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Computer
Aided
Welding
Analysis
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CAWA |
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Computer
Aided
Welding
Analysis
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CAWA -Questions that frequently arise subsequent to welding may be categorized into two areas: thermal related problems and fracture related problems.
(i) Thermal Related Problems
The thermal cycles produced during welding cause physical and metallurgical changes in the weldment, transient thermal stress and metal movements, and result in the creation of residual stress and distortion in the finished product. In addition, during the cooling cycle, discontinuities may form because of excessively rapid weld solidification.
The weld pool created by the heat source during welding has a direct impact on the welding result. Various monitoring hardware, such as vision systems, have been developed to observe the change of the weld pool during welding. Nevertheless, the relationship between the observed weld pool condition and the resulting quality of the solidified weld is not well understood.
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The use of mainframe computers in the modeling of heat and fluid flow in the pool and the consequences of the thermal flows on the weldment have been the basis for major research efforts by numerous researchers. Precise control of heat input is very critical in welding heat or dimensional sensitive materials.
Pulsing welding current provides two additional control variables, peak current and its duration, for heat input control. Welding power supplies, using a microcomputer for synergic control and process synchronization, are the highest priority in the developmental effort of many welding equipment manufacturers and represent the new technological area in welding.
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The correlation between control parameters and welding results is not readily available. An understanding of the thermal responses of various materials to these control variables, must be obtained before the new technology can make a real impact on welding productivity. Computer aided thermal analysis of welding processes is a solution to this need and is imminent.
In the area of welding metallurgy, weld solidification structures and microstructures in the heat affected zone (HAZ) are direct consequences of welding thermal flows during welding. Service performances such as impact strength and fatigue life of the weldment are the end results of this welding thermal process and are of great concern for welding engineers. More frequently, computer aided analysis provides the answer.
Residual stresses in weldment and weldment distortion are important physical nonhomogeneities in the welded structure and the source of great concern. The most effective way to minimize these problems is to understand the formation mechanisms and to prevent them in the design stage.
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Undesirable thermal conditions and the resulting inelastic strains in the weldment during welding must be avoided through proper control of the welding process and proper joint design. These can be achieved by computer aided welding analysis.
(ii) Fracture Related Problems
A "perfect" joint may not be desirable, nor .practical. Even if all the necessary technology is available to us, the time and cost required to obtain a perfect joint may be unfeasible. The more important question should be asked: Does this welded product fit its intended purpose? If the answer is yes, the least expensive design and fabrication must be the most acceptable solution to the industry.
To address the need for a service evaluation of the welded product, a better understanding of the product itself is necessary. This is to identify the fracture related problems due to the physical and metallurgical nonhomogeneities in the weldment. The effect of residual stresses and weldment distortion on the fracture behaviour of the welded joints is one area that needs to be determined.
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Cracking or embrittlement of the weldment must be evaluated to assess the effect on the service life of the product. Are physical discontinuities existing in the weldment weld defects? Their significance on the fracture behaviour of the welded product must also be assessed.
All of these questions need to be answered in order to use the byproducts from welding technology and to create a more fruitful manufacturing environment. Computer aided welding analysis has been the foundation for recent technological developments in this area and will have great impact in the future.
The computer aided welding analysis (CAWA) has its disadvantages. The requirement for an expensive mainframe computer makes the solution economically unfeasible for medium or small size companies. Even with the possibility of purchasing the computing time from centralized computer facilities, the cost of running a large scale simulation job for process modeling or fracture analysis is a prohibiting factor.
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The other major difficulty in CAWA is the present technology limitations. The welding process and its associated phenomena are extremely complex and little known to the scientific community. The accuracy of the modeling and simulation is often questioned. However, the validity of the approach should not be doubted.
The theoretical foundation of the mathematical formulations always reflects the true behaviour of the process within its assumed domains. The systematic investigations of the various parameters provide a means to reveal the formation mechanism of different welding phenomena, which are extremely difficult to obtain experimentally, if not impossible.
The correct attitude of looking at the computer analysis should be toward understanding the process, rather than putting the emphasis on the exact comparison of the experimental data. As a matter of fact, very few experimental techniques are available for scientific investigations of many complex welding phenomena. The computer solution may still be the only answer to technological advancement in welding.
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