Influence Of Welding Sequence Of Bend Box Structure On Residual Stress

Welding is widely applied as a basic connection approach, having important signification to the development of national economy. But welding is a transient heat input process, heat is highly concentrated, inhomogeneous heating and cooling will cause considerable residual stress after welding process. Welding residual stress has a detrimental effect on manufacturing process and usability of welded structure, inducing fracture failure of structure. Tensile residual stress will reduce fatigue strength and corrosion resistance of welded structure, compressive residual stress can reduce stability limits. Reasonable techniques are important measure of controlling residual stress, adjustment of welding sequence is a technique of low-cost and simple.Excavator’s boom, bucket, high-speed powered car’ bogie are all large box structures, if influence laws of welding sequence of large box structure on residual stress is studied by experiment, the experiment may not be completed because of high cost and multiple parameter. At present, finite element method(FEM) is applied most extensively for the analysis of complicated welding process. The FEM is more efficient and accurate for study of welding process of large box structure.This paper regards large bend box structure as the research object. Firstly, threedimension solid model of this structure is established by UG, then model is meshed by Hypermesh, bend box structure is numerical simulated by MARC which is a nonlinear finite element simulation software. Local to global hierarchical optimization is applied, control of residual stress is the optimization goal, combinations of welding sequences of multi-pass welding are optimized, then the optimum welding scheme is proposed. Influence mechanisms of different welding sequences on residual stress areexplained. Lastly, influence laws of different constraints on residual stress are also studied. Theoretical reasons are provided by numerical simulation for optimization of practical welding technology, these help improve production efficient and quality of welding structure, so this paper has important signification in theoretical research and practical application. The research results are listed as follows:(1)Welding sequences which decrease transient heat input concentration(the fifth scheme) can reduce residual stress for single weld bead significantly, reduction rate of maximum of residual tensile stress is 17.7% comparing with continuous welding sequences for three welding subsections, the effect is obvious.(2)Welding sequences which can reduce inhomogeneity of heat distribution(the second scheme) should be adopted for mufti-pass welding, this scheme could decrease temperature gradient, and reduction rate of maximum of residual tensile stress is 6.7%comparing with continuous-heating welding sequences for adjacent welds(the fourth scheme).(3)Parallel welds should be welded reversely with each other(the fourth scheme),this scheme has opposite influence on stress and deformation, reducing superposition of residual stress and plastic deformation, so stress is small, and reduction rate of maximum of residual tensile stress is 5.1% comparing with same-direction welding sequences for four long welds.(4)The change of welding sequences of global structure can’t alter value of residual stress prominently. This shows that,if reducing residual stress is selected as the object,the influence of global welding sequences should be ignored in actual welding, and optimization should be focused on the first three hierarchy or other aspects.(5)The residual stress of box structure will decrease with reducing of constraints,but reducing constraints doesn’t always decrease maximum of residual tensile stress by contrasting four-constraint(the third scheme) and two-constraint(the fifth scheme).Reasonable constrained control should be enforced in actual welding, too much or too less constraints are not ideal.