MICROSTRUCTURE AND MECHANICAL MICROSTRUCTURE AND MECHANICAL PROPERTIES OF DIRECT METAL LASER...
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MICROSTRUCTURE AND MECHANICAL PROPERTIES OF DIRECT METAL LASER SINTERED TI- 6AL-4V†
T.H. Becker1*, M. Beck2 & C. Scheffer3
1,3Department of Mechanical and Mechatronic Engineering Stellenbosch University, Stellenbosch
2Zentralinstitut für Medizintechnik (IMETUM) Technische Universität München, München
Direct metal laser sintering (DMLS) is a selective laser melting (SLM) manufacturing process that can produce near net shape parts from metallic powders. A range of materials are suitable for SLM; they include various metals such as titanium, steel, aluminium, and cobalt-chrome alloys. This paper forms part of a research drive that aims to evaluate the material performance of the SLM-manufactured metals. It presents DMLS-produced Ti-6Al- 4V, a titanium alloy often used in biomedical and aerospace applications. This paper also studies the effect of several heat treatments on the microstructure and mechanical properties of Ti-6Al-4V processed by SLM. It reports the achievable mechanical properties of the alloy, including quasi-static, crack growth behaviour, density and porosity distribution, and post-processing using various heat-treatment conditions.
Direkte metaal-laser-sintering is ‘n selektiewe lasersmeltvervaardigingsproses wat naby aan netto-vorm onderdele van metaalpoeiers kan produseer. Verskeie materiale is geskik vir lasersmeltvervaardiging, onder andere titaan, staal, aluminium en kobaltchroom legerings. Die doel van dié navorsing is om die materiaaleienskappe van lasersmeltvervaardigde onderdele te ondersoek. ‘n Titaan legering (Ti-6Al-4V) wat dikwels biomediese en ruimte toepassings het, word voorgehou. Verder word die effek van verskeie hittebehandelings op die mikrostruktuur en meganiese eienskappe van die titaan legering, na dit lasersmeltvervaardiging ondergaan het, ondersoek. Die quasi-statiese kraakvoortplanting, digtheid- en poreusheidsverspreiding en die verwerking met verskeie hittebehandelingtoestande word bespreek.
† This is an extended version of a paper presented at the 14th International RAPDASA conference held at the Central University of Technology in South Africa in 2013.
* Corresponding author South African Journal of Industrial Engineering May 2015 Vol 26(1), pp 1-10
Direct metal laser sintering (DMLS), a selective laser melting (SLM) process, is a laser-based additive manufacturing (AM) technique that uses CAD models to create three-dimensional parts. The technique uses a high-powered laser to fabricate dense components from metal powder [1-3]. DMLS is capable of producing geometrically complex designs to high tolerances and with minimal material waste, while avoiding lengthy machining times. Furthermore, no tooling changes are required for different components to be manufactured on the same machine. Over the past decade, increased material performance of SLM-manufactured components has allowed for a wide range of applications of the technique [4-6]. One such example is the use of DMLS to fabricate medical implants using Ti-6Al-4V, a titanium alloy that is typically characterised by high strength, low density, high corrosion resistance, and good biocompatibility [7-9]. The use of DMLS has demonstrated its versatility here, as it allows for the manufacture of geometrically complex and customised patient-specific implants. And due to the seamless CAD-to-manufacture transition, fast manufacturing of parts is possible . There is significant concern, however, about the application of SLM-produced parts. For example, medical implants require strict material properties that have not yet been completely matched by SLM products . The concerns abut SLM-manufactured metals relate to internal stresses (resulting from steep temperature gradients and high cooling rates) that occur during the manufacturing process , the microstructure of as-built components and the resultant material performance , and the occurrence of pores . DMLS parts do not normally have full density (although 99.8 per cent density can be achieved [10,11]), and they have an anisotropy due to the inherent layer-wise building procedure. Furthermore, little has been reported in the literature on heat-treatments, particularly for the application of Ti-6Al-4V in the biomedical industry. According to ASTM F1472 , as-built SLM-manufactured Ti-6Al-4V implants are currently may not be used in biomedical implants due to the presence of a martensitic microstructure  and the occurrence of porosity . Previous studies by the author have indicated that high, non-uniform residual stresses are present in as-built DMLS Ti-6Al-4V samples that approached the yield strength of the material [11,15]. These stresses could easily be