Xinhua Tang

Porosity sensitivity of A356 Al alloy during fiber laser welding

Abstract In order to decrease the metallurgical porosity and keyhole-induced porosity during deep penetration laser welding of Al and its alloys, and increase the mechanical properties of work-piece, the effects of welding parameters such as laser power, welding speed and defocusing value on both kinds of porosities were systemically analyzed respectively, and the shape and fluctuation of plume of the keyhole were observed to reflect the stability of the keyhole. The results show that increasing laser power or decreasing laser spot size can lead to the rising of both number and occupied area of pores in the weld; meanwhile, the plume fluctuates violently over the keyhole, which is always companied with the intense metallic vapor, liquid metal spatter and collapsing in the keyhole, thus more pores are generated in the weld. The porosity in the weld reaches the minimum at welding velocity of 2.0 m/min when laser power is 5 kW and defocusing value is 0.

Evolution behavior of TiB2 particles during laser welding on aluminum metal matrix composites reinforced with particles

Abstract The evolution behavior of TiB 2 particles was studied during laser welding on aluminum metal matrix composites (MMCs) reinforced with TiB 2 particles by a high power laser. X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy spectrum analysis (EDS) were used to analyze the phase, thermodynamics and morphology characteristics of the particles in the weld seam, and the interface reactions between TiB 2 particles and aluminum matrix were also discussed. The results show that partial TiB 2 particles melt and merge into larger ones in the middle of the weld seam and that fracture can be found in larger particles when the scale of TiB 2 cluster is larger than the spot size of laser. Al 3 Ti and AlB 12 are produced by the interface reaction between TiB 2 particles and Al matrix. The interface reaction of broken TiB 2 particles in the middle of the weld seam is stronger than that at the edge of the weld seam.

Migration behavior of solidification nuclei in pure Al melt under effect of electric current pulse

Abstract A mathematical model considering free nuclei was developed to reveal the migration behavior of the free nuclei. Numerical simulation results show that most of the nuclei on the top surface of the melt move downwards and distribute randomly inside the Al melt, which induces more nucleation sites resulting in grain refinement. At the same time, the effect of nuclei size on the nuclei distribution and refinement employing electric current pulse (ECP) was also investigated. The smaller nuclei migrate a short distance with the Al melt at lower speed. But for the larger nuclei, the migration downwards with higher speed benefits the refinement of interior grains of the melt. The research results help to better understand the refinement process and provide a more reasonable explanation of the grain refinement mechanism using ECP.

Mechanical properties and wear resistance of aluminum composite welded by electron beam

The welding property of TiB2/ZL101 composite was investigated using electron beam (EB) welding experimental system with a function generator. The fine defect-free welding seam was obtained under proper processing parameters and scanning rate. The reinforcement particles TiB2 distributed homogeneously in welding seam without any segregation. The tensile results show that fracture occurs at the base metal and elastic modulus increases compared with base metal. Wear resistance of welding seam is improved greatly compared with base metal. The results show that the TiB2/ZL101 composite can be successfully welded by EB technology.

Investigation on thermal inertia of GMAW-P welding on Al alloy

Abstract Thermal inertia relates to the rate at which the plasma cools and heats, which plays important role in improving the arc stability at base time of pulsed gas metal arc welding (GMAW-P). A three-dimensional (3D) transient model is established to investigate the thermal inertia due to current rapid variation of GMAW-P. The model is validated by comparison of measured and predicted arc profile and voltage. Thermal inertia increases as the current decent rate improves at the same current difference. Two high thermal inertia zones exist in arc region, one locates right under the wire tip, and another is above molten pool surface. The arc region with high temperature is broadened due to thermal inertia compared to constant current, which is more obvious as the current decent rate increases. The results show that thermal inertia remarkably influences the arc temperature and stability while current decent rate is higher than 130 A ms−1.

