M. L. Kuntz

Bio: M. L. Kuntz is an academic researcher from University of Waterloo. The author has contributed to research in topics: Spot welding & Welding. The author has an hindex of 13, co-authored 16 publications receiving 828 citations.

Effects of weld microstructure on static and impact performance of resistance spot welded joints in advanced high strength steels

Abstract: Evaluating the impact performance of resistance spot welded joints in advanced high strength steels (AHSS) is critical for their continued integration into the automotive architecture. The effect of strain rate on the joint strength and failure mode is an important consideration in the design of welded structures. Recent results suggest that the failure mode is dependent upon the strength, chemistry, and processing of AHSS. Current literature, however, does not explain the effects of weld microstructure and a comprehensive comparison has yet to be conducted. The present study details the fracture paths within the joint microstructure of spot welded AHSS, including dual phase (DP), transformation induced plasticity (TRIP) and ferritic–bainitic (FB), in comparison to new high strength low alloy steels. Quasi-static and impact tests were conducted using a universal tensile tester and an instrumented drop tower respectively. Results for elongation, failure load and energy absorption for each material ...

Resistance and friction stir spot welding of DP600: a comparative study

Abstract: Efforts to reduce vehicle weight and improve crash performance have resulted in increased application of advanced high strength steels (AHSS) and a recent focus on the weldability of these alloys. Resistance spot welding (RSW) is the primary sheet metal welding process in the manufacture of automotive assemblies. Friction stir spot welding (FSSW) was invented as a novel method to spot welding sheet metal and has proven to be a potential candidate for spot welding AHSS. A comparative study of RSW and FSSW on spot welding AHSS has been completed. The objective of this work is to compare the microstructure and mechanical properties of Zn coated DP600 AHSS (1 . 2 mm thick) spot welds conducted using both processes. This was accomplished by examining the metallurgical cross-sections and local hardnesses of various spot weld regions. High speed data acquisition was also used to monitor process parameters and attain energy outputs for each process. Results show a correlation found among microstructure, failure loads, energy requirements and bonded area for both spot welding processes.

Influence of microstructure and weld size on the mechanical behaviour of dissimilar AHSS resistance spot welds

Abstract: Resistance spot welds were produced in dissimilar combinations of advanced high strength steels. A 600 MPa dual phase (DP) steel was welded to a high strength low alloy, a 780 MPa DP, and a 780 MPa transformation induced plasticity steel. The microstructure and mechanical properties were characterised using metallurgical techniques and lap shear and cross-tension testing. The results show that a pullout failure mode with improved mechanical properties is obtained when DP600 is paired with other advanced high strength steels, compared to the DP600 welded to itself, which is prone to interfacial failure and poor mechanical properties, given the same weld size. An in depth comparison of the interfacial to pullout failure transition in similar DP600 and DP780 and dissimilar DP600–DP780 welds was performed. The results show that the interfacial to pullout transition for the DP600–DP780 welds is significantly lower than with DP600 welded to itself. Increased fusion zone strength through dilution with th...

Microstructure and Mechanical Properties of Resistance Spot Welded Advanced High Strength Steels

Abstract: Integration of advanced high strength steels (AHSS) into the automotive architecture has brought renewed challenges for achieving acceptable welds. Resistance spot welding (RSW) is the primary method used in welding automotive structures, which has resulted in a demand to better understand RSW of AHSS. The varying alloy contents and processing techniques used in their production has further complicated this initiative. The current study examines resistance spot welding of AHSS including 590R, DP600, DP780 and TRIP780. HSLA material is also included to represent conventional high strength steels and benchmark AHSS performance. The mechanical properties and microstructure of these resistance welded steel alloys are detailed. Furthermore, a relationship between chemistries and fusion zone hardness is produced.

Review: friction stir welding tools

Abstract: Friction stir welding (FSW) is a widely used solid state joining process for soft materials such as aluminium alloys because it avoids many of the common problems of fusion welding. Commercial feasibility of the FSW process for harder alloys such as steels and titanium alloys awaits the development of cost effective and durable tools which lead to structurally sound welds consistently. Material selection and design profoundly affect the performance of tools, weld quality and cost. Here we review and critically examine several important aspects of FSW tools such as tool material selection, geometry and load bearing ability, mechanisms of tool degradation and process economics.

