Igor Yadroitsev

Evaluation of residual stress in stainless steel 316L and Ti6Al4V samples produced by selective laser melting

Selective laser melting (SLM) has great potential in additive manufacturing because it enables the production of full-density complex parts with the desired inner structure and surface morphology. High temperature gradients as a result of the locally concentrated energy input lead to residual stresses, crack formation and part deformation during processing or after separation from the supports and the substrate. In this study, an X-ray diffraction technique and numerical simulation were used for investigating the residual stress in SLM samples fabricated from stainless steel 316L and Ti6Al4V alloy. Conclusions regarding directions and values of stresses in SLM objects are given.

Residual Stress in SLM Ti6Al4V Alloy Specimens

Selective Laser Melting (SLM) presents a modern manufacturing process with an innovative technology which allows the production of full-density objects or fine-structured parts with complex geometry and inner structures. Stability and certification of the properties of SLM parts are important tasks for all producers and end-users. One of the drawbacks of this technology is high residual stress in as-made SLM objects. In this study X-ray diffraction technique was used for investigating the residual stress induced into SLM Ti6Al4V alloy samples. Principal stresses were estimated for the cut rectangular specimen. Two types of the cantilevers were produced and numerical simulation of the stress was performed. The bending of cut cantilevers was measured before and after heat treatment. Next series of the samples had rectangular shapes and different thicknesses from 1 to 46 layers. All as-manufactured specimens attached to the substrate showed the presence of tensile residual stresses near the top surface. Residual stress along the laser scanning direction had magnitudes twice that of the stress in the perpendicular direction. Conclusions regarding directions and values of stresses in SLM objects from Ti6Al4V powder are given.

MICROSTRUCTURAL AND THERMAL STABILITY OF SELECTIVE LASER MELTED 316L STAINLESS STEEL SINGLE TRACKS

To remove residual stresses, an as-built SLM object is usually post- treated. This treatment can affect the microstructure, changing the final mechanical characteristics. This investigation is focused on the microstructural characterisation of 316L austenitic stainless steel in as-built and annealed conditions. The SLM microstructure was relatively stable up to 900°C, when cell boundaries start to disappear. At higher temperatures, an insignificant grain coarsening was detected. These microstructural changes caused a gradual drop in the hardness. The obtained result is background for the future development of post-treatment regimens to achieve a high level in the final mechanical properties of SLM objects.

Tensile properties and microstructure of direct metal laser sintered Ti6Al4V (ELI) alloy

Direct metal laser sintering (DMLS) is an additive manufacturing technology used to melt metal powder by high laser power to produce customised parts, light-weight structures, or other complex objects. During DMLS, powder is melted and solidified track-by-track and layer-by-layer; thus, building direction can influence the mechanical properties of DMLS parts. The mechanical properties and microstructure of material produced by DMLS can depend on the powder properties, process parameters, scanning strategy, and building geometry. In this study, the microstructure, tensile properties, and porosity of DMLS Ti6Al4V (ELI) horizontal samples were analysed. Defect analysis by CT scans in pre-strained samples was used to detect the crack formation mechanism during tensile testing of as-built and heat-treated samples. The mechanical properties of the samples before and after stress relieving are discussed.

Optical Monitoring and Numerical Simulation of Temperature Distribution at Selective Laser Melting of Ti6Al4V Alloy

Selective laser melting (SLM) is a modern method for producing objects with complex shape and fine structures in one working cycle from metal powders. Combination of the advanced technology of SLM with unique properties of Ti6Al4V alloy allows creating complex 3D objects for medicine or aerospace industry. Since properties of SLM parts depend on the geometrical characteristics of tracks and their cohesion, optical monitoring is actually used to for control the process. Temperature gradient determines the microstructure and mechanical properties of the SLM part, so studies about temperature fields are primarily important. On-line monitoring during laser scanning of Ti6Al4V alloy and formation of a single track in real-time with high-speed IR camera was studied. Numerical simulation allowed estimation the temperature distribution during processing. Conclusion regarding control system based on the online monitoring of deviations of the signal from IR camera during the SLM process was done.

Residual stress in Ti6Al4V objects produced by direct metal laser sintering

Direct Metal Laser Sintering produces 3D objects using a layer-by- layer method in which powder is deposited in thin layers. Laser beam scans over the powder fusing powder particles as well as the previous layer. High-concentration of laser energy input leads to high thermal gradients which induce residual stress within the as- built parts. Ti6Al4V (ELI) samples have been manufactured by EOSINT M280 system at prescribed by EOS process-parameters. Residual stresses were measured by XRD method. Microstructure, values and directions of principal stresses inTi6Al4V DMLS samples were analysed.

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

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.

Numerical comparison of lattice unit cell designs for medical implants by additive manufacturing

ABSTRACT The aim of this study was to compare traditional strut-based lattices with minimal surface designs using morphological analysis and image-based simulations of design files. While the two types have been studied widely, no direct comparison has ever been done. Surprisingly, there are no major differences in performance between the two types generally, but minimal surface designs do outperform slightly on angular load simulation. However, minimal surface designs in this density range are shown to have very thin walls, potentially making their accurate production more challenging, or more suitable for applications where larger pore sizes and sheet thicknesses may be applicable. Interesting results such as dual pore size distributions and variations in tortuosity of pore networks are demonstrated, with differences between various designs. The results show that all the tested designs are suitable for bone implants, but the best design might be selected based on its specialised performance requirements.

Functionalization of Biomedical Ti6Al4V via In Situ Alloying by Cu during Laser Powder Bed Fusion Manufacturing

The modern medical industry successfully utilizes Laser Powder Bed Fusion (LPBF) to manufacture complex custom implants. Ti6Al4V is one of the most commonly used biocompatible alloys. In surgery practice, infection at the bone–implant interface is one of the key reasons for implant failure. Therefore, advanced implants with biocompatibility and antibacterial properties are required. Modification of Ti alloy with Cu, which in small concentrations is a proven non-toxic antibacterial agent, is an attractive way to manufacture implants with embedded antibacterial functionality. The possibility of achieving alloying in situ, during manufacturing, is a unique option of the LPBF technology. It provides unique opportunities to manufacture customized implant shapes and design new alloys. Nevertheless, optimal process parameters need to be established for the in situ alloyed materials to form dense parts with required mechanical properties. This research is dedicated to an investigation of Ti6Al4V (ELI)-1 at % Cu material, manufactured by LPBF from a mixture of Ti6Al4V (ELI) and pure Cu powders. The effect of process parameters on surface roughness, chemical composition and distribution of Cu was investigated. Chemical homogeneity was discussed in relation to differences in the viscosity and density of molten Cu and Ti6Al4V. Microstructure, mechanical properties, and fracture behavior of as-built 3D samples were analyzed and discussed. Pilot antibacterial functionalization testing of Ti6Al4V (ELI) in situ alloyed with 1 at % Cu showed promising results and notable reduction in the growth of pure cultures of Escherichia coli and Staphylococcus aureus.

On the impact of different system strategies on the material performance of selective laser melting- manufactured TI6AL4V components

Selective Laser Melting (SLM) is a powder-based additive manufacturing process that has gained substantial interest in recent years due to its feasibility of producing geometrically- complex metallic components for end-use in various industries, with or without post-treatment procedures. This paper presents recent research undertaken on different scanning strategies and process parameters with the purpose of providing an overview of the achievable material performance of Ti6Al4V components, and comparing its properties with the conventionally-produced parts. In order to understand their output, differences in the building strategies of the systems studied are analysed, and their influence on the resulting mechanical and metallurgical properties is highlighted.