PSI - Issue 14

Digendranath Swain et al. / Procedia Structural Integrity 14 (2019) 337–344 Swain et al./ Structural Integrity Procedia 00 (2018) 000 – 000

338

2

Beam Melting (EBM) and Lase Metal Deposition (LMD). DMLS falls into the category of LBM often known as Selective Laser Melting (SLM) (Herzong et al. (2016)). Metal is deposited in thin layers with the help of selective scanning of the laser source over a bed of feedstock powders. Thereby, the laser source sequentially, track – by-track, and layer-by-layer melts the powders and fuses it to the previous layer to sinter a 3D part (van Zyl et al. (2016)). Additive method of fabrication has attracted more attention in the last decade owing to the imminent advantages of (i) shorter lead-time, (ii) freedom of component design (complex geometry with internal structure), and (iii) no wastage of raw materials (Herzog et al. (2016), Ali et al. (2017), Anderson (2017)). Despite of these advantages, it has got a key disadvantage of inducing large RS into the final products due heterogeneous temperature fields associated with high heating and cooling rates. The development of RS leads to a compromise in the structural integrity of the components fabricated through this route, if it amplifies the service stresses significantly. Hence, RS may pose undue restrictions on the service life and functional requirement of the components. Moreover, the distortions and warpage of the 3-D printed block, due to excessive RS, limits the size of the part to be fabricated (Li et al. (2018)). In this paper, a Titanium alloy (Ti-6Al-4V) is considered as the pilot material to study the RS behavior in DMLS samples. This material is very popular in aerospace industries owing to its high specific strength and corrosion resistance (van Zyl et al. (2016)). Moreover, Ti and Ti alloys are favorable for 3-D printing since it would reduce the high machining costs and long lead times in conventional processing (Herzog et al. (2016), Frazier (2014)). A short literature survey on the residual stress behavior of Ti-6Al-4V DMLS samples is reported below. Van Zyl et al. (2016) have recently measured residual stresses in Ti-6Al-4V (ELI) DMLS samples (cubes and parallelepiped printed using EOSINT M280) employing X-ray diffraction (XRD) technique. RS have been measured along the full depth by electrolytic removal of layers in the print plane (xy-plane) normal to the build direction (z). In a similar study, Yadroitsava and Yadroitsev (2015) have also noticed that DMLS samples had RS equivalent to the ultimate strength of wrought materials. The major component in the print plane was along the direction of scanning. They suggested that all DMLS samples must be heat-treated before removal from the substrate to avoid severe deformation or warpage. Lim et al. (2017) characterized RS in 30 mm (x) × 30 mm (y) × 10 mm (z- build direction) Ti-6Al-4V DMLS samples (EOS M290 machine) and electron beam welding (EBM – ARCAM A2XX machine). They used hole drilling, XRD and contouring method for RS measurement wherein both heat treated and non-heat treated DMLS conditions were studied. The heat-treatment was carried out in an Argon atmosphere at 650  C for 3 hours keeping the specimen on the printing base plate. The RS measurement was carried out in different specimens before and after heat treatment. They found non-equibiaxial state of stress near the surface of the non heat treated DMLS samples and equibiaxial state in case of heat-treated samples. However, they noticed a disagreement of hole drilling and XRD measurements. Anderson (2017) investigated the capability of three RS measurement systems, namely (a) neutron diffraction (ND), (b) XRD, and (c) Focused Ion Beam (FIB) stress relaxation method coupled with DIC, for evaluating RS in Ti-6Al-4V SLM samples. ND was able to obtain volumetric Type-I (macroscopic) RS distribution through the full depth of the sample. Long testing time and sophisticated facility with limited accessibility restricts its wide application in industries. XRD, in tandem with electro-polishing, could resolve the in-plane stress distribution through individual build layers. However the results get biased with surface roughness, hence it may lead to inaccurate measurements. The FIB-DIC technique could measure the strains due to stress relaxation to a depth more than XRD; however sophisticated SEM/FIB facility and more research are required in this direction. Vrancken (2016) explored various strategies to reduce RS in SLM fabricated samples, i.e. (a) process parameters, (b) material properties, and (3) post heat treatment. They used contouring method for the purpose of RS measurement. The process parameters viz. base plate preheating, smaller layer thickness, high laser power, and large scan speeds were studied among other factors. Eight different SLM printed materials have been studied to interlink the material specific behavior to RS, which concluded that the knowledge of one material cannot be extrapolated to other materials. The post heat-treatment study concluded that it improves the material properties coupled with lowering RS. They have also studied the effect of RS on the mechanical behavior by observing the crack growth behavior of Ti6Al4V SLM compact tension (CT) specimens. Ali et al. (2017) have studied the effect of powder bed pre-heating on the properties and residual stress behavior of 3-D printed Ti-6Al-4V via SLM process. They noticed a decrease in RS with increase in pre-heating of feedstock 1.1. Literature review and scope of work