Effect of Lack of Fusion Formed during Electron Beam Powder Bed Fusion of Ti-6Al-4V Alloy on Impact Toughness
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Effect of Lack of Fusion Formed during Electron Beam Powder Bed Fusion of Ti-6Al-4V Alloy on Impact Toughness Mona Aziziderouei, Zhan Chen, Timotius Pasang, Martyn Newby, and Yuan Tao (Submitted March 9, 2020; in revised form June 30, 2020; published online August 14, 2020) Selective electron beam melting (EBM), a powder bed-based fusion process of Ti-6Al-4V alloy, is now being applied more and more widely particularly in biomedical and aerospace manufacturing industries. However, how Charpy impact energy (IE) of Ti-6Al-4V is affected by the build direction and lack of fusion (LOF) formed during the EBM process are not sufficiently understood. This information is generally very important in the applications of EBM. In this study, the build direction (BD) and LOF on IE have been studied and the effective factors have been found depending on the notch direction (ND) and thus the crack propagation direction in the microstructure. In this study, the influence of the BD and LOF on Charpy IE has been studied. Three effective factors have been found associated with the ND and thus crack propagation direction. The first factor is the BD which is the IE increases as the angle between the ND and the BD increases. This leads to the result of cracks propagating readily along with the a (hcp) grain boundaries, absorbing less energy. The second factor is LOF to assist crack propagation and this effect is maximized when the ND and the BD coincide. The last factor is the microstructural variances. In vertical samples, the crack has been found to propagate mostly through the a + b colonies and barely propagate in the prior b grain boundaries which leads to an increase in IE. While in the horizontal and 45° built directed samples, the crack propagates in both a + b colonies and the prior b grain boundaries which lead to lower the IE. Keywords
additive manufacturing, Charpy impact, crack propagation, defect, EBM, lack of fusion, mechanical properties
1. Introduction Selective electron beam melting (EBM) and selective laser melting (SLM), both of which are powder bed fusion metal additive manufacturing (AM), can potentially be applied widely in industries. This is because of their capability to manufacture parts with highly complex shapes. High-strength Ti-6Al-4V is commonly known being used for making aerospace parts and medical implants, which are often either complex or unique in a shape. Thus, AM technologies are particularly suited for Ti6Al-4V parts manufacturing. However, a common product feature in many aerospace and medical implant applications is the requirement of excellent mechanical properties and thus AM Ti-6Al-4V parts must satisfy the demand of critical loading conditions. During EBM solidification of Ti-6Al-4V alloy, fcc b-Ti (with Al and V in solution) grows in columnar form and subsequently transforms into hcp a-Ti (Al rich) plus b-Ti (V rich) phases at lower temperatures. Transformation starts by Mona Aziziderouei, Department of Mechanical Engineering, University of Alberta, Edmonto
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