Abstract
This research study investigated the hybrid processing of 316L stainless steel using laser powder bed (LPB) processing with high-speed machining in the same build envelope. Benchmarking at four laser powers (160 W, 240 W, 320 W, and 380 W) was undertaken by building additively with machining passes integrated sequentially after every ten deposited layers, followed by the final finishing of select surfaces. The final geometry was inspected against the computer-aided design (CAD) model and showed deviations smaller than 280 µm for the as-built and machined surfaces, which demonstrate the good efficacy of hybrid processing for the net-shape manufacturing of stainless steel products. The arithmetic average roughness values for the printed surfaces, Ra (linear) and Sa (surface), were 11.4 um and 14.9 um, respectively. On the other hand, the vertical and horizontal machined surfaces had considerably lower roughness, with Ra and Sa values ranging between 0.33 µm and 0.70 µm. The 160 W coupon contained layered, interconnected lack of fusion defects which affected the density (7.84 g·cm−3), yield strength (494 MPa), ultimate tensile strength (604 MPa), Young’s modulus (175 GPa), and elongation at break (17.3%). By contrast, at higher laser powers, near-full density was obtained for the 240 W (7.96 g·cm−3), 320 W (7.94 g·cm−3), and 380 W (7.92 g·cm−3) conditions. This, combined with the isolated nature of the small pores, led to the tensile properties surpassing the requirements stipulated in ASTM F3184—16 for 316L stainless steel.
Highlights
The lower accuracy of the as-built surfaces concurs with findings from thermal simulations that have pointed to the key laser powder bed (LPB) process parameters (P, ν, hL, height spacing (hS) ) affecting the part geometry [44]
The evolution in the density and porosity with increasing laser power clearly indicated that the 160 W (Edensity = 38.1 J·mm−3 ) condition was insufficient for achieving near-full density in 316 L stainless steel (316LSS), and the impact on the mechanical performance was evaluated through tensile testing and fractographic analysis
The current study examined the characteristics and properties of 316LSS additively manufactured using laser powder bed (LPB) processing in-envelope with micro-machining
Summary
The review by Baja et al [11] on the microstructure and properties of steels processed by AM covered the research and challenges over the last decade to develop process windows for 316LSS using LPB technologies. The A/SM of 316LSS brings new challenges, including the lack of know-how on the efficacy of dry micro-milling for the finish-machining of surfaces, the allowable engineering design properties, and the material–process–structure–property (MPSP) interrelationships. Additively built 316LSS using a hybrid machine but undertook out-of-envelope milling to study the effectiveness of energy consumption as a process signature that could be correlated with the microhardness. The present study was devised to explore hybrid manufacturing of 316LSS to identify a robust process window for A/SM through an evaluation of the powder characteristics, part distortion, surface finish, density, porosity features using X-ray micro-computed tomography (μCT), macro/microstructure, bulk hardness, microhardness, and tensile properties. A/SM process was conducted on a Matsuura LUMEX Avance-25 system
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