Experimental determination of influencing factors on the humping phenomenon during laser micro welding of thin metal sheets

Abstract

Industrial applications such as joining pressure sensors or battery cells often demand short processing times for economic reasons. Thin metal sheets of thickness smaller than 100 μm are suitable for this purpose. The possible maximum feed rate for an efficient welding process is limited by weld defects, which occur at a certain threshold value of feed rate. Materials such as stainless steel, aluminum, and titanium were welded in bead-on-plate welds in order to generate a full penetration weld. Here, our attention is focused on understanding this instability. In this paper, we performed tests to clarify the influence of the thermophysical properties of the applied materials and the process factors laser power and focal diameter on the humping effect. Due to these attributes, the weld seam formation and hydrodynamic behavior of the melt change. By means of microscopical surface line scans and high-speed imaging, the observations were analyzed depending on the feed rate. The results from the line scans provide the possibility to analyze the surface topography of the weld seam. In particular, the distance, height, and axial frequency of the solidified humps can be categorized in order to get a deeper understanding of the solidified hump structure and the phenomenon in general. To avoid the occurrence of humping, a criterion is defined by the ratio of laser power to weld seam cross section for the applied materials.

Fiber Laser Welding of Dissimilar 2205/304 Stainless Steel Plates

“…The effect of surface deformation and melt flow on weld bead formation was first investigated by Bradstreet [57] when correlated the bead profile of a convex-shaped profile to the surface tension, which controls edge wetting in the weld pool. Similarly, several reasons, such as the aspect ratio, heat transfer, melt flow, pressure balance, and welding speed account for the development of concave-shaped weld bead profiles [53,[58][59][60].…”

The Role of Surface Topography on Deformation-Induced Magnetization under Inhomogeneous Elastic-Plastic Deformation

“…Yet, the physical origins of the magnetic stray field formation have so far mainly been attributed to (i) macroscopic stress gradients in the material (often referred to as “stress concentration zones”, SCZ) [ 13 , 17 , 18 , 19 , 26 , 29 ]; (ii) (locally) increased dislocation densities [ 13 , 16 , 18 , 19 , 26 ]; and (iii) the associated local magnetic permeability reduction due to plastic deformation [ 14 , 17 , 29 ]. Note that topography must be expected for most of the damage states being within the scope of MMM testing, such as welded joints (seam topography) [ 51 , 52 , 53 ], bulging, buckling, and barreling under non-uniform compressive loading [ 54 , 55 , 56 ], corrosion pits and stress corrosion cracking [ 57 , 58 , 59 , 60 ], inhomogeneous growth and spalling of (e.g., iron) oxide scales [ 61 ], and slip bands, and even intrusions and extrusions resulting from fatigue loading [ 62 , 63 , 64 , 65 , 66 ]. Our results demonstrate that disregarded surface topographies may provoke safety-relevant misinterpretations when using the MMM technique for quantitative stress analyses.…”

Understanding humping formation based on keyhole and molten pool behaviour during high speed laser welding of thin sheets

“…Even if no clear description of the two key questions has been proposed for understanding humping formation mechanism of high speed laser welding, it is interest to recognize that the keyhole and the molten pool behaviour are associated with humping formation process [21]. Firstly, through modifying the shape of the keyhole, it is interesting to find the humping tendency decreases [22].…”