Papers by Author: Li Han

Abstract: A study was carried out to investigate the effect of governing metal thickness (GMT) on weld quality and strength of resistance spot welded (RSW) AA5754 aluminium. Quasi-static joint strengths were evaluated for 27 different joint stack-ups in three test geometries: lap-shear, coachpeel and cross-tension; whilst micro examination was conducted on some of the samples to assess weld quality. The results derived from over 1000 samples show the importance of GMT and its various effects: the GMT has a significant effect on welding quality and joint strength by controlling the attainable weld diameter, regardless of stack-ups; depending on loading conditions, its effect may differ.

Abstract: A pilot study has been carried out to examine the effect of specimen dimensions on the obtained tensile properties of aluminium and steel sheet. The materials used were DP600 grade steel and AA5754 grade aluminium sheet. Four types of dog-bone samples with varying dimensions were tested for both materials. Standard tensile test procedures were performed using a universal test machine together with contacting extensometry. The GOM Aramis photogrammetric 2D strain mapping technique was also applied. The results suggest that for both steel and aluminium sheet materials, differing specimen dimensions have little effect on the obtained mechanical properties. Depending on the gauge length of extensometer chosen and the position at where necking occurred on the sample, the extensometry results and 2D strain mapping results slightly differed towards to the end of stress-strain curve. The failure mode between the chosen grades of steel and aluminium samples was observed to differ, as did the percentage of failures that occurred within the gauge length. All steel samples fractured across the specimen perpendicularly to the test direction; whilst fracture of aluminium samples occurred approximately 30 degrees from the perpendicular.

Abstract: The increased application of lightweight materials, such as aluminium has triggered many investigations into new joining techniques for aluminium alloys. The Resistance Spot Welding concept for aluminium has always attracted many researchers from different organizations. Self-piercing riveting is the major production process used to join aluminium sheet body structures for the automotive industry. Mean while, interest in laser welding and application as a candidate for joining aluminium is also growing. These key technologies for aluminium assembly are therefore being investigated by the research team at the University Of Warwick. The paper reported here looks at the feasibility of each process for joining thin gauge aluminium and compares the mechanical behavior for each joint type. The results suggested that all three joining technologies are feasible for joining aluminum and their mechanical behaviour is strongly dependent on the selection of process parameters

Abstract: Self-piercing riveting as an alternative joining method to spot-welding has attracted considerable interest from the automotive industry and has been widely used in aluminium intensive vehicles. Pressing and stamping are important processes in automotive production and result in additional straining on the vehicle body sheet material. It is therefore important to have knowledge of the effect of sheet pre-straining on the quality of the self-piercing riveted joints and on the mechanical behaviour of the riveted aluminium alloy sheets. This paper reports the influence of sheet pre-straining on the static and fatigue behaviour of self-piercing riveted aluminium alloy sheet. Wrought aluminium alloy sheet, NG5754 with a nominal thickness value of 2mm was used to obtain pre-strained NG5754 sheets with pre-straining levels of 3%, 5% and 10%. The pre-strained NG5754 sheets were then joined in pair to create single riveted lap joints. Lap-shear and fatigue tests were performed on the self-piercing riveted joints. Microscopic inspection showed that the joint quality was satisfactory despite the increasing sheet straining levels. The results showed that by increasing the pre-straining level up to 10%, the shear and fatigue strength also increased. The rate of increase of the static and fatigue strength differed as the pre-straining levels varied.