The Study on Mechanical Behavior of Butt Joints and Optimization of Process Variables in Al-Mg Alloys by Hybrid (CW Nd:YAG Laser+MIG) Welding

Abstract

최근 수송기계의 경량화를 통한 연료비 절감 및 자원재생을 목적으로 알루미늄 합금의 사용이 증가되고 있으나 알루미늄 합금에 대한 기존 용접법 (아크 및 저항용접법 등) 적용의 어려움으로 인해 새로운 용접법의 개발이 요구되고 있다. 따라서 본 논문에서는 수송기계(선박)용 5XXX 계열 고강도 알루미늄합금에 대한 고속 용접기법을 개발하기 위해 3.5 kW 연속파 Nd:YAG 레이저와 MIG 용접기법을 복합한 하이브리드 용접의 개발과 더불어 맞대기 용접이음부의 역학적 거동과 용접 공정변수 최적화에 관해 기술하고 있다. 이를 위하여 하이브리드 용접시 용접 공정변수(레이저-아크 간 거리, 선행공정, 레이저 및 아크 출력 등) 최적화와 용접부 역학적 거동을 이해하기 위해 각종 실험과 수치해석을 실시하였으며 각종 실험을 통하여 선정된 최적용접 공정변수를 통해 기존 MIG용접법에 비해 공정단축과 더불어 약 2배의 고속에서 건전한 용접부를 얻을 수 있었다. 또한, 하이브리드 용접변수가 용접부의 형성에 미치는 실제 영향을 고속촬영을 통하여 이해할 수 있었으며 이를 토대로 하이브리드 용접부의 유한요소해석에 필요한 하이브리드 용접열원 (체적분리형 반 타원 복합모델)을 제안하였으며, 이를 이용하여 계산을 수행한 결과 실제 하이브리드 용접부의 용융부 및 열 영향부의 경계와 잘 일치됨을 확인할 수 있었으며 하이브리드 용접에 있어서 선행 용접공정에 의해 용접부의 형상과 잔류응력의 분포가 변화함을 실험과 수치해석을 통해 확인할 수 있었다. 더불어, 용접부에 대한 금속 및 기계적 실험을 통하여 강도 측면에 있어서도 기존 용접법 (레이저 및 MIG 용접)에 비해 하이브리드 용접법이 장점을 가짐을 확인하였으며 용접구조물의 설계 및 제작 시 필요한 요소기술 정보를 얻을 수 있었다. 따라서, 향후 구조 경량화 등을 목적으로 고강도 알루미늄합금이 사용되는 관련분야(자동차 및 항공 등)에 대해서도 본 용접기술을 적용함으로써 제품의 품질향상은 물론 생산성 측면에서도 기여할 수 있을 것으로 생각된다.|The application of aluminium alloy is increasing for lightweight and high quality transport vehicle. However, it requires that new welding method has to be developed for productivity and high quality of products because of the difficulty in applying the conventional welding processes (resistance and arc welding, etc) to aluminium alloy. In this study, therefore, it is intended to develop the high speed welding method for 5XXX series marine grade aluminium alloy (A5083) used for ship applying that consists of 3.5 kW CW Nd: YAG laser and MIG welding process. For this purpose, fundamental welding phenomena of hybrid process using CW Nd:YAG laser and MIG is investigated by the experiments. In order to calculate temperature and residual stress distribution in hybrid welds, finite element heat source model is developed on the basis of experiment results and characteristics of temperature and residual stress distribution in hybrid welds are understood from the result of simulation. Mechanical and metallurgical characteristics of hybrid welds are also analysed and compared with other results in order to supply the fundamental information for the criteria of welding design and construction. From the result of this study, order of leading heat source, welding speed and laser-arc distance are selected as main process parameters. It is understood that heat input mechanism including variation of energy coupling effects according to the change of parameters. Also their relation is understood from the experiments. In this study, optimum heat transfer is occurred between LAD = 3 mm and LAD = 5 mm in to workpiece. Through optimization of welding process parameter, sound weld seam is achieved for gap=1.1 mm condition with increase of welding speed up to 1500 mm/min. Deposit rates control of molten wire is a useful parameter to get the sound weld seam, less depression of bead surface without lack of fusion, in hybrid butt joint welding. Welded joints are successfully passed the initial visual test above moderate condition (Quality D) is evaluated according to EN ISO 13919-2. Also, amount of contraction and distortion of hybrid weldments is not large compared to one of MIG weldments in the visual inspection. It is considered that reduction of assembly process such as turn-over and backing process is possible in the production line by one (1) pass running. From the results of welding experiment and high speed image observation, it is understood that existences of gap in the joint changes heat transfer condition and electrical circuit condition. Therefore, stabilization processes are needed before laser irradiation to overcome obstacles in the gap condition. In this viewpoint, it is considered that MIG+Laser process is suitable for the gap condition above beam diameter although Laser+MIG hybrid process has the better mechanism to combine heat energy in the bead on plate welding condition. Synergy effect of heat source combining is maximized when laser beam is located between arc centre and droplet point of molten wire. It is understood that final bead shape in hybrid welding is dominated by the volume of molten pool before the impingement of laser beam. Combination volume heat source model of split type is considered to develop the heat source model for hybrid welding. From the result of simulation using this model, cooling rate of WM in each process shows in the order of heat input quantity of each welding process as follows: CW laser > GMAW > Hybrid. Calculated FZ and HAZ boundary in hybrid welds is expected well compared to experiment result. Residual stress distribution in hybrid welds shows mixed shape, stress level in weld metal is increased, with GMA welds and laser welds. It is understood that bead shape and volume variation have different effect on structural strengths of component. The characteristics of hardness distribution in hybrid welds show the intermediate value between MIG and laser process. Hardness value in welds shows in the order of WM < HAZ < BM. All specimens for tensile test have been failed in the weld. However, joint efficiency in hybrid welds is better than laser welds and its value is estimated close to one of GMA welds. From the test result, it is understood that effect of hardness variation in WM and HAZ on the strength is not severe and geometric bead shape is important in the strength point. Strength of WM should be considered as a design criterion with the safety margin of structure at the design stage. Also, welds also should be located at places at which the service stresses are low in general design. Distribution of microstructure in MIG welds, Laser welds, and MIG+Laser hybrid welds is uniform all over weld metal. But, one of Laser+MIG hybrid welds is not uniform compared microstructure at the upper part of WM with microstructure of other area. It is thought to be caused by difference of weld pool formation mechanism. That is, it is estimated that impingement of laser beam on the pool after molten wire droplet affects on the formation of microstructure at the upper part of WM. Coarsening of microstructure is occurred at the upper part of WM. In the case of microstructure in HAZ, grain growth is observed with recrystallization in the all specimen. Further coarsening of microstructure is occurred in BOND by growth of the grains adjacent to the weld fusion line. The second-phase particles containing magnesium, iron, manganese and silicon were found to segregate to the grain boundaries. Therefore, comprehensive investigation is needed in order to apply the hybrid welding process in the production of aluminium structures. Through the development of high speed hybrid welding method for aluminium alloys applied to transport vehicle, the core technology to increase the application of aluminium is ship structures is acquired. Since high quality of products, reduction of fuel by lightweight structure and high value added is achieved by applying the obtained core technology into the industrial applicants, the competitiveness in related industries (automobile and aerospace, etc) will be secured.