INVESTIGATION OF MECHANICAL PERFORMANCE AND FORMABILITY OF WELDED AND BRAZED SHEET MATERIALS

Abstract

In the last two decades or so, hybrid structures from dissimilar materials and/or sheet gauges have been developed to achieve weight reduction while maintaining or even improving structural performance such as stiffness, crash and impact behavior. In particular, welded and brazed sheet materials in the form of tailor blanks (TBs) are being increasingly used or considered for future applications in different applications such as automotive, aerospace and marine constructions as they offer attractive combination of strength and performance in applications where weight reduction is desirable. However, technical problems are often encountered during forming of TBs from dissimilar base sheet materials with different thickness and/or strength. These include weld line movement and non-uniform deformation. Additionally, there are premature weld failures due to the presence of softening zone (as in TBs made from advanced high strength steels), and brazed interface failure due to insufficient bonding and wetting (as in TBs made from steel and aluminum). These areas of forming of TBs need to be scientifically studied to advance the use of dissimilar materials. The current research involves an understanding of deformation and forming behavior of steel-to-steel tailor welded blanks (TWBs) made from advanced high strength steel (AHSS) such as dual phase (DP780) steel. The research also involves a study of deformation behavior of steel-to-aluminum tailor brazed blanks (TBBs). TWBs have been successfully joined using a relatively new welding techniques such as defocused fiber laser welding. TBBs, on the other hand, have been successfully produced by fiber Laser/MIG hybrid brazing and Cold Metal Transfer brazing (CMT). In addition, the formability of TWBs of different gauges and/or strengths was tested by using a new, simple and reproducible method of formability testing using a double-layer blank method. This method was devised and assessed for testing various steel combinations in different strain paths such that the weld line stayed in position with respect to forming tools and is subjected to the same stress and strain state as the parent material in the weld and its vicinity. Moreover, results from conventional stretch forming tests, single-layer blank, and the double-layer method were compared at the macroscopic level (such as weld line movement, forming limit etc.) as well as at the microscopic level (such as failure location within the weld and failure mode) to isolate the advantages of the proposed double-layer method. With regard to TBBs made by fiber Laser/MIG and CMT brazing methods, a fundamental knowledge and understanding of the local deformation behavior and material plastic ow in and around the brazed steel-aluminum interfaces were obtained by conducting miniature tensile mechanical tests that focus on continuous observation of the brazed region under a high magnification optical microscope to assess the ductility of the brazed joint and its capacity to carry the load during a material shaping process.