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Research Projects
Current Projects
- Thermal and solidification modeling of resistance spot welds
J. A. Kahn- Mechanics of joints: modeling and analysis
X. Deng- Welding distortion measurement/analysis
M. A. Sutton, Y. J. Chao- Impact strength of joints
Y. J. Chao- Modeling of distortion and residual stress during welding process
Y. J. Chao- Effect of mismatch and residual stress on fracture resistance of welded joints
Y. J. Chao- Welding and weld-bonding for production of built-up structure: aerospace applications
A. P. Reynolds- Comparison of welding processes for production of tailor welded blanks
A. P. Reynolds- Friction stir joining
A. P. Reynolds- NDE techniques for spot-welded and weldbonded structures
V. Giurgiutiu
Department of Mechanical Engineering
College of Engineering
University of South Carolina
Please direct questions or comments regarding the project to Dr. Chao, chao@sc.edu
For comments or suggestions on the web pages, contact the webmaster.
© 1997 Department of Mechanical Engineering. All rights reserved.
Thermal and solidification modeling of resistance spot welds
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
stresses.
 Velocity Field |
 Temperature Profile |
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
pieces).
Publications
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)
Comparison of Welding Processes for Production of Tailor Welded Blanks
Professor A. P. Reynolds
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.
Welding and Weld-Bonding for Production of Built-up Structure: Aerospace Applications
Professor A. P. Reynolds
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.
Mechanics of
joints: modeling and analysis
Finite element simulations of arc and spot welding processes;Static and fatigue failure analyses of welded joints; CAE modeling techniques for spot-welded structures; Damage detection methods and applications to welded joints and structures. |
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| Modeling of distortion and
residual stress during welding process
Professor
Yuh J. Chao Computer code using finite element method is being developed to model the gas metal arc welding process which includes transient thermal analysis, melting process, solidification and mechanics. The code is three-dimensional, transient, and non-linear and runs on a personal computer platform. Weld quality, residual stress and distortion due to welding processes are being analyzed. Current work includes extending the modeling capability, e.g. friction stir welding and laser beam welding, and improving the computational efficiency. Publications Chao, Y.J. and Qi, X. " Three-dimensional Thermal-Mechanical Analysis of Gas Metal Arc Welding Process," submitted to the Twenty-seventh North American Manufacturing Research Conference, May 25-28, 1999, University of California at Berkeley, Berkeley, California. Y. J. Chao and X. Qi, "Heat transfer and Thermo-Mechanical Analysis of Friction Stir Joining of AA 6061-T6 Plates" submitted to International Symposium on Friction Stir Welding, Thousand Oaks, California, 15 &16, June 1999. |
 Thermal modeling of friction stir joining |
NDE Techniques for Spot-Welded And Weldbonded Structures
Professor V. Giurgiutiu
| Unlike most homogeneous solid structures where cracks initiate at the surface, spot-welded and weldbonded joints are subject to cracks initiation and incipient damage propagation starting from internal regions (nugget rim, heat affected zone, adhesive burn-out boundary, etc.). These critical regions are not readily accessible, and stiffness, strength, fracture toughness, crack initiation and crack propagation cannot be measured directly with conventional techniques. In lieu of direct measurement techniques, indirect NDE techniques need to be developed in order to monitor the joint "health" and in-service condition, and to predict its durability and long-term performance. In the first stage, our research is focusing on developing novel NDE techniques for spot-welded and weld-bonded structures utilizing smart sensors technology and piezo-electric transducers, based on the electro-mechanical impedance approach. |
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Schematic representation of the proposed NDE technique using the E/M impedance technique and its comparison with conventional ultrasonic NDE. |
Impact
strength of joints
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| Structural joints are often
used in transportation systems. The performance
of these joints under static and dynamic loads
are crucial to the design of the overall
structure. For instance, the crashworthiness of
an automobile is directly affected by the
strength of the many resistance spot-welded
joints embedded within the auto body. In this
project, strength and performance of structural
joints under static and impact loads are
investigated. Environmental effect on the
performance of joints is also studied. The type
of joints studied includes resistance spot welded
joints, adhesive bonded joints, mechanical
fastener, rivet joints, etc. A pendulum impact
device is developed to study the force and
deformation history as functions of time under
impact load. A gas gun is used to deliver higher
impact energy and speed. Fracture of joints under
impact speeds ranging from low to eighty miles
per hour is being studied. Publications Chao, Y.J., K.W. Miller, and P.C. Wang, " An Investigation of Static and Dynamic Strength of Structural Joints," Proceedings of the Taiwan International Welding Conference on Technology Advancements and New Industrial Applications in Welding, pp. 705-712, September 7-9, 1998, Taiwan, ROC. K. W. Miller, Y.J. Chao, and P. C. Wang, "Performance Comparison of Spot-Welded, Adhesive Bonded and Self-Piercing Riveted Structural Joints," Proceedings of the 5th International Conference on Trends in Welding Research, Callaway Gardens Resort, Pine Mountain, Georgia; June 1-5, 1998. Y.J. Chao, K. Miller and P. C. Wang, "Impact Strength of Spot Welded Joints," Proceedings of the Sheet Metal Welding Conference VIII, American Welding Society, Detroit, Michigan, October 14-16, 1998. K. W. Miller, Y. J. Chao and P.C. Wang, "Impact Strength of Spot-Welded Joints," SPC2, poster session, 1988 American Welding Society International Welding and Fabricating Exposition and Annual Convention, April 26-30, Detroit, Michigan. K. W. Miller and Y.J. Chao, "Development of an Impact Tester for Studying the Dynamic Strength of Structural Joints," 353-354, Proceedings of the SEM Spring Conference on Experimental Mechanics, Bellevue, Washington, June 2-4, 1997. |
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Friction
stir joining Friction stir welding (FSW) is a
new technique currently being developed to join
aluminum (and other low melting materials)
components. FSW has the potential to reduce
manufacturing costs for aluminum structures and
make welding a viable joining method for aluminum
alloys previously considered to be un-weldable
(e.g. 7XXX series precipitation hardening
alloys). FSW is a solid state joining process
with greater similarity to metal cutting or |
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