Effect of multiple laser peening on surface integrity and microstructure of laser additive manufactured Ti6Al4V titanium alloy

• Published in: Rapid prototyping journal 2019-09, Vol.25 (8), p.1379-1387
• Main Authors: Hu, Yongxiang, Lai, Mengqi, Hu, Zonghao, Yao, Zhenqiang
• Format: Article
• Language: English
• Subjects: Additive manufacturing; Alloying additive; Anisotropy; Cold; Compressive properties; Construction; Crystal defects; Dislocations; Electron backscatter diffraction; Evolution; Fatigue strength; Grain boundaries; Integrity; Investigations; Laser deposition; Lasers; Mechanical properties; Metal fatigue; Microhardness; Microstructure; Microtexture; Misalignment; Peening; Porosity; Post-production processing; Process controls; Rapid prototyping; Residual stress; Strain hardening; Stress concentration; Stress state; Surface properties; Surface roughness; Surface treatment; Titanium alloys; Titanium base alloys
• ISSN: 1355-2546; 1758-7670
• DOI: 10.1108/RPJ-09-2018-0250

Purpose Laser additive manufacturing is widely utilized to fabricate the Ti6Al4V alloy, but it requires post-processing to improve its performance. This paper aims to propose laser peening (LP) as an effective way to improve the surface characteristics of the Ti6Al4V alloy fabricated by direct laser deposition (DLD). Design/methodology/approach Surface integrity including surface roughness, porosity, residual stress and microhardness are investigated in detail before and after LP treatment. Microstructure evolution is characterized by the electron backscatter diffraction (EBSD) to analyze crystal phase, grain boundary misorientation and texture. Findings Multiple overlapping layers of LP treatment result in slight influence on the polished surface of DLD-built samples. Porosity measured by the Archimedes test is found to be greatly decreased after LP treatment. Compressive residual stresses are significantly induced, the magnitude of which is greatly increased by increasing layers of LP treatment. And, local weakening or enhancement of residual stress in depth is observed because of pore and inclusion defects in the DLD-built Ti6Al4V alloy. Favorable hardness property can be obtained after multiple overlapping layers of LP treatment. EBSD analysis shows that LP treatment with multiple layers can introduce a large amount of lower-angle boundaries, indicating that dislocations beneath the top surface could induce a strain-hardened layer. The microtexture of the DLD-built Ti6Al4V alloy cannot be eliminated to decrease the anisotropy of the mechanical property. Research limitations/implications The variation of porosity observed after LP inside the DLD-built Ti-Al-4V is attractive but requires more detailed work to analyze the evolution of pore geometry. Practical implications Surface treatment of an additive manufactured titanium alloy was carried out to improve its fatigue resistance. Originality/value This work is original in proposing LP as an effective post process for the surface treatment of an additive manufactured titanium alloy through analyzing the surface integrity and microstructure evolution.