We investigate the process-structure-property relationships for 316L stainless steel prototyping utilizing 3-D laser engineered net shaping (LENS), a commercial direct energy deposition additive manufacturing process. The study concluded that the resultant physical metallurgy of 3-D LENS 316L prototypes is dictated by the interactive metallurgical reactions, during instantaneous powder feeding/melting, molten metal flow and liquid metal solidification. The study also showed 3-D LENS manufacturing is capable of building high strength and ductile 316L prototypes due to its fine cellular spacing from fast solidification cooling, and the well-fused epitaxial interfaces at metal flow trails and interpass boundaries. However, without further LENS process control and optimization, the deposits are vulnerable to localized hardness variation attributed to heterogeneous microstructure, i.e., the interpass heat-affected zone (HAZ) from repetitive thermal heating during successive layer depositions. Most significantly, the current deposits exhibit anisotropic tensile behavior, i.e., lower strain and/or premature interpass delamination parallel to build direction (axial). This anisotropic behavior is attributed to the presence of interpass HAZ, which coexists with flying feedstock inclusions and porosity from incomplete molten metal fusion. The current observations and findings contribute to the scientific basis for future process control and optimization necessary for material property control and defect mitigation.

The length-scale effects on the load bearing capacity of reinforcement particles in an ultrafine-grained metal matrix composite (MMC) were studied, paying particular attention to the nanoscale effects. We observed that the nanoparticles provide the MMCs with a higher strength but a lower stiffness compared to equivalent materials reinforced with submicron particles. The reduction in stiffness is attributed to ineffective load transfer of the local stresses to the small and equiaxed nanoparticles.

•Volumetric Energy Density (VED) affects track shape, values lower than 100 J/mm3 are insufficient to fully melt the alloy.•Surprisingly, under some conditions, tracks deposited with sufficiently high VED values still had an undesirable morphology.•VED fails to capture melt pool physics, hence it poorly predicts both melting condition and track morphology.Energy density is often used as a metric to compare components manufactured with Selective Laser Melting (SLM) under different sets of deposition parameters (e.g., laser power, scan speed, layer thickness, etc.). We present a brief review of the current literature on additive manufacturing of 316L stainless steel (SS) related to input parameter scaling relations. From previously published work we identified a range of Volumetric Energy Density (VED) values that should lead to deposition of fully dense parts. In order to corroborate these data, we designed a series of experiments to investigate the reliability of VED as a design parameter by comparing single tracks of 316L SS deposited with variable deposition parameters. Our results show the suitability of VED as a design parameter to describe SLM to be limited to a narrow band of applicability, which is attributed to the inability of this parameter to capture the complex physics of the melt pool. Caution should be exercised when using VED as a design parameter for SLM.Download high-res image (335KB)Download full-size image

•High speed imaging captures formation of morphological defects resulting from the interaction between powder and melt pool.•Metal powder is observed to melt before interacting with the beam: measuring laser absorptivity of liquid phase is crucial.•Cooling rates of the solidification front are in the ~ 106 K/s range and show an inverse dependence on the power/speed ratio.•Physicochemical differences between powders did not significantly affect morphology and thermal history of the melt pool.Detailed understanding of the complex melt pool physics plays a vital role in predicting optimal processing regimes in laser powder bed fusion additive manufacturing. In this work, we use high framerate video recording of Selective Laser Melting (SLM) to provide useful insight on the laser-powder interaction and melt pool evolution of 316 L powder layers, while also serving as a novel instrument to quantify cooling rates of the melt pool. The experiment was performed using two powder types – one gas- and one water-atomized – to further clarify how morphological and chemical differences between these two feedstock materials influence the laser melting process. Finally, experimentally determined cooling rates are compared with values obtained through computer simulation, and the relationship between cooling rate and grain cell size is compared with data previously published in the literature.Download high-res image (273KB)Download full-size image

