The Optimization of Process Parameters and Microstructural Characterization of Fiber Laser Welded Dissimilar HSLA and MART Steel Joints
Nowadays, environmental impact, safety and fuel efficiency are fundamental issues for the automotive industry. These objectives are met by using a combination of different types of steels in the auto bodies. Therefore, it is important to have an understanding of how dissimilar materials behave when they are welded. This paper presents the process parameters' optimization procedure of fiber laser welded dissimilar high strength low alloy (HSLA) and martensitic steel (MART) steel using a Taguchi
... el using a Taguchi approach. The influence of laser power, welding speed and focal position on the mechanical and microstructural properties of the joints was determined. The optimum parameters for the maximum tensile load-minimum heat input were predicted, and the individual significance of parameters on the response was evaluated by ANOVA results. The optimum levels of the process parameters were defined. Furthermore, microstructural examination and microhardness measurements of the selected welds were conducted. The samples of the dissimilar joints showed a remarkable microstructural change from nearly fully martensitic in the weld bead to the unchanged microstructure in the base metals. The heat affected zone (HAZ) region of joints was divided into five subzones. The fusion zone resulted in an important hardness increase, but the formation of a soft zone in the HAZ region. Metals 2016, 6, 245 2 of 17 side access welding, low process cost and suitability of automation, laser welding is becoming an attractive and economically advantageous joining technique in the automotive industry  . Joints of dissimilar steel combinations in auto body structures are widely utilized for several applications requiring a special combination of properties besides cost saving and weight reduction. However, due to different metallurgical, thermal and physical properties of the materials, dissimilar material welding is more challenging than similar materials welding. Due to low and concentrated heat input and high speed properties, laser welding has also advantages on joining dissimilar materials over other conventional methods  . Thus, reduced distortion and a narrower heat affected zone (HAZ) with limited microstructural changes can be obtained. There are several studies in the literature concerning the laser welding of similar or dissimilar DP and HSLA steels. Saha et al.  examined the mechanical and microstructural properties of laser welded DP980 and HSLA steel sheets. They stated that the tensile strength of the dissimilar welds was lower than DP welds. Xu et al.  investigated microstructural and mechanical properties, and Parkes et al.  reported the fatigue properties of laser welded DP and HSLA joints with varying weld geometries. Parkes et al.  evaluated the tensile properties of laser welded HSLA and DP steels at cryogenic, room and elevated temperatures. They reported that with the temperature increase, the tensile properties were decreased. In addition, several research works investigated laser welding of higher degree DP steels and AHSS. Wang et al. [9, 10] investigated the effect of energy input and softening mechanism on the laser butt welded DP1000 steel. They found that the weld bead width and softening zone width become narrowed at lower energy input levels. Additionally, the mechanical properties were increased. The study of Rossini et al. , concerned with laser welding of dissimilar AHSS types, has shown that a fully martensitic microstructure was present in the 22MnB5, DP and TRIP steels close to the fusion zone (FZ), while mainly tempered martensite and ferrite zones were close to the base metal. Although there are many research works about laser welding of DP and HSLA steels, only limited work has been reported on the laser welding of MART steels. Nemecek et al.  compared the microstructural and mechanical properties of MART steel joints made by laser and metal active gas (MAG) welding. They stated that the strength of the laser welded joints was higher than arc welding, and the HAZ width and grain coarsening in the HAZ were minimal. Zhao et al.  investigated the effect of welding speed on weld bead geometry and the tensile properties of the laser welded MART steel. They observed that, due to the fast cooling rate, the FZ of the joints contained predominantly martensite. Furthermore, the tensile load gradually increased with decreasing welding speed. Due to welding process parameters directly affecting the quality of the weld joints, it is necessary to work in the suitable range. However, defining the suitable parameters to obtain the required quality welded joints is a time-consuming process. Several optimization methods are utilized in order to solve this problem. The Taguchi method is one of the most common design of experiment (DOE) techniques that allows the analysis of experiments with the minimum number [14, 15] . In the literature, several researchers have used DOE methods to optimize quality characteristics in laser welding parameters. Benyounis and Olabi  have presented a review of the application of optimization techniques in several welding processes. Anawa and Olabi  used the Taguchi method for the purpose of increasing the productivity and decreasing the operation cost of laser welding ferritic-austenitic steel sheets. Another study of the authors  analyzed the optimized shape of dissimilar laser welded joints and fusion zone area depending the process parameters. Sathiya et al.  carried out the Taguchi method and desirability analysis to relate the parameters to the weld bead dimension and the tensile strength of the joints with various shielding gasses. Fiber laser welding has demonstrated its capability of welding dissimilar steel joint with and without the help of a synergic power source like the arc . Acherjee et al.  used Taguchi, response surface methodology (RSM) and desirability function analyses in laser transmission welding, and they investigated the optimal parameter combination for the joint quality.