Showing papers by "Igor Yadroitsev published in 2014"

Evaluation of Residual Stress in Selective Laser Melting of 316L Steel

Abstract: Selective Laser Melting (SLM) has great potential in additive manufacturing methods because it allows producing full density complex parts with desired inner structure and surface morphology. Mechanical properties of SLM objects depend strongly on the material properties as well as strength of the connections between tracks and layers since all objects made by SLM are superposition of the single tracks and single layers. High temperature gradient as a result of the locally concentrated energy input can lead to residual stresses, crack formation and part deformations both during laser processing and after cutting objects from the substrate. X-ray diffraction method was used for investigation of residual stress in SLM samples from 316L steel fabricated by one-zone strategy with 50% overlap of the tracks. Samples had rectangular shape and different thickness: 50 m (one layer), 0.2 mm (5 layers) and 1 mm (25 layers). All as-made samples attached to the substrate had the tensile stress. Normal residual stress along the scan direction was 1.2-1.7 times higher than perpendicular direction. In some areas residual stress was about and exceeded the yield strength of 316L wrought material.

Microstructure of slm manufactured 316l and 420 grades stainless steel

Abstract: Selective laser melting (SLM) is an additive manufacturing process, involving track-by-track powder material melting on the previous fabricated layer. Because of the complete remelting, SLM objects have a cast microstructure and as a result of high cooling rates the microstructure itself is fine. During manufacturing, each next track heats up already solidified materials beneath the surface. Therefore, inner layers are subjected to multiple heating-cooling cycles. In this investigation, microstructural characterization of SLM parts made of AISI 316L and 420 stainless steel grades were performed in order to understand the influence of the SLM process parameters on the final steel microstructure. It was shown that rapid solidification after track melting resulted in the formation of colonies grown epitaxially from the fusion boundary surface. Each colony consists of very fine cells with submicron cell spacing, coherent and grown in the same direction. In AISI 316L steel, this structure remains unchanged since austenite is stable and is not transformed at cooling down to room temperature. In AISI 420 steel, martensite is formed with rapid cooling and undergoes in-situ heat-treatment at further thermal cycling. The effect of thermal cycling on the final microstructure of the built part, i.e. the depth of the heat-affected zone of each next single track, is dependent on the material properties and the SLM process parameters. In the case of AISI 420 martensitic stainless steel, thermal cycling initiated a partitioning heat treatment process, which resulted in high amounts of austenite in the microstructure. The in-situ heat treatment conditions in the solidified inner parts were numerically simulated using a time-dependent “Heat transfer in solids” Comsol module, and a comparison of modeled isotherms with experimentally observed microstructures showed a good agreement between simulation and experiment. Since the model demonstrated a good agreement with experimental results, it was used to evaluate the influence of laser power and laser scanning speed on the in-situ heat treatment. This gives useful background to predict the microstructure of SLM products at manufacturing.

Strategy toward the manufacturing of fully dense parts from aisi 420 stainless steel by selective laser melting

Abstract: Selective laser melting (SLM) is an additive manufacturing technique that produces functional parts from metal powders. SLM is a powerful technology for automotive, medical, chemical, aerospace and other hi-tech industries. SLM is a well-suited technology for the mould industry: it can considerably reduce the time for engineering and fabrication of moulds. SLM provides extraordinary freedom to validate design of moulds and to develop new materials. The extension of applications requires different materials with specific properties. To show the potential and features of SLM for mould manufacturing applications, fully-dense samples from AISI 420 stainless steel powder were fabricated. AISI 420 stainless steel is a material widely used in the plastics-moulding industry where high hardness and wear resistance is required. The study discusses the influence of laser power, scanning speed, layer thickness and hatch distance on the formation of single tracks and layers. Algorithm for finding optimal process parameters is indicated with respect to features of single tracks, layers and 3D objects. Width of the single track determines the hatch-distance to form a single layer. Height of the tracks and scanning strategy define the morphology of the synthesized layer. Morphology of the layer has a significant impact on the variation of the thickness of the next delivered powder layer. This should be considered when choosing the optimal process parameters. An energy input should be adequate to melt powder layer and to achieve remelted depth providing good cohesion between the layers. The pore analysis (shapes and sizes) can also contribute to the identification of optimal process parameters. To produce non-porous objects from AISI 420 (–32 µm) stainless steel powder, 60 W laser power, 70 µm spot diameter, 0.12 m/s scanning speed, 120 µm hatch-distance and 40 µm thickness of powder layer and two-zone scanning strategy with following 90 turning for each next layer were applied. Microhardness of the SLM samples was 513±19.5 HV0.3 for inner regions; upper layers had 700-750 HV0.3.