What are the most common post-processing used for stainless steel parts produced by SLM?
Stress Relief and Heat Treatment
The most critical and common post-processing step for Stainless Steel parts produced by Selective Laser Melting (SLM) is stress-relief annealing or full heat treatment. The rapid heating and cooling cycles of the SLM process lock in significant residual stresses, which can lead to part distortion or reduced fatigue life. A controlled heat treatment cycle relieves these internal stresses, stabilizes the microstructure, and can be tailored to optimize mechanical properties, such as enhancing the strength and corrosion resistance of grades like 316L or precipitating hardening in 17-4 PH.
Support Removal and Surface Finishing
Following heat treatment, the removal of support structures is mandatory. This is typically done via manual breaking, cutting, or machining. The as-built SLM surface, characterized by partially melted powder particles and layer lines, is often unsuitable for functional applications. Common surface finishing techniques include:
Machining: CNC machining is used to achieve tight tolerances on critical functional surfaces and interfaces.
Abrasive Finishing: Processes like vibratory finishing, bead blasting, or stream finishing are employed to reduce surface roughness (Ra), remove adhering powder, and improve aesthetics.
Advanced Polishing: For applications requiring ultra-smooth surfaces, such as in the pharmaceutical and food industries, electrochemical polishing or manual polishing may be applied.
Densification and Integrity Improvement
For parts subjected to high pressure, cyclic loading, or critical structural roles, Hot Isostatic Pressing (HIP) is a vital post-process. HIP applies high temperature and isostatic gas pressure to the component, effectively closing internal microscopic pores and voids that are inherent to the SLM process. This results in near-theoretical density, significantly improved fatigue strength, fracture toughness, and more isotropic mechanical properties, which is essential for demanding sectors like aerospace and aviation or nuclear.
Final Treatments and Validation
The final post-processing steps ensure functional readiness and compliance. For stainless steels, passivation is frequently used to remove free iron particles from the surface and enhance the natural chromium oxide layer, restoring maximum corrosion resistance. Finally, comprehensive inspection and material testing and analysis are conducted. This includes dimensional checks, dye penetrant inspection for surface defects, and mechanical testing to validate that the post-processed part meets all specified performance criteria for its intended application.
