metals Article Microwave Sintering of Ti6Al4V: Optimization of Processing Parameters for Maximal Tensile Strength and Minimal Pore Size Dilpreet Singh 1 , Abhishek Rana 1 , Pawan Sharma 2 , Pulak Mohan Pandey 2 and Dinesh Kalyanasundaram 1,3, * 1 Centre for Biomedical Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India; dilpreet.iitdelhi@gmail.com (D.S.); Abhishek.rana130@gmail.com (A.R.) 2 Department of Mechanical Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India; scholarpawansharma@gmail.com(P.S.); pmpandey@mech.iitd.ac.in (P.M.P.) 3 Department of Biomedical Engineering, All India Institute of Medical Sciences, New Delhi 110016, India * Correspondence: dineshk.iitdelhi@gmail.com or dineshk@cbme.iitd.ac.in; Tel.: +91-11-26597344 Received: 23 November 2018; Accepted: 13 December 2018; Published: 19 December 2018   Abstract: Pressureless sintering is a powder metallurgical process wherein the powder particles are sintered without the aid of any compressive force. Though this additive manufacturing process is economical, the strength of the component is undermined due to the presence of pores; the elimination of which is a challenge. In this work, the optimal process parameters for the pressureless microwave sintering of a grade 5 titanium alloy that yields higher tensile strength and minimum sizes of pores were obtained. The three process parameters (sintering temperature, heating rate, and holding time) were experimented at five different levels using the design of experiments (DOE). Post sintering, the tensile strength was assessed as per ASTM standard B925-15, while the pore size was evaluated, non-destructively, using micro-computed tomography (μ-CT). The optimal process parameters that yielded minimum size pores were: sintering temperature—1293 ◦ C, heating rate—6.65 ◦ C/min; and holding time—72 min. Keywords: microwave sintering; Ti6Al4V; tensile strength; pore; computed tomography; RSM; additive manufacturing 1. Introduction Titanium alloy Ti6Al4V, a highly-preferred material in aerospace and biomedical industries due to its mechanical and biocompatibility properties, suffers from poor machinability. Given the higher cost of titanium, the use of subtractive processes is not an economical approach due to the material wastage [1]. Additive processes produce near net-shape components and can replicate complex shapes with simple tooling. Among additive processes, metal sintering is an efficient method for producing complex-shaped components of titanium and titanium alloys, wherein the powder is heated below the melting point (~80% Tm) with or without the application of mechanical pressure [2,3]. The pressureless microwave-based sintering of metal is a rapid process and requires a shorter thermal treatment time as compared against the conventional sintering processes [4]. However, in the pressureless microwave sintering, the mechanical properties of the components are affected by the presence of pores that are inevitable. Various research groups have evaluated the mechanical properties in the additive manufactured components [5–7]. Kumar et al. [6] observed that ductility decreases with the increase of pore size in conventional sintering of the Ti6Al4V alloy. On the contrary, strain localization and early crack initiation were also observed as a result of increased pore size. Further, the stress concentration Metals 2018, 8, 1086; doi:10.3390/met8121086 www.mdpi.com/journal/metals