Stainless Steel Resistance Spot Welds Research Articles

Enhancing load-bearing capacity of martensitic stainless steel resistance spot welds: Microstructural vs. geometrical approaches

The load-bearing capacity of resistance spot welds is determined by a combination of weld geometrical attributes, such as the size of the weld nugget (WN), and weld metallurgical features. When resistance spot welds are made on martensitic stainless steels (MSSs), their load-bearing capacity is reduced due to the formation of a brittle martensitic microstructure within the WN. In this study, we investigated two distinct approaches to enhance the mechanical properties of AISI420 MSS spot welds. The results indicated that manipulating the WN size did not effectively produce strong welds in both tensile-shear (TS) and cross-tension (CT) tests. Therefore, we focused on examining the impact of the weld microstructure by employing two different interlayers: low carbon steel (LCS) and pure nickel. The analysis using electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA), along with equilibrium and non-equilibrium phase diagrams, revealed that the microstructure of the WN with low carbon steel interlayer of various thicknesses exhibited similarities with spot welds made without an interlayer, and the solidification mode was still δ-ferrite. This microstructure primarily consisted of martensite (M) and likely residual delta ferrite (δ) phases, resulting in no significant improvements in mechanical properties. In contrast, the utilization of the nickel interlayer led to remarkable enhancements in mechanical properties. This improvement can be attributed to a shift in the solidification mode from primary delta (δ) to austenite (γ) within an optimal interlayer thickness. This transition facilitated the development of a predominantly austenitic (γ) microstructure, contributing to the formation of an exceptionally tough microstructure within the weld nugget. Consequently, the load-bearing capacity was significantly improved.

Transitioning solidification mode via electroplated Ni coatings in martensitic stainless steel resistance spot welds: new insights into fabricating tough microstructure

The present study addresses the enhancement of fracture toughness of martensitic stainless steel (MSS) spot welds by utilizing through electroplating of Ni on MSS sheets. The equilibrium and non-equilibrium solidification modelling showed that by Ni coating with 50 μm thick on 1.5 mm thick MSSs, the solidification mode changes from δ-ferrite to γ-austenite, leading to a weld nugget (WN) dominated by austenite grains. Moreover, electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) showed that the other phases (martensite, δ-ferrite) appeared in band areas of WN owing to incomplete mixing of MSS and the Ni-coating. The tough microstructure in the Ni-coated MSS spot welds provided superior mechanical properties compared to non-coated welds, both in cross-tension (CT) and tensile-shear (TS) tests. Notably, the TS and CT strengths of the Ni-coated MSS spot welds showed a remarkable increase of 57% and 127%, respectively, in comparison to the conventional bare MSS spot welds. Furthermore, in terms of failure energy, the Ni-coated MSS spot welds demonstrated a substantial enhancement of 296% in TS and 520% in CT, when compared to their non-coated counterparts. This research study showcased the effectiveness of Ni electroplating as an industrial method for improving the spot weldability of MSSs.

Effect of Ni addition on the microstructural evolution and lap-shear performance of AISI 630 martensitic stainless steel resistance spot welds

Martensitic stainless steels exhibit poor resistance spot weldability, with high susceptibility to interfacial failure due to the formation of brittle martensite in the fusion zone (FZ). To alter the composition of the FZ and improve the lap-shear performance, Ni interlayer was employed during the resistance spot welding (RSW) of 2-mm-thick 630 martensitic stainless steel (MSS). The microstructures of the joints with and without Ni addition were characterized using optical microscopy and EBSD analysis. Calculation of phase diagrams (CALPHAD) technique was used to analyze the phase evolution in the joints as a function of temperature. The FZ of the joint without Ni addition consisted of predominantly martensite with small amount of dispersed retained austenite. Under lap-shear loading, the crack propagated through the martensitic FZ, leading to interfacial failure (IF), irrespective of the welding time or current. The joints could only reach a peak load of 8.26 kN and energy absorption of 6.83 J. The Ni addition changed the solidification mode from ferritic-austenitic to fully austenitic mode and significantly lowered the Ms. temperature (from 108 to −188.7 ° C). Accordingly, the Ni-added joints exhibited fully austenitic microstructure and failed largely through pullout mode with significantly improved energy absorption. They exhibited a peak load of 13.27 kN and energy absorption of 27.22 J.

Investigation of thin sheet stainless steel resistance spot welds: Effect of weld current on nugget failure and microstructure

Thin stainless-steel sheets commonly used in food and beverage industry is resistance spot welded in the process of making leak proof containers. The spot-welding parameters are optimized to obtain maximum nugget size and weld strength. In contrast, least importance is given to the temperature developed at the sheet/electrode interface and subsequent oxidation of the weld. It is well known that the oxidation of the stainless-steel results in the depletion of the chromium in the material affecting its corrosion resistance properties. In this context, this paper is aimed at investigating the effect of resistance spot welding current on the chromium depletion and subsequently on the microstructure of 0.4 mm 304 stainless steel sheet weldments. Comparative study between the microstructures of the heat affected and weld zones were carried using optical microscope and scanning electron microscope. The effect of weld current on the morphology of the delta ferrite formed in the weld zone is presented in the paper with help of pseudo-binary phase diagram. An attempt is made to correlate the local chromium depletion with the microstructure and defect formation.

