Post-processing of additively manufactured metals

Abstract

In this thesis, the most common metal AM methods are covered. The covered methods are directed energy deposition, powder bed fusion, binder jetting and nozzle-based processes for metal-polymer composites. The additive manufacturing process forms a part by building material up layer-by-layer and joining specific areas either by sintering or melting metal powder together, or by joining metallic particles with adhesive binding agent. Alternatively, metal wire can be used instead of powder in wire-and-arc manufacturing, which is a directed energy deposition method. Post-processing methods available for AM parts are explored and evaluated with emphasis on increasing material properties, especially material hardness, strength, ductility, and fatigue life. Post-processing methods covered in this thesis cover surface polishing and finishing methods, such as thermal and mechanical surface treatments. Heat treatments, that improve mechanical properties of the material are also discussed, as well as residual stress relief heat treatment and densification processes used for powder AM. Due to different manufacturing principle, AM parts have microstructures and defects that are different from those manufactured with conventional manufacturing processes. Porosity, lack-of-fusion pores, and unmelted particles are defects that affect especially AM parts. By reviewing literature, it is apparent that surface polishing and finishing methods can be used to increase part fatigue life by reducing surface roughness of the part. Heat treatments can be further used to improve fatigue life by altering the amount, size, and morphology of internal defects, and by increasing material strength and hardness with heat treatments, such as nitriding and precipitation hardening. Residual stresses caused to the part by rapid cooling of the melt pool can be relieved with stress relief heat treatment or annealing, which both can be used to increase material ductility. The research literature suggests that post-processing may be used to increase part fatigue life and mechanical properties of AM part.