Abstract
This paper extends our previous work to investigate the effect of heat treatment on the microstructure of Ti-6Al-4V fabricated by selective laser melting. A post-heat treatment at 930 °C for 15 min followed by three cooling rates before and after hot isostatic pressing (HIP) treatment was applied. The findings illustrated that the microstructure of the quenched samples before the HIP treatment was characterized by a mixture of α + α′ phase with a microhardness value of 336 ± 6 HV0.3. Air cooling produced a structure dominated by the α phase, with ~ 7.5% of the β phase and a microhardness value of about 330 ± 4 HV0.3. Furnace cooling led to a mixture of α phase and ~ 17% of the β phase and hardness of 327 ± 6 HV0.3. After HIP followed by post-heat treatment, acicular α′ martensite with microhardness value 377 ± 2 HV0.3 dominated the quenched specimen microstructure. Following air cooling, the microstructure consisted of a mixture of α-lamella and β with some needles of the α with a microhardness value of 336 ± 3 HV0.3. In the case of the furnace cooling, a complete transformation of β to a mixture of α + β phase was observed. The β volume fraction formed in the microstructure was estimated at ~ 8.5%, having microhardness 322 ± 4 HV0.3. Reasons for such behaviors are discussed.
Highlights
The aerospace and medical sectors are constantly developing new designs to achieve weight reduction and higher productivity, performance and safety requirements
The SR + hot isostatic pressing (HIP) specimens, before applying any further solution treatment, demonstrated coarser grains composed of a mixture of a with a typical lath width size of about [2,3,4,5] lm and $ 6% b lamellar microstructure, with a reduction of volume fraction of porosity from 0.38 ± 0.24% to 0.20 ± 0.09 and a microhardness value of 346 ± 8 HV0.3
The microstructural examination of the stress-relieved sample followed by solution treatment at 930 °C indicated that the quenched samples were characterized by a mixture of a and a¢ phases identified as needles with size 1.8-80 lm in length and 0.3-2.4 lm in width, with a microhardness value of 336 ± 6 HV0.3
Summary
The aerospace and medical sectors are constantly developing new designs to achieve weight reduction and higher productivity, performance and safety requirements. Many studies on the mechanical properties like hardness, tensile, and fatigue of SLM samples before and after solution heat treatment have been carried out by different authors (Ref [12, 20, 21]). Hot isostatic pressing (HIP) treatment was applied on as-built SLM samples in numerous studies to improve the fatigue property of the SLM part and reduce porosity (Ref [22, 23]). In order to generate different microstructures, the samples underwent heat treatment at 930, 45 °C below the b-transus temperature This process lasted for 15 min, after which they were cooled at different rates, including furnace cooling (FC), air cooling (AC) and water quenching (WQ).
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