Thick Section Laser Beam Welding of Structural Steels: Methods for Improving Welding Efficiency

摘要

Laser beam welding (LBW) is applicable for a wide range of industrial sectors and hasa history of fifty years. However, it is considered an unusual method withapplications typically limited to welding of thin sheet metal. With a new generation ofhigh power lasers there has been a renewed interest in thick section LBW (also knownas keyhole laser welding). There was a growing body of publications during 2001-2011that indicates an increasing interest in laser welding for many industrial applications,and in last ten years, an increasing number of studies have examined the ways toincrease the efficiency of the process.Expanding the thickness range and efficiency of LBW makes the process a possibilityfor industrial applications dealing with thick metal welding: shipbuilding, offshorestructures, pipelines, power plants and other industries. The advantages provided byLBW, such as high process speed, high productivity, and low heat input, mayrevolutionize these industries and significantly reduce the process costs. The researchto date has focused on either increasing the efficiency via optimizing processparameters, or on the process fundamentals, rather than on process and workpiecemodifications.The argument of this thesis is that the efficiency of the laser beam process can beincreased in a straightforward way in the workshop conditions. Throughout thisdissertation, the term “efficiency” is used to refer to welding process efficiency,specifically, an increase in efficiency refers an increase in weld’s penetration depthwithout increasing laser power level or decreasing welding speed. These methods are:modifications of the workpiece – edge surface roughness and air gap between thejoining plates; modification of the ambient conditions – local reduction of the pressure in the welding zone; modification of the welding process – preheating of the weldingzone. Approaches to improve the efficiency are analyzed and compared bothseparately and combined. These experimentally proven methods confirm previousfindings and contribute additional evidence which expand the opportunities for laserbeam welding applications.The focus of this research was primarily on the effects of edge surface roughnesspreparation and pre-set air gap between the plates on weld quality and penetrationdepth. To date, there has been no reliable evidence that such modifications of theworkpiece give a positive effect on the welding efficiency. Other methods were testedin combination with the two methods mentioned above. The most promising -combining with reduced pressure method - resulted in at least 100% increase inefficiency.The results of this thesis support the idea that joining those methods in one modifiedprocess will provide the modern engineering with a sufficient tool for many novelapplications with potential benefits to a range of industries.