•Nano, submicron, and micron-sized B4C enhances Al-B4C composite hardness.•Nano B4C enhances abrasive wear resistance of Al-B4C composites.•Submicron and micron B4C are subject to pull-out.•Nano B4C forms coherent interfaces within Al grains that inhibit pull-out.The use of ceramic nanoparticle reinforcements has shown significant promise for enhancing the mechanical properties of metal matrix composites due to the high specific surface area and superior intrinsic mechanical properties of nanoparticles. In this study, the effect of B4C reinforcement particle size on the abrasive wear behavior of Al-B4C composites was investigated. Composites with a homogenous dispersion of micrometric-B4C, submicron-B4C, and nano-B4C in a nanostructured Al alloy 5083 (AA5083) matrix were fabricated using cryogenic mechanical alloying and dual mode dynamic forging. Hardness was seen to increase with decreasing B4C reinforcement size, with the Al-nanoB4C composite exhibiting a 56% enhancement over unreinforced AA5083. The abrasive wear resistance of the Al-nanoB4C composite was 7% higher than the unreinforced AA5083. The other Al-B4C composites exhibited equivalent or reduced abrasive wear resistance as compared to AA5083. Analysis of the abrasive wear scars demonstrated that larger B4C reinforcements are prone to particle pull-out, thereby negating the benefit of higher hardness. The Al-nanoB4C composite has superior wear resistance due its high hardness and greater interfacial area, which hindered pull-out of nano-B4C particles.Download high-res image (314KB)Download full-size image

The mechanical response of heterophase interfaces has attracted substantial attention in recent years. Here, we utilized an in situ transmission electron microscopy (TEM) technique to isolate an individual nanoscale ceramic/metal interface and characterize its nanomechanical response. The interface, at which there was a Mg-rich segregation nanolayer between the single crystal ceramic (B4C) and the polycrystalline metal (Al alloy, AA5083), was determined to have a bond strength greater than 1.5 GPa. Bimodal failure and metallic grain rotation occurred in the metallic region, allowing the interface to accommodate a deformation strain of 5.4%. The roles of elemental segregation and nanoscale dimensions on interfacial debonding mechanisms are discussed.

Nanostructured Al 5083-based composites with nano-TiB2 reinforcement particles were fabricated via cryomilling and spark plasma sintering (SPS). TEM observation revealed that the Al matrix consists of equiaxed nano-grains (average size, ∼74 nm), and the reinforcement, TiB2 nanoparticles (n-TiB2), was distributed discretely and homogeneously in the Al matrix. The interface between the Al-matrix and n-TiB2 appears to be free of defects, and no obvious discontinuities were observed. The composite exhibits a compressive strength of 817 MPa with 6.0% strain-to-failure. The strength is 20% higher than that of an equivalent SPS consolidated Al 5083 without reinforcement. Nanoindentation was used in our study to provide fundamental insight into the local microscopic mechanical properties. The strengthening mechanisms of the composites are analyzed taking into account the grain boundaries, the Orowan strengthening from the n-TiB2 particles and dispersoids such as Al2O3, AlN and Al6Mn, as well as geometrically necessary dislocations induced in the matrix by the nano-TiB2 particles.

Metal additive manufacturing (AM) is commonly promoted as a sustainable technology because of its capability to produce engineering components of complex geometries in a single step. Prior studies on environmental impact assessment of metal AM have underestimated the resource consumption due to the overestimation of the material utilization factor and neglecting the energy consumption needed to produce the desirable feedstock powder. This paper aims to address the role of feedstock powder and the actual material utilization factor in environmental impact assessment of laser metal deposition. Recycling and reuse of the unfused powder as the feedstock for subsequent depositions are proposed.

•WC-Co/nanodiamond coatings were successfully fabricated via thermal spray methods.•Addition of nanodiamonds enhanced wear resistance at room temperature.•Nanodiamond phase is degraded during 300 °C sliding wear tests.•Composite coatings exhibit reduced wear resistance during 300 °C wear tests.This study investigates the effects of nanodiamonds (ND) on the wear behavior of WC-Co coatings during dry sliding under ambient and elevated temperature environments. The nanometric dimensions and exceptional hardness of ND are envisioned to enhance hardness while maintaining toughness, thereby enhancing wear resistance. ND reinforced WC-Co coatings were successfully fabricated by high velocity oxygen fuel spray (HVOF) and air plasma spraying (APS). The tribological behavior of WC-Co-ND composite coatings was evaluated at room temperature and at 300 °C using reciprocating dry sliding wear tests. At room temperature, the addition of ND led to an enhancement in wear resistance of 8.5% and 13% in HVOF and APS coatings, respectively. The composite coatings exhibited increased formation of a protective silica tribolayer, which was attributed to enhanced heat transfer induced by the excellent thermal conductivity of diamond. At 300 °C, however, the composite coatings exhibited poorer wear resistance than the counterpart WC-Co coatings as a result of the degradation of the ND phase. The loss of the diamond phase was believed to decrease hardness and weaken splat interfaces, which led to more facile delamination in HVOF coatings, as well as severe brittle wear and fracture in APS coatings.