Professor Jamil A. Khan
The spot welding process involves the interaction of electrical, thermal, mechanical and metallurgical phenomena. During the spot welding process, the materials to be joined are brought together under pressure by a pair of electrodes and electrical current, typically 30 to 75 kA for aluminum alloys, is passed through the work pieces. A portion of each work piece is melted in the region between the electrodes by the heat generated at the faying surface. The work pieces are then joined as the weld pool solidifies through conduction of heat to the electrodes. Accurate prediction of the microstructure within the HAZ and of the residual stress field in the weld requires that a physically sound thermal-melting/solidification model be developed and experimentally verified.
Although resistance spot welding has been extensively
studied, nearly all efforts have focused on steel plates
and its application to the automotive industry. The
special aluminum alloys employed in air travel exhibit
differing properties that affect the spot welding process
and the resulting welded joint. Aluminum alloys
typically have much higher thermal and electrical
conductivities, varying oxide layers (influencing the
faying surfaces), and metallurgical properties which
result in drastically different melting and
solidification rates, nugget sizes, microstructure, grain
growth, heat affected zone, and consequently, residual
A three-dimensional finite element weld nugget growth model employing coupled thermal-electrical-mechanical analysis of resistance spot welding is presented. The welding parameters considered include heat generation, heat transfer coefficient at the faying surface and the workpiece-electrode surface;, and Joule heating at the workpiece and the electrode; and the thermal contact conductance between the elerctrode and the workpiece.. The latent heat of phase change due to melting is accounted for. The effect of friction coefficient on pressure at thethe workpiece interface is also studied. The computed results agree well with the experimental data. Heat generation at the faying surface in the early stages of welding dominates the nugget development, and Joule heating at long times governs the weld-nugget growth. A parametric study is done for the nugget growth with specific consideration of resistance spot welding of Al-Alloys.
The objective of this research is also to further
develop and enhance an existing control volume finite
difference model which predicts melting for pure
aluminum. The first phase of study involves
simulation of weld pool solidification. After the
model for the pure metal has been successfully developed
and verified, a model for alloys with species
transportation will be developed. This model will
predict transient temperature history, melting and
solidification rates, nugget size, and micro-segregation
of species during resistance welding as a function of
process variables. The thermal model will be used
to predict the weld microstructure so that the structural
performance of spot welded joints can be estimated using
the properties of individual joint components (work
Xu, L.and Khan, J.A., "The Finite Element Modeling of Axisymmetric Nugget Development during Resistance Spot Welding", Proceedings of the 5th International Conference on Trends in Welding Research, Callaway Gardens Resort, Pine Mountain, Georgia; June 1-5, 1998
Khan, J.A., Xu, L. and Chao, Y.J., "Prediction of Nugget Development during Resistance Spot Welding Using A Coupled Thermal-Electrical-Mechanical Model", submitted to Science and Technology of Welding and Joining (1998 accepted)
Xu, L. and Khan, J.A., "Nugget Growth Model for
Al-alloys during Resistance Spot Welding", submitted
to Welding Journal (1998)
Many steel body parts start out as "tailor welded blanks"; that is, two sheets of differing thickness are welded together and then the resulting assembly is formed into a single part (e.g. a door panel) in one step. The resulting structure provides weight advantages over a part formed from a single thickness. In order to economically produce auto bodies from aluminum alloy, it must be demonstrated that tailor welded blanks made from aluminum alloys possess adequate mechanical properties, formability, and corrosion resistance. In addition, manufacturers of tailor welded blanks will want to use the best available welding process for this application.
Currently, we are studying the relative merits of tungsten gas arc welding (TGA) and friction stir welding (FSW) for the production of aluminum alloy tailor welded blanks. We will compare the formability, the corrosion behavior, and the fatigue resistance of tailor welded blanks made from 5XXX and 6XXX aluminum alloys. In addition, we will attempt to derive constitutive data for the microstructural constituents of the weld region by correlating nominal applied stresses with high resolution, full field strain measurements and residual stress measurements.
The majority of commercial and military aircraft are manufactured using stiffened skin/stringer design concepts. This is known as built-up structure. In the conventional production of built-up structure, joining of skin to stringers is accomplished by mechanical fastening. Disadvantages of mechanical fasteners include the expense of the fasteners, their weight, a multi-step assembly process, and the required presence of numerous stress concentrations in the form of rivet holes. If a welding process could be qualified to substitute for mechanical fastening, several advantages could be realized. Weight, part count and cost could be reduced; however, aerospace structure is generally made from high strength, precipitation hardened aluminum alloys which are normally considered to be poor candidates for welding due to the deleterious microstructural changes which accompany the welding process.
In this project we will examine the suitability of resistance spot welding, weld-bonding, and friction stir welding for production of skin stringer assemblies to be used in aerospace structure. Damage mechanisms for shear and tension loading will be determined during both fatigue and monotonic loading conditions. In addition, the effect of adhesive cure cycles on the mechanical properties of the weld bonded joints will be investigated.