Methods And Models Of Residual Stress Reduction
Residual stress due to welding is an ongoing problem in engineering which affects multiple material and structural failure mechanisms. The current development of residual stress reduction technologies is driven by the high cost and inherent limitations of conventional processes such as post-weld heat treatment, and has been aided by continual advances in measurement methods and computer modelling capability. Good agreement was found between FE predictions and experimental measurements in terms of trends and magnitudes of the residual stress measurements and numerical predictions.
The FE models predict the correct location of the peaks for the longitudinal tensile stress and capture the mechanical redistribution of the stress after weld pass is performed. Since stress formation during welding is affected by thermal, mechanical and material factors, there has been a proliferation of different approaches to the problem based on these different physical mechanisms. Some techniques (notably the mechanical methods – rolling and global mechanical tensioning) have been proven capable of comprehensively changing the distribution of stress in a weld, but all come with their own practical limitations. Consequently, it is likely that a greater degree of specialization will occur as these processes develop, since each is more suited to particular materials and applications. Using the processes described above, some very encouraging results in terms of the residual stress distribution have been achieved in recent years. It is now necessary to consolidate such results with research into the accompanying material properties and microstructure, to show that these new stress reduction techniques can be safely applied to structural welds outside of the laboratory. In general, compressive zones are typically seen at repair weld ends, and a tensile zone within the midlength of a repair weld. Depending on repair depth, an elevated membrane tension can be seen in the through thickness direction. Some of the techniques have demonstrated enormous potential in developing high performance structures and advanced fabrication technologies in today’s competitive environment. However, one should keep in mind that welding process modelling, at the present time and for the foreseeable future, requires a high degree of expertise in welding process physics, thermoplasticity, computational mechanics etc. , depending on the specific objectives to be addressed.
In order to obtain meaningful solutions, proper problem definition, assumptions and simplifications are always necessary. Recent Experimental investigation of bead-on-plate welds by means of weld macrography, thermocouples and neutron diffraction measurements with the aim to investigate the effects of the interaction between weld induced residual stresses. The second weld pass was shown to cause are distribution of stresses, resulting in a reduction in the longitudinal peak stresses in the location of the first and also, the highest tensile transverse stresses to move to the location of the second weld pass. The magnitude of the transverse stress increased as a result of the weld repair. Although the plates are relatively thick, both the samples appear to be in a plane stress condition, showing a normal stress of approximately zero. To answer the question whether it is acceptable or not to neglect the initial stress in simulating the fusion welding process is simulated, the authors suggest considering the purpose of the numerical model. If the objective of the simulation is to evaluate the local stress distributions, a good representation of the stress might be obtained by neglecting the pre-existing stresses into the component. If the aim is to evaluate the global stress distributions in a complex geometry, including critical areas in the far weld region, and/or study the effect of different welding conditions (welding power, length pass, multiple bead deposition) it is strongly recommended that any pre-existing stresses in the component be considered, properly Representing the actual pre-existing stress distributions into the FE model. The applicability of laser ultrasonic for both defect detection and residual stress measurement was demonstrated. Ultimately, the approach could allow fast scanning for weld assessment along the tool path. When combined with F-SAFT for defect detection, discontinuities such as wormholes, hooking and LOP were clearly detected in the lap, butt or T-joint configurations. Moreover, the detection of kissing bonds could be possible in lap joints with frequencies up to 200MHz. Laser ultrasonics could also be used to measure residual stresses induced by the FSW process.
The method is based on monitoring the small velocity change of the P-wave with the SAW correction for other effects. The residual stress profile measured across the weld line was in fairly good agreement with results from simulations and strain gauge measurements. However, the cross point from tensile to compressive stress was different for the different methods. This will be further investigated in future work with results from Neutron diffraction technique on more specimens and the use of a more complete Numerical model. An experimental investigation of the plasma welding process in the transversal direction are tensile and relatively close to the material yield strength (~300 MPa), which can have a large effect on the useful life of the component. However, the vibratory motion was effective in reducing the residual stresses in the DP600 steel plate. It was noteworthy that its effectiveness for the stress in the longitudinal direction of the weld was around 40 % for four test specimens. In the transversal direction, most of the test specimens presented reduction levels around20 %. Additionally, the residual stress profile for plasma welding is also presented. It was observed that residual stress reduction is heterogeneous and more significant in the region of the weld and near of the electromechanical shaker. Though many technologies comes into existence the maximum residual stress that can be removed Is only up to 75% and it is up to 40%for vibratory stress relief more over the authors said that the exact value of the residual stress cannot be found due to microstructural,material,defects in the parent weldment.
