. Examined parameters include powder flow, rheology of the melt and shrinkage and hardening behavior. It is found that, even though the particle morphology is not optimum, smooth and dense powder layers can be deposited for the investigated powders. Low melt viscosity and low shrinkage upon hardening further enable these materials to be easily processed into functional parts. Remaining issues, however, are part porosity and material degradation. The findings in this study provide clear links between material properties and behavior during laser sintering, and result in guidelines for future selection of TPU grades. more...

Owing to the high optical reflectivity of copper, silver, and gold in the infrared region, high laser power is required for laser-based additive manufacturing (L-AM). This increases the risk of damaging the laser optics due to sustained back-reflections and renders L-AM of reflective metals an unsustainable technology. To tackle this issue, a novel, industrially upscalable powder surface modification method is proposed and validated using a CuCr1 alloy. The surface of CuCr1 powder is modified by the outward diffusion of chromium in a nitrogen atmosphere, forming a rim around the powder particles. This doubled the optical absorption of the powder. Consequently, a mere 20% of the laser energy is required to process the surface-modified powder by laser powder bed fusion compared to the virgin CuCr1 powder. The fabricated parts demonstrate a very high thermal conductivity of 370 ± 15 W/(m·K) and tensile strength of 439 ± 19 MPa, after applying a suitable post-heat treatment. ispartof: Additive Manufacturing vol:35 issue:October 2020 status: Published online more...

The metal additive manufacturing industry is rising and so is the interest in new lattice structures with unique mechanical properties. Many studies have already investigated lattice structures with different geometries and their influence on mechanical properties, but little is known about the effect of specific processing characteristics that are inherent to metal additive manufacturing. Therefore this study investigates the effect of two crucial steps in the manufacturing process: the build orientation selection and heat treatment. In total the microstructure and static mechanical properties of five different orientations and three heat treatment conditions were evaluated using Ti6Al4V diamond like lattice structures. The results show a significant decrease in mechanical strength for samples that are built diagonally and a transformation of the microstructure after a HIP (hot isostatic pressing) treatment, resulting in a lower maximum strength, but higher ductility. In general, horizontal struts should be avoided during manufacturing, unless the applied load after manufacturing can be properly supported by other struts. Both a stress relief heat treatment and a HIP treatment can be used in statically loaded applications, whereas a HIP treatment is believed to be beneficial for dynamically loaded applications. This study enables an appropriate selection of build orientation and heat treatment of lattice structures for different applications. more...