What Post-Processing Steps Are Necessary for SLM-Printed Ti-6Al-4V Parts?

Mandatory Thermal Treatment for Stress Relief and Microstructure

The first and most critical step for SLM-printed Ti-6Al-4V parts is thermal treatment to address the metastable, acicular martensitic (α') microstructure resulting from rapid cooling. A stress relief anneal is performed immediately after the build to prevent distortion or cracking. This is typically followed by a Hot Isostatic Pressing cycle. Hot Isostatic Pressing (HIP) is crucial for Ti-6Al-4V as it simultaneously eliminates internal porosity and transforms the brittle α' martensite into a more ductile and stable equilibrium α+β lamellar microstructure. This dramatically improves fatigue life, fracture toughness, and ductility, bringing the material properties in line with, or even exceeding, wrought standards.

Support Removal and Surface Finishing

Following thermal processing, the rough removal of support structures is conducted. This is often done via wire EDM or careful band saw cutting. The as-built surface of SLM parts is characterized by adhered powder particles and high surface roughness, which act as stress concentrators. Therefore, abrasive processes like vibratory finishing, shot peening, or blasting are used to clean and smooth general surfaces. For critical fatigue-prone areas, more aggressive techniques like CNC machining or grinding are essential to achieve a superior surface finish and remove the outer layer where subsurface defects may be present.

Precision Machining for Critical Features and Tolerances

Despite the high dimensional accuracy of SLM, functional surfaces like sealing faces, threading holes, and mating interfaces require precision machining to meet final tolerances and achieve the necessary surface integrity. This superalloy CNC machining is always performed *after* HIP and primary heat treatment. This sequence ensures that any minor distortions from thermal processing are corrected and that machining is done on the part's final, stable microstructure, guaranteeing long-term dimensional stability under load.

Final Validation and Cosmetic Enhancement

The last stage involves validation and final finishing. Components undergo rigorous material testing and analysis, which for Ti-6Al-4V often includes dye penetrant inspection to verify surface integrity and ultrasonic testing to confirm internal soundness post-HIP. For medical implants or consumer-facing components in the aerospace sector, electropolishing is a common final step. This electrochemical process not only provides a smooth, mirror-like finish but also passivates the surface, enhancing the titanium's natural corrosion resistance for applications in demanding environments.