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Effects of part-to-part gap and the direction of welding on laser welding quality

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Title
Effects of part-to-part gap and the direction of welding on laser welding quality
Author
Kim, Hyun-Ki
Advisor
Kim, Duck Young
Issue Date
2013-02
Publisher
Graduate School of UNIST
Abstract
The use of laser welding has become quite widespread because it can achieve higher productivity than spot welding. This is due to its desirable features, which include high power density, faster welding speed, highly accurate welding, and excellent repeatability. In addition, laser welding can minimize the distortion in heat-affected zones (HAZs), and there is no tooling that wears out or must be changed over. In spite of these advantages, laser welding still causes many problems when used on compositions such as galvanized steel and aluminum alloy. Galvanized steel, for example, is composed of a zinc layer whose physical parameters differ from those of steel as a base material. Zinc vaporizes at a temperature of 907 K, whereas steel begins to melt at 1510 K. This phenomenon causes serious defects in welds because the pressure of zinc is more powerful than that of steel. As a result, a certain manipulable control is needed in order for the zinc coating to be able to evaporate. To prevent this circumstance, the following solutions have been proposed: (i) a de-gassing method that induces the zinc fumes to escape from the part-to-part gap between two materials; (ii) the removal of zinc layers that will be joined together; (iii) a pulsed laser method that minimizes zinc vaporization using a high energy per pulse and a short pulse duration; (iv) a laser hybrid method; and (v) the addition of additional elements to the surface, which form a compound with the vaporizing zinc. Despite these suggestions, applications involving zinc-coated steels are rarely used in the automotive industry because the shapes of the materials to be welded are not always uniform. In this study, we ascertain the effects of the part-to-part gap and the direction of welding on the quality of laser welding. Using a 2 kW fiber laser and galvanized steel sheets (with thicknesses of 1.4 mm and 1.8 mm), our experiments employed lap welding, which has been applied to side members in the automotive industry. The experimental design was used with a 33 factorial design with 3 replications. The three types of welding direction used are ascendance, descendance, and a uniform gap. Based on the experiments, using analysis of variance (ANOVA) it was determined that the direction of welding is an important factor that can affect the weld quality. In addition, the differences between the shear tensile strengths in the ascendance and descendance directions were determined using a t-Test. The maximum shear tensile strength in the ascendance direction was achieved with a laser power of 2000 W and a welding speed of 2100 mm/min, followed by a part-to-part gap of 0.32 mm/min as the steepest ascent method. Moreover, we analyzed cross-sections of sampling specimens, varying the gap differences in order to verify the differences in shear tensile strength based on two different directions of welding.
Description
Engineering & Systems Design
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