PSI - Issue 14

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

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powders. However, it is known that large preheating of powders diminishes the yield strength of the printed sampled (Vrancken (2016)). Chauke et al. (2013) have characterized the tensile properties and hardness of DMLS (EOSINT M270 printed) non heat-treated and post heat-treated samples of Ti-6Al-4V. The samples were solution heat-treated by keeping the tensile specimens at 1200  C for 3 hours and furnace cooled. The heat-treated samples showed lower tensile properties, lower elongation, an increased hardness and a brittle fracture behavior. Li et al. (2018) in a recent review proposed that RS in metal 3-D printing can be mitigated through in-process (e.g. preheating of base plate, pre-heating of feedstock, laser peening, and better process planning) and post-process methods (e.g. proper machining, heat treatment, and shot peening). From the aforesaid review, it is evident that RS not only depends upon material properties, build parameters and post-process conditions, but also on the geometry of the samples and support structures (van Zyl et al. (2016)). Hence these parameters are intimately interlinked (Vrancken (2016)). Since many process variables (scan speed, beam spot size, beam power, pre-heating, and post heat treatment) detect the emergence of RS, it is essential to characterize RS for specific parameter sets. Moreover, since the measurement range, accuracy and resolution is a prime factor in understanding the behavior of RS, it is necessary to know in what scenarios these measurements are valid. In this paper, incremental hole drilling stain gauge technique has been used to measure the surface and subsurface RS. In literature, RS measurement on Ti-6Al-4V has been carried out mostly using methods other than incremental hole drilling except Lim et al. (2017). The dimension along the built direction in Lim et al. (2017) was only 10 mm in contrast to 65 mm in this paper. Moreover, in this paper RS in the same specimen has been measured before and after heat-treatment, which was not the case with Lim et al. (2017). In addition to that the heat-treatment conditions and the fabrication parameters used in this study are different from Lim et al. (2017) and Chauke et al. (2013). Furthermore, Lim et al. (2017) have not clearly brought out the directional nature of the RS; hence it becomes difficult to interpret the results. The current knowledge of RS and its impact in finished products through3-D printing is in its incipient stage. More research is needed to make functional worthy product with best structural integrity. At this juncture, Indian Space Research Organization (ISRO) is on the verge of inducting 3-D printed products into its main stream launch vehicle and satellite programs. Towards this objective, it is utmost important that the 3-D printed materials are well characterized and qualified before it is used in the space structures. Since the emergence of RS is inherent to the AM process, a good understanding of its origin and nature is necessary. This would help in chalking out plans to mitigate the deleterious effects of the RS and in easier qualification of these products. This paper is an attempt to characterize the RS behavior of DMLS (EOS M280 machine) Ti-6Al-4V samples manufactured in an Indian industry. Table 1. The built parameters (equipment specifications) of the DMLS process. Process parameters Specimen V3 & H3 Energy source Yb-fibre laser Max scan speed 950 mm/s Max. beam Power 300 W Beam spot size 80 µm

2. Material and Methods

2.1. Details of sample fabrication

Two Ti-6Al-4V specimens, namely specimen V3 and H3 (Figs. 1a and 1b), were 3-D printed using the DMLS machine EOS M280. These specimens were manufactured using Ti-6Al-4V (ELI) powders supplied by EOS in the pre-alloyed gas-atomized condition. The chemical composition of the feedstock powder was: Al – 6.2%, V – 3.9%, O – 0.12%, N – 0.009%, H – 0.003%, Fe – 0.20%, C – 0.005%, and Ti – remaining (weight %). The built parameters like scan speed, beam spot size and beam power play a major role in the development of RS. Therefore it is necessary to mention these printing variables used for the fabrication process. These parameters are reported in Table 1. The samples were fabricated using a back-and-forth scanning with a hatch distance of 0.1 mm to obtain the prismatic blocks. The scanning direction in each current layer was varied by 67  to the recently built layer. The base