Overview of transient liquid phase and partial transient liquid phase bonding

Abstract: Transient liquid phase (TLP) bonding is a relatively new bonding process that joins materials using an interlayer. On heating, the interlayer melts and the interlayer element (or a constituent of an alloy interlayer) diffuses into the substrate materials, causing isothermal solidification. The result of this process is a bond that has a higher melting point than the bonding temperature. This bonding process has found many applications, most notably the joining and repair of Ni-based superalloy components. This article reviews important aspects of TLP bonding, such as kinetics of the process, experimental details (bonding time, interlayer thickness and format, and optimal bonding temperature), and advantages and disadvantages of the process. A wide range of materials that TLP bonding has been applied to is also presented. Partial transient liquid phase (PTLP) bonding is a variant of TLP bonding that is typically used to join ceramics. PTLP bonding requires an interlayer composed of multiple layers; the most common bond setup consists of a thick refractory core sandwiched by thin, lower-melting layers on each side. This article explains how the experimental details and bonding kinetics of PTLP bonding differ from TLP bonding. Also, a range of materials that have been joined by PTLP bonding is presented.

Critical review of automotive steels spot welding: process, structure and properties

Abstract: Spot welding, particularly resistance spot welding (RSW), is a critical joining process in automotive industry. The development of advanced high strength steels for applications in automotive industry is accompanied with a challenge to better understand the physical and mechanical metallurgy of these materials during RSW. The present paper critically reviews the fundamental understanding of structure–properties relationship in automotive steels resistance spot welds. The focus is on the metallurgical characteristics, hardness–microstructure correlation, interfacial to pullout failure mode transition and mechanical performance of steel resistance spot welds under quasi-static, fatigue and impact loading conditions. A brief review of friction stir spot welding, as an alternative to RSW, is also included.

Tempering of Martensite in Dual-Phase Steels and Its Effects on Softening Behavior

Abstract: The isothermal and nonisothermal tempering of martensite in dual-phase (DP) steels was investigated mainly by analytical transmission electron microscopy, and the effect on softening behavior was studied. The isothermal tempering resulted in coarsening and spheroidization of cementite and complete recovery of laths. However, nonisothermal tempering manifested fine quasi-spherical intralath and platelike interlath cementite, decomposition of retained austenite, and partial recovery of laths. The distinct characteristic of nonisothermal tempering was primarily attributed to the synergistic effect of delay in cementite precipitation and insufficient time for diffusion of carbon due to rapid heating that delays the third stage of tempering. The finer size and platelike morphology of cementite coupled with partial recovery of lath resulted in reduced softening in nonisothermal tempering compared to severe softening in isothermal tempering due to large spheroidized cementite and complete recovery of lath substructure. The substitutional content of precipitated cementite in nonisothermal tempering was correlated to the richness of particular steel chemistry. Softening resistance during nonisothermal tempering was related to DP steel chemistry, i.e., Cr and Mn content. Fine cementite and less decomposed martensite in rich chemistry confer high resistance to softening compared to leaner chemistries, which indicated severe decomposition of martensite with coarser cementite.

Failure mode transition in AHSS resistance spot welds. Part I. Controlling factors

Abstract: Failure mode of resistance spot welds is a qualitative indicator of weld performance. Two major types of spot weld failure are pull-out and interfacial fracture. Interfacial failure, which typically results in reduced energy absorption capability, is considered unsatisfactory and industry standards are often designed to avoid this occurrence. Advanced High Strength Steel (AHSS) spot welds exhibit high tendency to fail in interfacial failure mode. Sizing of spot welds based on the conventional recommendation of 4t0.5 (t is sheet thickness) does not guarantee the pullout failure mode in many cases of AHSS spot welds. Therefore, a new weld quality criterion should be found for AHSS resistance spot welds to guarantee pull-out failure. The aim of this paper is to investigate and analyze the transition between interfacial and pull-out failure modes in AHSS resistance spot welds during the tensile–shear test by the use of analytical approach. In this work, in the light of failure mechanism, a simple analytical model is presented for estimating the critical fusion zone size to prevent interfacial fracture. According to this model, the hardness ratio of fusion zone to pull-out failure location and the volume fraction of voids in fusion zone are the key metallurgical factors governing type of failure mode of AHSS spot welds during the tensile–shear test. Low hardness ratio and high susceptibility to form shrinkage voids in the case of AHSS spot welds appear to be the two primary causes for their high tendency to fail in interfacial mode.