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

On the formation of AlSi10Mg single tracks and layers in selective laser melting: microstructure and nano-mechanical properties

Selective laser melting (SLM) is a relatively new manufacturing technique that can be used to process a range of materials. Aluminum alloys are potential candidates for SLM but are more difficult to process than the titanium alloys more commonly used with this technique. This is because of the former’s physical properties that can result in high levels of porosity in the final parts. Although the majority of studies to date into the processing of Al alloys by SLM have considered the development of load bearing objects, in particular porosity reduction and mechanical characterization of the parts, it is also important to study the single tracks formed during the process. This paper studies the effect of changing the scan speed on the formation of fusion lines and single tracks from an Al alloy, as well as their overlap to form a single layer. The geometrical features of the melt pools as well as the boundaries of continuity and/or irregularities were defined and showed dependence on scan speed. Keyhole mode melting domination was observed. The scan tracks and layers were porosity-free suggesting pores to form with layer accumulation. Investigations showed that increasing the layer thickness should be avoided as it promoted defects. Energy dispersive X-ray (EDX) mapping was implemented to compare the chemical composition distribution in the SLM material and its as-cast counterpart. A fine microstructure with homogenous distribution of the alloying elements was observed. Nanoindentation and EDX were used to establish an understanding of the hardness profile across melt pools of single tracks and their interrelation to the chemical composition. The elemental distribution yielded uniform high nano-hardness with no spatial variation across the SLM material

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 STRESS IN TI6AL4V OBJECTS PRODUCED BY DIRECT METAL LASER SINTERING

Published ArticleDirect 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

METAL ADDITIVE MANUFACTURING OF BLENDED ELEMENTAL Ti-6Al-4V POWDERS

ArticleSouth Africa primarily produces titanium raw material as a TiO2 rich slag of which most is exported, without further value addition to the mineral. Therefore, powder development becomes a significant aspect of research with possibilities of growth within the titanium metal industry in South Africa. Commercially pure titanium has been successfully blended in conventional powder metallurgy processing, but the use of blended elemental powder to produce Ti-6Al-4V powder for metal additive manufacturing alloy parts has not been demonstrated yet. The objective of this study is to determine the feasibility of using blended elemental Ti-6Al-4V powder for use in a powder bed additive manufacturing (AM) system. In this paper a literature review and proposed methodology are presented and the expected outcomes are discussed

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

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

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