Experimental and numerical analysis of solidification cracking behaviour in fibre laser welding of 6013 aluminium alloy

Abstract Experimental observation and numerical modelling were employed to investigate the solidification cracking behaviour during fibre laser welding of 6013 aluminium alloy. The solidification cracking initiation location and propagation path were studied using a high speed camera system and via metallurgical analysis. A three-dimensional thermomechanical finite element model of fibre laser welding of aluminium alloys was developed, which considered cylindrical volumetric heat source, temperature dependent material properties, solidification shrinkage and stress relaxation in the weld molten pool. The transient evolution and distribution of mechanical strain in the brittle temperature range (BTR) were analysed in detail to find the factors which drove the crack initiation and propagation. The results showed that the solidification cracking initiated near the fusion line and then propagated along the centreline of the weld, which was the result of the strain distribution characteristic in BTR.

Investigation of molten pool oscillation during GMAW-P process based on a 3D model

In order to better reveal the oscillation mechanism of the pulsed gas metal arc welding (GMAW-P) process due to an alternately varied welding current, arc plasma and molten pool oscillation were simulated through a self-consistent three-dimensional model. Based on an experimental analysis of the dynamic variation of the arc plasma and molten pool captured by a high-speed camera, the model was validated by comparison of the measured and predicted results. The calculated results showed that arc pressure was the key factor causing the molten pool to oscillate. The variation in arc size and temperature from peak time to base time resulted in a great difference in the heat input and arc pressure acting on the molten pool. The surface deformation of the molten pool due to the varying degrees of arc pressure induced alternate displacement and backflow in the molten metal. The periodic iteration of deeper and shallower surface deformation, drain and backflow of molten metal caused the molten pool to oscillate at a certain frequency. In this condition, the arc pressure at the peak time is more than six times higher than that at the base time, and the maximum surface depression is 1.4 mm and 0.6 mm, respectively, for peak time and base time.

Effects of shielding gas composition on arc profile and molten pool dynamics in gas metal arc welding of steels

In gas metal arc welding, gases of different compositions are used to produce an arc plasma, which heats and melts the workpiece. They also protect the workpiece from the influence of the air during the welding process. This paper models gas metal arc welding (GMAW) processes using an in-house simulation code. It investigates the effects of the gas composition on the temperature distribution in the arc and on the molten pool dynamics in gas metal arc welding of steels. Pure argon, pure CO2 and different mixtures of argon and CO2 are considered in the study. The model is validated by comparing the calculated weld profiles with physical weld measurements. The numerical calculations reveal that gas composition greatly affects the arc temperature profile, heat transfer to the workpiece, and consequently the weld dimension. As the CO2 content in the shielding gas increases, a more constricted arc plasma with higher energy density is generated as a result of the increased current density in the arc centre and increased Lorentz force. The calculation also shows that the heat transferred from the arc to the workpiece increases with increasing CO2 content, resulting in a wider and deeper weld pool and decreased reinforcement height.

Welded seam 3D calculation and seam location for welding robot system

Abstract Using laser scanning techniques, scanning lens rotating around a laser diode and a circular laser trajectory was projected onto the surface of a weldment to detect it. Furthermore, one novel vision sensor based on the circular laser is developed. The three-dimensional (3D), circular laser based seam location sensor is investigated. On the basis of the light path system, 3D calculation algorithm is brought forward and used to locate the welded joint. After image denoise, filter, segmentation and thinning, characteristic points of the welded joint could be detected real time using a proposed vision sensor and confirmed by real experiments of butt welded joints with I groove and V groove, and lap, and fillet and ramp welded joints. The results show that circular laser based vision sensor can be used in seam locating and its calculation precision meets the requirement of seam tracking.

Spatial distribution characteristics of plasma plume on attenuation of laser radiation under subatmospheric pressure

The attenuation of a laser by plasma plume can be restrained for laser welding under subatmospheric pressure. Based on the experimental obtained spectra, the extinctions of a probe laser under different subatmospheric pressures and the spatial distribution of probe laser extinction were measured. The role of subatmospheric pressure on plasma plume was analyzed. The results show that, with decreasing ambient pressure, the extinction of a probe laser decreases, and the welding penetration depth increases. The maximum attainable value of extinction gets far away from the keyhole in the transversal and vertical directions. The attenuation of a fiber laser is about 10% under normal atmosphere, and it reaches only about 1% when the ambient pressure is reduced to 3 kPa.