Abstract
The butt welding tests of 5.5mm thick pure nickel N6 plate were carried out by plasma arc welding (PAW) and ERNi-1wire-filled PAW. And the differences in microstructure and properties between two kinds of joints were studied. The results show that the wire-filled pure nickel N6 joint has good appearance without defects such as pores, cracks, and undercuts. Pure nickel N6 weld is single-phase austenite structure, and no phase transformation occurs during heating or cooling. The weld zone of PAW joint is characterized by coarse columnar grain with average grain size of 125.8 μm, while the weld zone of wire-filled joint is equiaxed grain with average grain size of 54.8 μm. The tensile strength of the wire-filled PAW joint is 337MPa and the elongation after fracture is 44%, which is 11.6% and 85.5% higher than that of the PAW joint. The corrosion rate of wire-filled PAW joint is 2.6908g/m2h, which is lower than that of PAW joint of2.8820g/m2h, and the corrosion rates of both joint are higher than the base metal. The alloy elements in the welding wire can supplement the alloy elements burned in the welding process. As the filler metal, the melted wire forms the joint with the melted base metal, avoiding the defects such as undercut and depression. Al and Ti elements in ERNi-1 wire have deoxidation and nitrogen fixation effect during welding thermal cycle process, which improves the resistance of weld to carbon monoxide and nitrogen porosity. In the process of nitrogen fixation, TiN particles are formed in the crystal or grain boundary. TiN particles at the grain becomes heterogeneous nucleation particles in the grain, which makes a large number of nucleation and increases the nucleation rate. TiN particles at grain boundaries act as "nails" to hinder the migration of austenite grain boundaries. In the deoxidization process, the deoxidized products Al2O3 and TiO2 are formed. However, large-size inclusions are formed owing to the high content of Al and Ti in the molten pool, which affects the performance of the joint. Therefore, the structure and performance of filler wire-filled PAW joint are better than those of PAW joint.
Highlights
Pure nickel has good strength, plasticity, ductility and magnetic conductivity
The shape of wire-filled plasma arc welding (PAW) joint is more beautiful with the surface being continuous, smooth and even with fish scale ripple
The improvement of the appearance of wire-filled PAW joint is closely related to the filling of ERNi-1 wire in the welding process, which will be elaborated in the following part of the paper
Summary
Pure nickel has good strength, plasticity, ductility and magnetic conductivity. It has good machinability and special electrical, magnetic and thermal expansion properties with high chemical stability under cold and hot conditions[1]. Pure nickel is one of the most corrosion-resistant and good oxidation-resistance metals in heavy non-ferrous metals, especially for caustic soda with the corrosion rate being less than 25μm/year in 50% boiling caustic soda solution[2,3]. Pure nickel N6 is one of the most widely used materials in industry, which is widely used in chemical industry, mechanical electronics, food and other fields. Pure nickel N6 has small solid-liquid temperature range, poor fluidity, sensitivity to dissolve H2 and O2 to form pore in liquid state, and high resistivity and low thermal conductivity. It is easy to form defects such as thermal cracks, pore and rapid grain growth caused by welding superheat during welding process, which seriously
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