Microstructural analysis of additively manufactured Ti–6Al–4V subjected to duplex surface treatment

In this research, the impact of an innovative duplex surface treatment on the surface characteristics of additively manufactured Ti–6Al–4V was investigated. This duplex approach encompasses two distinct stages; the material is initially subjected to mechanical shot peening, followed by the application of a ceramic multilayer coating (consisting of Ti, TiN, TiAlN and TiAlCuN) through physical vapour deposition. The comprehensive analysis delves into the influence of the shot peening procedure, employing advanced techniques such as X-ray diffraction stress measurements, profile hardness assessments, and electron backscatter diffraction. The mechanical shot peening treatment induced a hardened surface layer, approximately 150 μm thick. This transformation was accompanied by the generation of compressive residual stresses, detected up to depths of 150 μm from the surface. Notably, the most substantial compressive residual stress, measuring 770 MPa, is located at a depth of approximately 27 μm beneath the surface. The existence of these stresses is further substantiated by average misorientation measurements of the cross-sections. The duplex treatment led to a remarkable advancement in the material's microhardness, exhibiting an increase of approximately 210% when compared to the untreated sample. Additionally, the ceramic coating itself demonstrates outstanding mechanical properties, with a nanohardness of 26 GPa, and an elasticity index (H/E) of 0.08. Furthermore, when subjected to scratch tests, the duplex-treated specimens exhibited enhanced durability attributed to the concurrent rise in surface roughness induced by the peening process.