Residual Stress in Metal Specimens Produced by Direct Metal Laser Sintering

Direct Metal Laser Sintering (DMLS) has great potential in additive manufacturing because it allows 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 deformations 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 investigation of the residual stress in DMLS samples fabricated from stainless steel 316L and Ti6Al4V alloy. Conclusions regarding directions and values of stresses in DMLS objects are given.Mechanical Engineerin

Multi material powder delivering systems for selective laser melting

Published ArticleSelective Laser Melting (SLM) is a powder bed fusion process which is an additive manufacturing (AM) process, whereby a laser beam selectively fuses regions of a powder bed to form complex objects. Growth in the SLM field has revealed the need for parts containing multiple materials for applications in the medical, tool making, aerospace and other hi-tech industries. By applying multiple materials, regions with different mechanical properties, thermal conductivity zones or corrosion-resistant coatings can be achieved in a single manufacturing cycle utilizing the SLM process. With the SLM process physical bonds can be formed between different materials by creating an interlocking interface due to the rapid solidification of the molten materials. With the current SLM equipment, multi material objects are possible but only with material differences between the layers. New approaches are needed to develop a method that allows multi material parts not only in the Z axis, but also allow material differences on a single layer (X-Y axis). Approaches such as powder feeding through a capillary tube, auger feed system, electrostatic charge or masks have all been proposed as solutions to multi material deposition. Multi material objects produced in a single cycle with complex geometry and prescribed properties has the opportunity of further growing the AM market

Titanium Alloys Manufactured by In Situ Alloying During Laser Powder Bed Fusion

Published ArticleThis work is focused on the investigation and understanding of in situ processes in Ti-15%Mo and Ti6Al4V-1.38%Cu alloys by laser powder bed fusion (LPBF). In both materials, Mo and Cu were introduced as elemental powders into the precursor powder mixture. The effect of process parameters, i.e., energy input on surface morphology and homogeneity, was investigated. The importance of different thermophysical properties of blended powders is also discussed. The chemical composition of phases and phase distribution in sintered materials were investigated by means of scanning electron microscopy. The mechanical properties of in situ alloyed as-built LPBF specimens were determined. The results obtained developed knowledge that is important for understanding the in situ alloying process during LPBF, and they create a base for synthesizing new materials

Laser-assisted synthesis in CueAleNi system and some of its properties

Published ArticleConditions of intermetallic phase synthesis during layerwise selective laser sintering/melting in CueAl eNi powdered system were explored. There to we used optical and scanning electron microscopy, EDX and XRD analysis for microstructure testing and intermetallide identification in porous 3D samples depending on the laser syntheses regimes. A high laser sintering ability of the prepared mixtures and shape memory effect in 3D samples by results of specific electrical resistance measurements were shown. Porosity of the laser-synthesized Al7Cu4Ni intermetallide led to a peak shifting by the temperature axis and the intervals of the AseAf austenite transformation and MseMf martensite transformation were 50 ÷ 80 C and þ25 ÷ 40 C respectively

Residual stresses in direct metal laser sintered parts

Published ArticleDirect Metal Laser Sintering (DMLS) fabricates parts using a track-by-track, layer-by-layer method in which powder is formed by melting and solidification of single tracks and thin layers. A laser beam scans over the powder layer thus creating a cross-sectional area of the 3D object. High-concentration of laser energy input leads to high thermal gradients which induce residual stress within the as-built parts. Methods for measurement residual stresses and stresses in DMLS parts were analysed

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

Published ArticleTo 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

NON-DESTRUCTIVE TESTING OF THE PARTS MANUFACTURED BY DIRECT METAL LASER SINTERING

Published Conference ProceedingsInterest in Additive Manufacturing (AM) has grown considerably in the past decades. The industry has gained the great benefits from this type of technologies. The main advantages being geometrical freedom that allows designing parts with complex shape, which are difficult or impossible to produce by conventional technology, shortened design to product time, customization and possible use of several materials in one process. Direct Metal Laser Sintering (DMLS) is one of the most promising AM techniques that utilize metal materials. Due to the complex nature of the DMLS process, one of the drawbacks is the high residual stress in the manufactured parts. This can result to the formation of internal cracks and eventually to a substantial deterioration of the mechanical properties of the products and their application properties. For this reason it is very important to identify defective parts before enrolling into service. Non-destructive testing (NDT) is effective for detection of internal defects without causing damage. NDT also covers a wide group of methods of analysis used to evaluate the properties of a material. NDT techniques like ultrasonic inspection, acoustic emission, visual inspection, thermography, X-ray and 3D computed tomography (CT) inspection, etc. are now widely used for various industrial applications. For the detection of defects and to study the properties of the material each of these methods uses different physical principles that have their advantages and disadvantages. In this study some of the NDT techniques in terms of their applicability to the inspection of parts manufactured by DMLS technology are considered

In situ heat treatment in selective laser melted martensitic AISI 420 stainless steels

Published ArticleThe article explores an evolution of a microstructure in AISI 420martensitic stainless steel during selective laser melting. Several upper layers had hardness of 750 HV and contained 21±12 vol.% austenite phase. The final bulk microstructure consisted of thermally decomposed martensitewith hardness of 500–550HV and unusually high, 57±8 vol.%,amount of austenite. Obtained results indicate that duringmanufacturing a partitioning and austenite reversion took place, owing to the thermal cycling of the inner regions duringmanufacturing. Numerical simulation was found plausible to analyze and explain thermally activated processes that occurred in situ. Results of numerical simulation of the thermal cycles in dependence on the processing parameters suggested a possibility to control the thermal processes by variation of the laser energy input

IN-SITU ALLOYING PROCESS OF TI6AL4V-xCU STRUCTURES BY DIRECT METAL LASER SINTERING

Published Conference ProceedingsIn this paper the fabrication of in-situ Ti6Al4V-xCu alloy structures by DMLS are investigated. Ti6Al4V is a commonly used biomedical alloy because of it suitable mechanical and biocompatible properties. Copper is a proven anti-bacterial agent and in small amounts is not toxic to the human body. Ti6Al4V-xCu implants can be constructed to have a biocompatible structure with copper additions to reduce the risk of bacterial infection and implant failure. Infection at the bone–implant interface is the most probable reason for implant failure directly after implantation. Ti6Al4V powder was mixed with Cu powder to form a master alloy. Optimal process parameters need to be established for in-situ alloying of Ti6Al4V-xCu to form dense parts with suitable surface quality. The effect of laser scanning speeds and hatch distance on surface characteristics was investigated. The surface roughness, chemical composition and distribution of Cu near the surface and within the synthesized layer, as well as micro hardness were considered. A rescanning strategy was employed and showed improved alloy homogeneity and surface quality

PECULIARITIES OF SINGLE TRACK FORMATION FROM TI6AL4V ALLOY AT DIFFERENT LASER POWER DENSITIES BY SELECTIVE LASER MELTING#

Published ArticleThis paper describes the geometrical characteristics of single tracks manufactured by selective laser melting (SLM) at different laser powers (20-170 W) and scanning speeds (0.1-2.0 m/s). Simulation of temperature distribution during processing is carried out. A conclusion about the optimal process parameters and peculiarities of selective laser melting of Ti6Al4V alloy at low and high laser powers and scanning speeds is reached. The analysis of temperature fields creates opportunities to build parts with the desired properties by using SLM