Wei Meng

An austenitic stainless steel 316L deposition specimen is fabricated by cold metal transfer‐based swing arc additive manufacturing (CMT‐SAAM), and its formation, microstructure, and tensile properties are investigated. The results showed that the 316L deposited samples exhibited a multi‐layer structure with a small aspect ratio, and the surface forming accuracy is improved with the increase in the swing amplitude. The remelted zone near the interface is dominated by long grains perpendicular to the fusion line, whereas the deposited zone is dominated by columnar dendrites and shows an orientation. Along the building and transverse direction of the swing deposits, the microhardness distribution is uniform. Compared with rollings and forgings, 316L deposited by using the CMT‐SAAM method has both a higher strength and toughness. The average ultimate tensile strength is 573 MPa, and the elongation at break is 52.6%. The high strength and toughness of 316L specimens are attributed to the fine microstructure, high‐density dislocations, and low‐angle grain boundaries (LAGBs) generated during the CMT‐SAAM process.

Cold metal transition wire‐arc deposition of S214 bronze/316 L stainless steel bimetals is investigated, and the effects of heat input and deposited sequence on the interfacial behavior of bimetallic structure are systematically characterized. It is demonstrated that 316 L and S214 bimetals are defect‐free, with no penetrating cracks or porosity, and that satisfactory metallurgical bonding at the interface is obtained. The interface thickness with low heat input is greater than that with high heat input, and the microstructure of the interface has a honeycomb pattern. The interfacial zone presents the highest microhardness due to the formation of Fe‐based solid solutions with high copper content and intermetallic compounds such as AlCrFe₂ and AlNi₃. As increasing heat input, microstructure of S214 deposited layer in the S214/316 L transforms from α‐Cu+γ + Fe‐rich phase to α‐Cu + Fe‐rich phase + K phase, and 316 L deposited layer changes from lath ferrite to skeletal ferrite with the determinations of copper and cooling rate. The tensile strength of S214/316 L specimen is substantially higher than that of 316 L/S214.

Narrow gap welding of 316L stainless steel thick plate was carried out using the cold metal transition pulsed wire-arc swing method. The effects of heat input, arc swing and shielding mode on weld shaping and lack of fusion were investigated, and microstructure and tensile properties of welded joints were analyzed and characterized. The results indicate that arc swing amplitude was the primary factor influencing sidewall fusion. The tendency to develop undercutting and incomplete fusion at the weld toes increased with increasing wire feed rate. Narrow gap welds are prone to porosity with built-in shielding. Microstructure of welded joints was mostly represented by twin crystals in HAZ and columnar dendrites in the weld. Under high heat input, solidification sub-boundaries form between dendritic crystal clusters in the middle of weld, resulting in severe regional segregation and crystalline cracks that extend along sub-grain boundaries. The microhardness of weld and HAZ was greater than that of base material. The tensile strength of welded joints was comparable to that of base material. Defect-free tensile specimens fractured in the base material and welded joints with good mechanical properties were obtained.

Metal matrix ceramic composites (MMCs) are widely applied materials in surface engineering due to their high hardness and excellent wear resistance. Recently, various MMCs have been successfully fabricated by a promising method named direct laser deposition. In this work, nickel-based hard surface coatings reinforced with multiple in-situ phases were deposited by wide-band laser. The strengthened phases were synthesized by varied content of Ti and B₄C precursor powders. The microstructure evolution, phase constitution and mechanical properties of the designed coatings were investigated. Results indicated the B₄C were decomposed and free C and B atoms were released in a molten pool. Multiple secondary phases such as TiC, Cr₇C₃, Cr₂₃C₆, TiB and CrB were in-situ synthesized. As the content of precursor Ti and B₄C powders increased, the microstructure of the laser-clad coatings was greatly refined due to the plentiful in-situ phases. Mechanical properties of the coatings revealed the maximum elastic modulus and microhardness reached 247 and 7.18 GPa in the experiment group. Friction tests indicated the average friction coefficient of optimized coating was about 0.50.