relieved, however, through heat-treatment. Studies by Vrancken et al.  on the microstructure and the influence of heat-treatment have shown that, due to the specific process conditions and thus the specific microstructure, SLM-produced parts require different heat-treatment from bulk alloy parts. They showed that the temperature, time, and cooling rate play an important role. Mechanical properties were dependent on the maximum heat treatment temperature where an increased maximum temperature resulted in a decline in the yield and ultimate tensile strength (UTS) and an increase in the fracture strain due to the transformation of the fine α’ needles to a more coarse mixture of α and β. Similarly, work undertaken by Leuders et al.  has shown that porosity influences the fatigue life of SLM-manufactured Ti-6Al-4V: they identified a correlation between porosity and the fatigue behaviour in the high-cycle fatigue regime, where porosity vastly decreases fatigue life. It follows that the successful industrial application of DMLS-manufactured Ti-6Al-4V components requires an investigation of material performance. Such a study should address the achievable mechanical properties of DMLS-manufactured Ti-6Al-4V, and post-treatment – such as heat-treatment – to improve the material performance. Characteristic quasi- static, crack growth and fatigue behaviour, residual stresses, and, most importantly, the microstructural interaction with the properties mentioned, should be investigated. In this work, DMLS Ti-6Al-4V is post-processed through heat-treatment and hot isostatic pressing (HIP). The material performance is compared with the as-built and the wrought
material condition. A thorough testing procedure, combining porosity investigations using X- ray computed tomography (CT), quasi-static and dynamic mechanical loading, and microstructural characterisation is undertaken. Based on the findings, conclusions are drawn on the applicability of DMLS Ti-6Al-4V for biomedical implants.
2 EXPERIMENTAL METHODOLOGY
Fully characterising the material properties of DMLS-manufactured Ti-6Al-4V and the link between the DMLS process and the material properties requires a vast number of test specimens and test data, and a great deal of time. In this study, the focus was directed toward heat-treatment and the link between microstructure, as well as their link to the achievable tensile and dynamic mechanical properties of DMLS Ti-6Al-4V. This study did not consider any anisotropic affects that arise due to the inherent building process; samples were thus built in the XY plane orientation, according to the ISO/ASTM52921-13 designations. All samples were made of Ti-6Al-4V, and were produced with an EOSINT M280 (EOS GmbH), which used a layer thickness of 30μm and a 200W Yb-fibre laser. The scanning strategy is multidirectional with no further parameter descriptions supplied by EOS. The machine is installed at the Centre for Rapid Prototyping and Manufacturing at the Central University of Technology in Bloemfontein. The experimental procedures presented in this study were conducted in ambient conditions. Heat-treatments and density determination (CT scans) were carried out at Stellenbosch University. Tensile tests and investigations in crack growth behaviour were conducted at the Centre for Materials Engineering at the University of Cape Town. Hot isostatic pressing (HIPing) was undertaken by Bodycote in Belgium.
2.1 Heat-treatment and microstructural evaluation
Ti-6Al-4V is an alpha-beta (α-β) alloy that is widely known to be suitable for heat- treatment, with many different microstructures obtainable through variations of heat- treatments. At room temperature, mill-annealed Ti-6Al-4V is about 90 per cent (volume) α, and the α phase thus dominates the physical and mechanical properties of this alloy; the β phase can be manipulated in amount and composition through heat-treatment. The overall effects of processing history and heat-treatment on microstructure are complex, where the microstructure depends on both processing history and heat-treatment . The microstructure that combines the highest static strength and ductility is not necessarily that which provides the optimum fracture toughness, fatigue strength, or resistance to crack growth. Typically, when wrought Ti-6Al-4V is heat-treated in the α-β temperature range and subsequently cooled, an equiaxed microstructure is formed that is categorised by the presence of globular (equiaxed) primary α in the transformed β (platelike) matrix. Similarly, a β structure is achieved by cooling from above the β transus to obtain an acicular or needle-like structure. The relative advantages of equiaxed and acicular titanium alloy microstructures include a higher ductility and formability, higher strength, and better low-cycle fatigue (initiation) prope