Abstract
Components produced by additive manufacturing (AM) via direct metal laser sintering (DMLS) have typical as-fabricated surface defects. As a result, surface properties of AM products should be modified to increase their strength, anti-wear behaviour, and at the same time to ensure their high corrosion resistance. Surface modification via shot peening is considered suitable for additive manufacturing of medical devices made of 17-4PH stainless steel. The objective of this study was to determine the effect of shot peening pressures (0.3 MPa and 0.6 MPa) and three types of blasting media (CrNi steel shot, nutshell granules and ceramic beads) on the tribological characteristics and corrosion resistance of specimens of DMLS 17-4PH stainless steel. Results demonstrated that shot peening caused steel microstructure refinement and—except for the nutshell shot-peened specimens—induced both martensite (α) formation and retained austenite (γ) reduction. 17-4PH specimens peened with steel and ceramic shots showed the highest increase in surface hardening by approx. ~ 119% (from 247 to 542 HV), which significantly improved their wear resistance. The highest mechanical properties (hardness and wear resistance) and corrosion resistance were obtained for the surfaces modified using the following media: ceramic beads > CrNi steel shot > nutshell granules. Adhesive and fatigue wear were two predominant mechanisms of tribological deterioration. Results demonstrated that the application of shot peening using ceramic beads led to grain size refinement from 22.0 to 14.6 nm and surface roughness reduction, which in turn resulted in higher corrosion resistance of the material. DMLS 17-4PH specimens modified by shot peening using ceramic beads and a pressure of 0.6 MPa exhibited the optimum surface morphology, hardness and microstructure, and thus improved wear and corrosion performance.
Highlights
17-4PH (1.4542, X5CrNiCuNb16-4) is precipitation-hardened martensitic stainless steel, strengthened with finedispersion precipitates of intermetallic phases [1]
The results demonstrate that the use of steel shot and nutshell granules leads to increased surface roughness for both tested peening pressures
An analysis of the surface morphology of the as-fabricated specimens reveals the presence of typical direct metal laser sintering (DMLS) surface features such as unmelted metal powder grains and weld pool collapse along the laser melted track
Summary
17-4PH (1.4542, X5CrNiCuNb16-4) is precipitation-hardened martensitic stainless steel, strengthened with finedispersion precipitates of intermetallic phases [1]. There are many studies investigating the problem of selecting appropriate shot type and shot peening parameters such as blasting pressure to obtain the required surface properties, mechanical and wear behaviour [15], and corrosion resistance [16]. The literature review shows that no study to date has synthesized the effects of shot type (ceramic, organic or steel particles) and peening pressure on the surface roughness, microstructural phase development, hardness, wear and corrosion behaviour of additive manufactured 17-4PH steel. To fill this literature gap, the present study will investigate the above problem
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