D.Q. Sun

Effect of tin on melting temperature and microstructure of Ag–Cu–Zn–Sn filler metals

Abstract To develop low melting point filler metals for brazing TiNi shape memory alloy (SMA) and stainless steel (SS), a series of Ag–22Cu–Zn–Sn (wt-%) filler metals have been studied. Using differential thermal analysis (DTA) analysis, the melting temperatures of Ag–22Cu–Zn–Sn filler metals were determined. The results show that the increase of zinc and tin contents drastically decreases the solidus and liquidus temperatures of the Ag–22Cu–Zn–Sn filler metals and the melting temperatures of the Ag–22Cu–18Zn–Sn filler metals with 5–8 wt-%tin are < 650°C. Metallographic observations indicate that the increase of zinc and tin in the Ag–22Cu–Zn–Sn filler metals helps the formation of eutectic structure and inhibits the formation of α-Ag and α-Cu solid solutions, but the increase of tin also causes the formation of Ag3Sn and Cu41Sn11 brittle compounds. The results of mechanical property tests of the laser brazed joints of TiNi SMA and SS show that the proper increase of zinc and tin in Ag–22Cu–Zn–Sn filler metals is favourable for improving the strength of the laser brazed joints of TiNi SMA and SS.

Comparison of microstructures and thermal insulation capability of plasma sprayed nanostructured and traditional YPSZ coatings

Abstract Microstructural features and thermal insulation capability of plasma sprayed nanostructured and traditional yttria partly stabilised zirconia (YPSZ: ZrO2–8Y2O3) coatings were investigated. Both nanostructured YPSZ coating and traditional YPSZ coating were mainly composed of non-transformable t-ZrO2 phases. The detailed microstructures of the nanostructured YPSZ coating were observed using FESEM, which presented three types of microstructures: columnar grains, equiaxed grains and nanosized zirconia particles embedded in the so called matrix formed by melted powders. However, the traditional YPSZ coating only contained columnar and equiaxed grains. The average porosities of nanostructured and traditional YPSZ coatings are 12·5 and 9·7% respectively. Compared with the traditional YPSZ coating, the nanostructured coating contained finer microcracks. The nanostructured YPSZ coating has higher thermal insulation capability than the traditional YPSZ coating. For YPSZ coating of 200 μm in thickness, the temperature drop ΔT (thermal insulation capability) of the nanostructured coating at 1350°C increased by 27% compared with that of the traditional coating.

Microstructures and mechanical properties of tungsten inert gas arc welded magnesium metal matrix composite (TiCp/AZ91D)

Abstract Magnesium metal matrix composite (TiCP/AZ91D) was joined by tungsten inert gas arc welding with addition of filler metal (TiCP/AZ91D). The weld metal (WM) consists of TiC particulates, primary α-Mg and eutectic phases (eutectic α-Mg and eutectic β-Mg17Al12). TiC particulates distribute at primary α-Mg grain boundaries or inside the grains depending on the particulate size. At the grain boundaries, there exist divorced eutectic and lamellar eutectic microstructures due to rapid cooling rates. The pores accumulated are observed in the WM. In the heat affected zone, its microstructures have a coarsening tendency and microcracks form at the grain boundaries. The WM has the tensile strength of 140–190 MPa and the elongation of 1–3%. The tensile strength and elongation of welded joint are 130–160 MPa and 0–1% respectively. The pores accumulated, microcracks and grain coarsening are the main reasons to affect the mechanical properties of WM and welded joint.

Transformation characteristics, microstructure and mechanical properties of austempered ductile iron welds

Abstract The isothermal transformation of austempered ductile iron (ADI) weld at 370°C for a time period ranging from 0·5 to 1440 min involves three stages. The welding process has the effect of accelerating the subsequent transformation during austempering to bainitic ferrite and high carbon austenite (stage 1) and delaying the transformation to carbide (Fe3C) and ferrite (stage 3). Within the austempering time range 15–240 min, the ADI welds consisting of bainitic ferrite and retained austenite had superior properties (namely tensile strengths between 1030 and 1060 MPa and elongations between 8·0 and 8·5%). As the austempering temperature increased from 310 to 400°C, the tensile strength of ADI weld decreased from 1141 to 1040 MPa and its elongation increased from 4·0 to 9·1%. Under tensile loading a large number of slip bands appeared in the retained austenite and the austenite/ferrite interfaces behaved as obstacles limiting the movement of dislocations. The superior properties of ADI weld are mainly associated with the presence of large amount of retained austenite (30–32%), grain boundary strengthening effect and solid solution strengthening effect.