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Straight polarity direct current method(DCSP A-TIG) was applied to join 2219 high strength aluminum alloy, and the effects of single-component(AlF3,LiF), three-component (AlF3+30%LiF+10%KF-AlF3) and four-component (AlF3+30%LiF+10%KF-AlF3+10%K2SiF6) activating flux on weld face forming, weld quality (porosity), arc shape, weld penetration, joint microstructure and mechanical properties were studied. The results show that adding activating flux helps to remove the oxide film on the weld face of the 2219 aluminum alloy, improve the weld surface forming quality; the four-component activating flux of weld face forming is the best; compared with the weld quality of variable polarity TIG welding (VPTIG), DCSP A-TIG welding method significantly reduces the porosity generation in 2219 aluminum alloy weld; AlF3 single-component activating flux obviously increases the weld penetration, which has obvious dragged arc phenomenon; DCSP A-TIG welded seam has the same structure component as the parent metal. Welding current has a greater influence on DCSP A-TIG weld microstructure, increasing current may result in the coarsening of the joint microstructure. The strength and elongation of the DCSP A-TIG welding joint, which are coated with four-component activating flux are the highest, and the mechanical properties are nearly the same as VPTIG welding. The DCSP A -TIG welding method of 2219 high strength aluminum alloy is of great value to the engineering application.

Zn-11%Al, Zn-11%Al-1.5%Mg, Zn-11%Al-3%Mg and Zn-11%Al-4.5%Mg alloy coatings were obtained by dipping pure iron in the alloy bath at 510℃ for different immersion time. The effects of Mg content on the solidification structure of the Zn-11% Al coating and the growth of the Fe-AI intermetallic layer in the alloy coating were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry. The results show that the solidification microstructure of the Zn-11% Al alloy coating is composed of Al-rich phase and Zn/Al ternary eutectic. With the increase of Mg content in the Zn-11% Al-x% Mg alloy, Zn/Al/MgZn2 ternary eutectic, blocky MgZn2 phase and AL/MgZn2 binary eutectic occur gradually. The intermetallic layers in the Zn-11% Al alloy coatings with different amounts of Mg consist of Fe2Al5Znx phase and FeAl3Znx phase, and the thickness of the inter-metallic layer increases with the prolongation of immersion time. The increase of Mg content in the hot dip bath reduces the growth-rate index and the growth rate of the intermetallic layer. At the beginning of the hot dip process, a compact and stable Fe-Al alloy layer in the Zn-11% Al alloy coating form on the iron surface. After immersing for 120 s, the Fe-Al intermetallic layer is instable and broken because of diffusion path movement. Mg element in the Zn-11% Al-x% Mg alloy can significantly delay the time of liquid zinc into the Fe-Al alloy layer, which makes the Fe-Al alloy layer be more stable and compact.

Fe/(Zn-6%Al-%Mg) solid-liquid diffusion couples were kept at various temperatures for different periods of time to investigate the formation and growth of the Fe-Al alloy layer. Scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD) were used to study the constituents and morphology of the Fe-Al alloy layer. It was found that the Fe2Al5Zn x phase layer forms close to the iron sheet and the FeAl3Zn x phase layer forms near the side of the melted Zn-6%Al-3%Mg in diffusion couples. When the Fe/(Zn-6%Al-3%Mg) diffusion couple is kept at 510C for more than 15min, a continuous Fe-Al alloy layer is formed on the interface of the diffusion couple. Among all Fe/(Zn-6%Al-%Mg) solid-liquid diffusion couples, the Fe-Al alloy layer on the interface of the Fe/(Zn-6% Al-3% Mg) diffusion couple is the thinnest. The Fe-Al alloy layer forms only when the diffusion temperature is above 475. These results show that the Fe-Al alloy layer in Fe/(Zn-6%Al-%Mg) solid-liquid diffusion couples is composed of Fe2Al5Zn x and FeAl3Zn x phase layers. Increasing the diffusing temperature and time period would promote the formation and growth of the Fe-Al alloy layer. When the Mg content in the Fe/(Zn-6%Al-%Mg) diffusion couples is 3%, the growth of the Fe-Al alloy layer is inhibited. These results may explain why there is no obvious Fe-Al alloy layer formed on the interface of steel with a Zn-6%Al-3%Mg coating. [ABSTRACT FROM AUTHOR]