Microstructure and high temperature oxidation resistance of in-situ synthesized TiN/Ti3Al intermetallic composite coatings on Ti6Al4V alloy by laser cladding process

In this paper, 12CrNi2 alloy steel samples were successfully fabricated by direct laser deposition (DLD) technology. The evolution of bainite in the DLD 12CrNi2 alloy steel process at different laser power of 1800 W, 2000 W and 2200 W and mechanical properties of as-deposited samples were investigated. The results showed that the middle-upper microstructure of the samples fabricated at different laser power transformed from lath bainite (LB) to granular bainite (GB) with increasing laser power. In addition, granular bainite was divided into blocky granular bainite (GB1) and lath-like granular bainite (GB2) according to different formation mechanisms, and the amount of GB1 increased and GB2 decreased in the range of laser power from 2000 W to 2200 W. No preferred texture was observed in the EBSD maps due to the complex heat flux direction which was resulted in the reciprocating scanning strategy. With the increase of laser power, the proportion of high-angle grain boundaries increased from 33.1% to 46.4% and that of low-angle grain boundaries decreased from 66.9% to 53.6%. The sample fabricated at 2000 W had the highest mean microhardness (331 HV0.2) and the best combination of ultimate tensile strength (757 MPa) and elongation (9.1%). However, the sample fabricated at 2200 W had the best impact toughness (aku = 100.0 J/cm2) because it contained a large amount of GB1 with dispersive and spherical-like island structures. This study provides the theoretical and experimental basis for the design of laser power and controllability of microstructure and properties in the DLD alloy steel process.

The evolution of bainite and mechanical properties of direct laser deposition 12CrNi2 alloy steel at different laser power

To construct reliable heterojunctions between metals and polymer matrix composites, friction self-riveting welding (FSRW) was proposed, characterized by multi-scale mechanical interlocking and adhesive bonding, to improve mechanical properties. Holes and porous oxide structures were pre-fabricated on the metal sheet. High-quality heterogeneous joints were achieved. Under the frictional heat and forging force, the composites were softened and flowed into the pre-fabricated holes and porous structures via squeezing behaviors from a welding tool, improving macro/micro-mechanical interlocking. The oxide layers also enhanced adhesive bonding. The maximum tensile shear strength reached 27 MPa based on multi-scale mechanical interlocking and adhesive bonding. Joint fracture located at the interfaces between composites and metals, the composites rivets and the intermixing region of fibers. This strategy has the potential to construct reliable heterojunctions between polymers or polymer matrix composites and metals in the fields of aerospace and automotive.

Friction self-riveting welding between polymer matrix composites and metals

The composite coating reinforced with TiC and Al2O3 ceramic particles were fabricated on the Cr12MoV steel by laser cladding. The cross-section, microstructure, phase, microhardness and wear resistance of the composite coating were investigated by the optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), Vickers microhardness tester and dry sliding wear testing machine, respectively. An obvious crack occurs in the coating when the content of the ceramic reinforcement is 15%. Composite coatings have good metallurgical bonding with the substrate when the content of the ceramic is less 15%. The phases of the composite coatings are composed of solid solutions (Ni–Cr–Fe and γ-(Fe,Ni)), chromic carbides (Cr3C2 and Cr2C), ferric carbide (Fe3C) and ceramics (TiC and Al2O3). The composite ceramics that Ti-rich alloy compound was coating unfused Al2O3 and TiC ceramics respectively were generated in the coating. A part of Ti element and C element, dissolved from the original TiC ceramic, precipitate from the matrix to form new TiC ceramic. These kinds of composite ceramic particles significantly improved the wear resistance of the coating via enhancing the bonding strength between the ceramic particles and the matrix.

The effect of TiC/Al2O3 composite ceramic reinforcement on tribological behavior of laser cladding Ni60 alloys coatings

Process optimization to prepare T-joints of poly methyl methacrylate (PMMA) and AA5754 alloy by friction stir welding (FSW) was performed. Effects of processing parameters including tool rotational velocity, traverse speed, tilt angle and plunge depth (TP) were studied. The material flow and formation of interaction layer upon FSW were evaluated and the tensile strength and hardness of the joints were examined. It is shown that by controlling the heat input into stir zone through increasing the rotational velocity or decreasing travelling speed, less defects are formed while the thickness of the interaction layer increases. The optimum condition is attained at 1600 rpm/30 mm min−1 with a tool tilt angle of 2° and plunge depth of 0.2 mm. Under this condition, the joint strength would reach ~71% (50 MPa) of the base polymer. It has been found that the thickness of the interaction layer and formation of internal voids have a remarkable influence on the strength and hardness of the T-joints. The presented results could be useful for development of dissimilar FSW of metal/polymer joints.

An investigation on the dissimilar friction stir welding of T-joints between AA5754 aluminum alloy and poly(methyl methacrylate)

Resistance spot-welded galvanized ultrahigh-strength steels are sensitive to liquid metal embrittlement (LME), which is manifested by surface cracks on the joints. LME occurs when a solid metal contacts a liquid metal under tensile stress, and the phenomenon has not been fully understood until now, especially for resistance spot welding. In this study, the susceptibility of hot-dipped galvanized Q&P980 steel to LME cracking during resistance spot welding was systematically investigated by an orthogonal experiment. Cracks were detected by fluorescent magnetic particle testing and cross-sectional microscopic observation. Cracks were mostly located at the indentation edge and slope, and a few cracks were also located at the indentation center and slope periphery. The severity of the cracking increased with the increasing welding current and welding time, and the decreasing electrode force. The sequence of influence degree from high to low was welding current > electrode force > welding time. Holding time had no obvious effect. Microstructural analysis revealed that the content of martensite in areas with cracking increased, indicating that a high temperature was experienced at these locations. Zinc accumulated inside the cracks, and the cracks were intergranular, which coincides with the characteristics of LME. The LME cracking was provoked by the simultaneous occurrence of a tensile stress, an appropriate temperature and liquid metal, and the cracks were influenced by the welding parameters. Under suitable conditions, Zn diffused along the grain boundaries and weakened them, resulting in LME cracking.

Min Wang

Papers

Liquid Metal Embrittlement Cracking During Resistance Spot Welding of Galvanized Q&P980 Steel

Towards an explanation of liquid metal embrittlement cracking in resistance spot welding of dissimilar steels

Microstructure evolution of Fe-based nanostructured bainite coating by laser cladding

Friction spot welding between porous TC4 titanium alloy and ultra high molecular weight polyethylene

Effects of isothermal heat treatment on nanostructured bainite morphology and microstructures in laser cladded coatings