Recently, additive/subtractive hybrid manufacturing has arisen to harness the merits of both additive manufacturing and traditional subtractive manufacturing. In this study, a novel hybrid machine tool that combines selective laser melting (SLM) and high-speed dry milling is employed to produce 316L stainless steel widely used in modern industries to produce shells, values, pipes, feeders, wristwatches, connectors, etc. Sliding wear properties are important for 316L stainless steel products in certain applications because they may have a vital impact on the service life of these products. This study investigates the effects of SLM/milling-based additive/subtractive hybrid manufacturing processing parameters on the relative density, hardness, surface roughness, coefficient of friction (CoF), and wear rate of 316L stainless steel. In addition, it analyzes the mechanisms of dry sliding wear of 316L stainless steel produced by additive/subtractive hybrid manufacturing.

Gas-atomized 316L stainless steel powder is used as the feedstock (Fig. 1), and samples are produced by hybrid manufacturing with the hybrid machine tool (Fig. 2), which have varied laser power, scan speed, resultant laser energy density ranging from 112.5 J·mm^-3 to 183.3 J·mm^-3, and feed per tooth ranging from 0.02 mm to 0.08 mm. For analysis and comparison, other samples are produced by the SLM method with the same additive processing parameters as those in hybrid manufacturing. Cast 316L stainless steel samples are produced by a machining center with the same milling parameters as those in hybrid manufacturing. Then, the relative density of the samples with varied SLM processing parameters is tested through Archimedes' method. The hardness of the samples is tested by a microhardness tester. A 3D profiler is used to test the surface roughness. Defects in microstructures and the surface morphology of the samples are investigated under an optical microscope to relate the characteristics of the samples to the processing parameters. Dry sliding wear tests are performed at room temperature by counter balls of Si₃N₄ to obtain the CoF of the samples. The wear rate and wear track morphology are explored under the 3D profiler and a scanning electron microscope (SEM). The element composition of the worn surfaces is analyzed by energy-dispersive X-ray spectrometry (EDS).

The relative density and hardness of the samples produced by SLM are in the ranges of 93.8%-99.2% (Fig. 4) and 232.3-283.6 HV_0.2 (Fig. 6), respectively. For E=150 J·mm^-3, the highest relative density and maximum hardness are obtained, and the number of pores and cracks in the microstructures is the smallest. Pores or cracks (Fig. 5) may result in a lower hardness value (Fig. 7). Defects, differences in re-melt times, and grain orientations-caused variables result in hardness tests. The grain size of the cast samples is much larger than that of the samples produced by SLM, and the hardness is about 208 HV_0.2. The surface roughness after milling is much better than that after SLM (Table 4), and there is some debris on the surface caused by the dry milling process. With varied, the CoF and wear rate of the polished SLM samples are in the ranges of 0.93-1.03 and 5.02×10^-8-7.51×10^-8 mm³·mm^-1, respectively, and they both obtain their minimum values when the highest density is obtained (Fig. 10). In the first 15 min of sliding, higher surface roughness causes a more significant fluctuation in CoF for the SLM samples. The total wear rate of the surface produced by hybrid manufacturing is lower than that of the surface produced by SLM, and it decreases as the feed per tooth declines. The milled surface of cast 316L stainless steel shows a slightly higher CoF, while with the same feed per tooth, it shows a slightly lower wear rate than that of SLM-manufactured 316L stainless steel. At the beginning of sliding, abrasive grooves occur on the processed surfaces, and other types of wear follow as the sliding goes on (Fig. 11). Abrasive wear and fractures that result in small craters are the elemental wear mechanisms on SLM-processed surface, while adhesive wear is mild. On surfaces processed by hybrid manufacturing, abrasive wear occurs near the edges of the wear track, while debris adhesion and surface flattening occur in the center of the wear track. On the milled surfaces of cast 316L stainless steel, abrasive wear and fractures govern material loss. Elastic deformation at the edges can be observed on the wear tracks of the milled surfaces, while on the SLM-processed surface, burrs are difficult to grow. After 30 min sliding, the main wear mechanisms for samples produced by SLM are abrasive wear and adhesive wear (Fig. 12). Pores in these samples play a vital role in material loss because they cause fractures during sliding, and they affect the CoF and wear rate (Fig. 13) in several ways. The cast 316L stainless steel shows similar wear mechanisms except that it exhibits more craters than the SLM-built 316L stainless steel, though no obvious pores are observed near the fractures. The processes of dry sliding in the air of 316L stainless steel, SLM-built and cast, are accompanied by prominent oxidation in places where adhesion occurs, which can be attributed to the severe plastic deformation of the adhered debris (Fig. 14).

A hybrid process has an advantage over a sole additive process in manufacturing parts with sliding wear resistance. A surface manufactured in a hybrid manner is comparable to a milled surface of cast 316L stainless steel in sliding wear resistance despite the fact that there are differences in their sliding wear mechanisms. In additive/subtractive hybrid manufacturing, the additive process parameters affect the wear characteristics of 316L stainless steel by regulating the number of defects in the microstructures, and a decrease in defects in the microstructures suggests an improvement in the dry sliding wear resistance. By modifying the state of the initial surface of 316L stainless steel after the additive process, the subtractive process improves the sliding wear resistance at the early stage of sliding, and a decreasing feed per tooth can enhance the dry sliding wear resistance.