The transient thermal stress and residual stress in bead-on-plate welding and laser surface hardening were analyzed by employing a new two-dimensional finite element model. In the formulation, the sliced solution domain possessing one element in the welding or hardening direction was introduced to allow the change of the longitudinal total strain along the width and depth of the specimen. The longitudinal total strain, which makes the resultant force in the longitudinal direction to be zero, was used as the boundary condition. To determine the longitudinal total strain, the equations of the compatibility, force equilibrium and moment equilibrium were considered. Calculation of the transient temperature distribution was based on the two-dimensional finite element model of the quasi-stationary condition.
By using the proposed model the thermal and residual stresses of the bead-on-plate GTA welding was successively calculated. The computed results were compared with those by the conventional two-dimensional model of the plane strain condition. The simulation results revealed that the present model could more reasonably describe the thermal and residual stresses than the plane strain model.
The thermal and residual stresses in the laser surface hardening treatment was also successively calculated. The thermal stress was induced mainly from the temperature gradient and martensitic phase transformation, which was found to have a greater influence on the residual stress than the former. The simulation results revealed that a compressive residual stress region occurs near the hardened surface of the workpiece and a tensile residual stress region in the interior of the workpiece, while the maximum tensile residual stress occurs along the center of the laser scanning path in the interior region. In the multi-pass laser surface hardening, the region of the tensile residual stress remains on the hardened surface, if the distance between the neighbouring passes is larger than the width of the hardened zone of each pass. The tensile residual stress on the surface, however, disappears when the distance between the passes is so short that the hardened zone of each pass is partially overlapped.
The effect of the transformation plasticity was considered by using an additional strain in the numerical analysis of laser of laser surface hardening treatments. The simulation results showed that the deviation between the residual stresses with considering the transformation plasticity and without considering it on the hardened surface is large, so that it cannot be considered as a negligible factor. In the non-hardened region, however the shapes of the iso-stress lines were very similar to each other.
Measured residual stress distribution in bead-on-plate GTA welding showed much better agreement with the residual stress predicted by the new two-dimentional finite element model than with those predicted by the conventional two-dimentional model of the plane strain condition. In laser surface hardening treatment, experimental results obtained by irradiating the test specimen with a 2.4 kW CW-$CO_2$ laser showed a better agreement with the residual stress predicted by considering the transformation plasticity than with those predicted by neglecting it.
아크나 레이저빔과 같은 고밀도 에너지를 이용하여 공작물을 가공 하면 국부적인 가열에 의해 불균일한 온도분포를 갖게되어 모재에 열응력이 발생한다. 모재가 열을 받으면 항복응력이 저하하기 때문에 발생한 열응력은 항복응력을 초과하여 소성변형을 일으키고 냉각, 수축후 최후에 재료내부에 잔류응력으로 남게된다. 생성된 잔류응력은 균열발생, 피로강도저하, 구조물의 형상변화등 많은 문제점을 야기시키므로 이에 대한 연구를 필요로한다.
열가공 공정시 발생한 잔류응력을 해석하기 위해서는 가열, 냉각중에 천이과정 온도분포를 계산한후, 이를 이용하여 열응력을 시간 변화에 따라 계산하면 최후에 남은 응력이 잔류응력이 된다. 이와같은 비선형, 천이과정 열응력 문제해석을 위해서는 수치해석 기법인 유한요소법(FEM)을 사용하는 것이 적절하다. 위의 과정을 유한요소법을 이용하여 3차원적으로 해석하기에는 컴퓨터 용량, CPU 시간이 문제됨으로 적절한 가정을 통하여 2차원 해석을 필요로 한다. 기존의 2차원 유한요소 열응력 모델로는 평면변형율 조건이 사용되고 있는데, 평면변형율 조건에서는 주어진 1방향 (열원 진행방향)으로 구속되므로 잔류응력이 힘의 자체평형 (self-equilibium)을 만족하지 못한다. 그러므로 본 연구에서는 힘의 자체평형을 만족하는 새로운 2차원 유한요소 모델을 제시하고, 새로운 모델을 GTA용접 공정에 적용하여 평면변형율 조건과 비교하였으며, 최근에 개발되어 연구가 활발히 진행되고 있는 레이저 표면경화 처리의 잔류응력 해석에 응용하였다.
레이저 표면경화 처리의 잔류응력 분포는 좁은 영역에 집중되어 냉각시 마르텐사이트 생성때문에 일반적인 용접공정과는 다른 응력 분포 양상을 나타내어, 마모저항, 피로강도등에 중요한 영향을 미치게 되므로 최적 공정변수를 결정할 때 잔류응력 분포를 고려해야 한다. 본연구에서는 sigle-pass 및 multi-pass 레이저 처리시 잔류응력을 해석하였으며, 상변태시 나타는 transformation plasticity 효과를 수치해석에 고려하여 계산하였다.
수치해석의 신뢰성을 입증하기 위해 bead-on-plate GTA 용접 및 레이저 표면경화처리 시편에 대해 잔류응력을 측정하여 계산치와 비교하였다.
