Xueming Hua

Relationship between the weld pool convection and metallurgical and mechanical properties in hybrid welding for butt joint of 10-mm-thick aluminum alloy plate

The ALHW and LAHW processes are applied to join 5083 aluminum alloy with thickness 10xa0mm; the differences of weld pool convection is studied, and the relationship between the weld pool convection and microstructural characterization of aluminum alloy in ALHW and LAHW processes are analyzed. The result shows that an inward flow pattern, furious mingling, and stirring effects in LAHW process will contribute to a better weld bead with lower porosity level. The shorter escaping route of the bubble, more difficult bubble captured, and better stability of the keyhole are the three effective factors for eliminating of porosity. In LAHW process, the grain size is larger, but the LAHW joints have higher tensile strength, which is greatly influenced by porosity and second-phase particles. Compared with ALHW process, the LAHW process is beneficial to get the less weld defects, better bead formation, and higher tensile strength, which is conducive to successfully weld aluminum alloy thick plates potentially.

Effect of Al-Si Coating on Weld Microstructure and Properties of 22MnB5 Steel Joints for Hot Stamping

Abstract22MnB5 hot stamping steels are gradually being used in tailor-welded blank applications. In this experiment, 1-mm-thick Al-Si coated and de-coated 22MnB5 steels were laser-welded and then hot-stamped. The chemical compositions, solidification process, microstructure and mechanical properties were investigated to reveal the effect of Al-Si coating and heat treatment. In the welded condition, the coated joints had an Al content of approximately 2.5xa0wt.% in the fusion zone and the de-coated joints had 0.5xa0wt.% Al. The aluminum promoted the δ-ferrite formation as the skeletal structure during solidification. In the high-aluminum weld, the microstructure consisted of martensite and long and band-like δ-ferrite. Meanwhile, the low-aluminum weld was full of lath martensite. After the hot stamping process, the δ-ferrite fraction increased from 10 to 24% in the coated joints and the lath martensite became finer in the de-coated joints. The tensile strengths of the coated joints or de-coated joints were similar to that before hot stamping, but the strength of the coated joints was reduced heavily after hot stamping compared to the de-coated joints and base material. The effect of δ-ferrite on the tensile properties became stronger when the fusion zone was soft and deformed first in the hot-stamped specimens. The coated weld showed a brittle fracture surface with many cleavage planes, and the de-coated weld showed a ductile fracture surface with many dimples in hot-stamped conditions.

Using the multi-wire GMAW processes for controlling the formation of humping

Three-dimensional numerical models are established to investigate the convection in multi-wire GMAW processes. A high-speed photography system is used to capture the transient images of the weld pools. Based on the simulation and experimental results, the humping formation and suppression mechanisms are discussed. The results show that both the “push-pull” and outward flow patterns exist in the multi-wire GMAW weld pools, which help decrease the momentum of the backward fluid flow and widen the weld width. In high-speed GMAW process, there are three main factors that cause the formation of humping: the high momentum of the backward fluid flow, the capillary instability, and the large variation of the capillary pressure of the liquid channel in the welding direction. In twin-wire GMAW process, the first two factors are suppressed, humping is disappeared, but the variation of the capillary pressure of the liquid channel in the welding direction is still large, which causes the weld width uneven. All of those three factors are suppressed in triple-wire GMAW process, so sound weld bead can be obtained; the distance between the middle wire and trailing wire has little influence on the weld bead formation.

Effect of keyhole mode on weld shape and mechanical properties of thin sheets of Al-Si coated 22MnB5 steel

The great change of plasma or plumes and weld pool flows existed between “blind” keyhole and open keyhole, especially for thin sheets. The keyhole mode could be changed by increasing the power density for deeper keyhole penetration. In this study, weld shape, aluminum content, δ ferrite fraction, and tensile strength were investigated in laser welding Al-Si coated steel. “Blind” keyhole led to “Y” weld shape, lower aluminum content, less δ ferrite, and higher strength at the optimized parameter. On the contrary, open keyhole led to “X” weld shape, higher aluminum content, more δ ferrites, and lower strength. The microstructure was more homogeneous in low speed “Y” weld. From the high speed images, the open keyhole was accompanied with fierce molten flow on the bottom side. The bottom coating was also important in the aluminum increase. In the suitable combination of welding parameters for “blind” keyhole, their strength could reach the level of decoated sample with no need for decoating preparation.The great change of plasma or plumes and weld pool flows existed between “blind” keyhole and open keyhole, especially for thin sheets. The keyhole mode could be changed by increasing the power density for deeper keyhole penetration. In this study, weld shape, aluminum content, δ ferrite fraction, and tensile strength were investigated in laser welding Al-Si coated steel. “Blind” keyhole led to “Y” weld shape, lower aluminum content, less δ ferrite, and higher strength at the optimized parameter. On the contrary, open keyhole led to “X” weld shape, higher aluminum content, more δ ferrites, and lower strength. The microstructure was more homogeneous in low speed “Y” weld. From the high speed images, the open keyhole was accompanied with fierce molten flow on the bottom side. The bottom coating was also important in the aluminum increase. In the suitable combination of welding parameters for “blind” keyhole, their strength could reach the level of decoated sample with no need for decoating preparation.

A study on laser weldability and formability of high thickness ratio tailor welded blanks

Cold-rolled steel sheets of thickness ranging from 0.7 (with zinc coating) to 2.0 mm were used to produce tailor laser welded blanks (TLWBs) with various thickness ratios, the maximum of which reaches 2.86:1. In this study, the laser weldability and formability of the TLWBs, as well as the mechanical characteristics of the weld zones, were analyzed experimentally under the effects of various thickness ratios of TWLBs. The results showed that the hardness of the weld beads are 1.9 times higher than respective parent metal at most but the formability of TLWBs as evaluated by the Erichsen cup test was lower than that of the parent material.