D. Liu

New technique of self-refilling friction stir welding to repair keyhole

Abstract A new technique of self-refilling friction stir welding (SRFSW) relying on non-consumable joining tool has been developed to repair the keyhole left at the end of 316L stainless steel friction stir welding/friction stir processing (FSW/FSP) seam. The conventional FSW process was transformed by adopting a series of non-consumable tools with gradual change in geometry to create a solid state refilled joint step by step. Using the combined plastic deformation and flow of the material around the keyhole, the SRFSW process is able to repair the keyhole with both metallurgical and mechanical bonding characteristics, and the FSW/FSP seam can be achieved without keyhole or other obvious macro defects. Microstructural observation results showed that the grains in the refilled zone were significantly refined by the tool. Tensile test results showed the refilled joint fractured at the base metal side, and the relative tensile strength and elongation are 112 and 82% of the base metal respectively. Moreover, no sigma phase but few Cr carbides were found in the refilled zone, which would not result in obvious corrosion resistance degradation of 316L stainless steel.

Effects of Mo, Ti and B on Microstructure and Mechanical Properties of Underwater Wet Welding Joints

Alloy components are designed and transferred into weld metal via electrode covering to address the deterioration of the microstructure and mechanical properties of underwater wet welds. Emphasis is placed on studying the effects that the Mo, Ti and B contents have on the microstructure, tensile strength and low-temperature toughness of the underwater wet welding joint. The as-welded metal obtained at a water depth of 3xa0m is analyzed. The results indicate that the addition of Mo depresses the formation of coarse pro-eutectoid ferrite. However, a higher Mo content (0.609xa0wt.%) results in the formation of lath-like bainite and martensite, which are harmful to the plasticity and toughness of the weld. Acicular ferrite nucleation increases with the combined addition of Ti and B. The deposited metal with the optimum alloy components ratio achieves good plasticity and toughness while maintaining its tensile strength of 592xa0MPa, with the impact toughness at 0xa0°C and elongation reaching 53.34xa0J and 16.2%, respectively.

Microstructure and mechanical properties of dissimilar welds between 16Mn and 304L in underwater wet welding

ABSTRACT Dissimilar joint between 304L austenitic stainless steel and low-alloy steel 16Mn was underwater wet welded using self-shielded nickel-based tubular wire. Microstructure, mechanical properties and corrosion behaviour of dissimilar welded joints were discussed. Ni-based weld metal was fully austenitic with well-developed columnar sub-grains. Type II boundary existed between Ni-based weld metal and ferritic base metal in underwater welds similar to that in air welds. Major alloying elements distributed non-uniformly across the austenitic weld metal/16Mn interface. Maximum hardness values in wet welding appeared in a coarse-grained heat-affected zone at the 16Mn side, which possessed very low impact toughness. Underwater Ni-based welded joints fractured at Ni-based weld metal under tensile test. Ni-based weld metal had favourable corrosion resistance similar to 304L base metal.