What Post-Processing Techniques are Commonly Applied to SLM Aluminum Alloy Parts?
Standardized Post-Processing Sequence
A standardized sequence of post-processing techniques is essential to transform as-printed Selective Laser Melting (SLM) aluminum parts into components with reliable mechanical properties, dimensional accuracy, and functional surface quality. For common alloys like AlSi10Mg, this process addresses the inherent characteristics of the SLM process—residual stress, support structures, surface roughness, and a non-equilibrium microstructure—to meet the stringent demands of industries such as aerospace and aviation and automotive.
Thermal Treatment: Stress Relief and Aging
Thermal processing is the foundational step to ensure part stability and performance.
Stress Relief Annealing: Performed immediately after the build to mitigate high residual stresses that can cause distortion or cracking. This stabilizes the part for subsequent handling and machining.
Solution Heat Treatment & Aging (T6): A full heat treatment cycle is standard. Solutionizing homogenizes the microstructure and dissolves alloying elements, while subsequent artificial aging precipitates strengthening phases (e.g., Mg₂Si in AlSi10Mg), optimizing the part's tensile strength, hardness, and dimensional stability.
Support Removal and Surface Finishing
These steps address the part's external geometry and aesthetics.
Support Structure Removal: Supports are carefully removed via cutting, clipping, or machining, followed by manual grinding or filing to clean the attachment points.
Surface Improvement: The as-printed surface has high roughness. Techniques like abrasive blasting (bead or sand blasting) and vibratory finishing are commonly used to reduce surface roughness (Ra), remove partially sintered powder, and improve uniform appearance.
Precision CNC Machining: Critical functional surfaces, mating interfaces, and features requiring tight tolerances are finish-machined. This is a non-negotiable step for achieving sealing surfaces, bearing fits, or aerodynamic profiles.
Advanced Treatments for High-Integrity Parts
For components in fatigue-critical or leak-proof applications, further advanced treatments are applied.
Hot Isostatic Pressing (HIP): HIP subjects the part to high temperature and isostatic pressure, effectively eliminating internal micro-porosity. This results in increased fatigue strength, improved ductility, and more isotropic mechanical properties.
Specialized Surface Treatments: Processes like abrasive flow machining (for internal channels), chemical polishing, or anodizing may be applied to enhance surface smoothness, corrosion resistance, or for aesthetic purposes.
Validation and Quality Assurance
The post-processing chain is validated through rigorous inspection.
Dimensional Verification: Using Coordinate Measuring Machines (CMM) or 3D scanners to ensure conformity to the CAD model.
Non-Destructive Testing (NDT): Dye penetrant inspection checks for surface defects. X-ray computed tomography (CT scanning) may be used for internal quality assessment of complex parts.
Mechanical Testing: Coupons from the same build undergo tensile, fatigue, and hardness testing as part of material testing and analysis to certify that the post-processed material meets all specified mechanical property requirements.