Influence of In Situ Thermal Processing Strategies on the Weldability of Martensitic Stainless Steel Resistance Spot Welds: Effect of Second Pulse Current on the Weld Microstructure and Mechanical Properties

Martensitic stainless steel (MSS) welds are notorious for their susceptibility to low-energy failure due to the formation of brittle martensitic structure in the fusion zone. The unique approach to enhance the mechanical properties of MSS resistance spot welds during both the tensile-shear and the cross-tension loading is to improve the fracture toughness of the fusion zone. In the present study, the effect of double-pulse welding on the microstructure-mechanical properties relationship of the AISI420 MSS resistance spot welds is investigated. Depending on the second pulse current level, various metallurgical phenomena was observed including (i) rapid tempering of martensite in the fusion zone featured by hardness reduction and precipitation of nano-sized carbide precipitates which resulted in remarkable improvement of the load-bearing capacity and energy absorption capability of the welds, (ii) re-austenitization of the fusion zone and reformation of the un-tempered martensite without improvement of the weld mechanical properties, (iii) temper embrittlement of the nugget edge which resulted in very low-energy grain-boundary failure, and (iv) nugget edge re-melting/enlargement which resulted in slight improvement of weld mechanical properties. Therefore, to enhance the mechanical properties of the MSS resistance spot welds using a two-pulse welding strategy, the second pulse parameters should be precisely controlled.

A pathway to produce strong and tough martensitic stainless steels resistance spot welds

ABSTRACTMartensitic stainless steel (MSS) resistance spot welds are prone to quasi-cleavage interfacial failure with very low energy absorption capability due to formation of hard and brittle carbon and chromium rich martensite in the fusion zone (FZ). In this work, a new pathway is proposed to enhance strength/toughness of the MSS resistance spot welds based on modification of the FZ composition/microstructure via introducing a nickel interlayer. This altered the FZ microstructure from dual phase microstructure of martensite and δ-ferrite to austenitic microstructure with finely dispersed ultra-fine chromium rich carbides. Formation of a tough predominately austenitic microstructure in the FZ enabled achieving a pullout failure with an unprecedented level of energy absorption.

Mechanical properties of martensitic stainless steel weld/adhesive hybrid bonds

ABSTRACTHybrid weld-adhesive bonding (AB) was used as an approach to improve the mechanical properties of martensitic stainless steel resistance spot welds. The weld fusion zone in both techniques was composed of predominantly martensite and δ-ferrite. It was found that the presence of adhesive does not induce excess hardening effect in the fusion zone. It was shown that hybrid AB/resistance spot welding) i.e. weld-bonding) technique can double the peak load and energy absorption of the joints compared to those of the resistance spot welds. To obtain high performance weld-bonded joints, heat generation during welding should be controlled to produce weld bonds with large fusion zone size, but without detrimental effect on the adhesive strength.

Fracture toughness of martensitic stainless steel resistance spot welds

The paper is focused on the strength and fracture toughness of AISI420 martensitic stainless steel resistance spot welds under the tensile-shear loading. The failure behavior of AISI420 spot welds was featured by quasi-cleavage interfacial failure with low load bearing capacity and weak energy absorption capability which was a function of the weld fusion microstructure, predominately carbon and chromium rich martensite plus δ-ferrite. Fracture toughness of the fusion zone proved to be the most important factor controlling the peak load of the spot welds made on AISI420 failed in interfacial mode. A geometry-independent fracture toughness of the weld nugget (c.a. 23 MPam0.5) was determined using fracture mechanics concept. Through modeling it was found that there is a critical fracture toughness which beyond that the pullout failure mode can be obtained which is a function of sheet thickness and tensile strength of the base metal.

Defect intelligent identification in resistance spot welding ultrasonic detection based on wavelet packet and neural network

In this paper, ultrasonic echo signals of four kinds of stainless steel resistance spot welds, namely failed weld, stick weld, good weld, and defective weld with gas pore, are analyzed in the time domain, frequency domain, and time-frequency domain based on wavelet packet transform. Fourteen ultrasonic characteristic signals which can reflect the different kinds of spot welds are extracted and can be automatically identified and classified by back-propagation (BP) neural network. The method of this paper can realize the intelligent identification of resistance spot welding defects, and the feasibility of this method has been verified in the experiment.

Effect of joint design on the failure behaviour of three-stack-up austenitic stainless steel resistance spot welds

Tensile shear tests were carried out on three-stack-up austenitic stainless steel resistance spot welds having four types of joint design. Mechanics-based criteria were applied to reveal the difference in the stress state at the microstructural level. Optical microscopy and scanning electron microscopy showed evidence of the different stress states for different joint designs. Nugget rotation generated combined tensile/shear stress at the microstructural level. The peak load and the energy absorption of the joints reduced with the growth of the normal component of the global loads. Joints for which both interfaces bear the load were more prone to failure in pull-out mode.

Welding metallurgy of stainless steels during resistance spot welding Part II –heat affected zone and mechanical performance

Implementation of new materials in automotive body-in-white requires through knowledge of their metallurgical response to welding process thermal cycle. This two-part paper aims at understanding the physical and mechanical metallurgy of stainless steels, as interesting candidates for automotive application, during resistance spot welding. The second part addresses the phase transformations in the heat affected zone of three types of stainless steels including austenitic, ferritic and duplex steels. Failure modes and mechanical properties of stainless steel resistance spot welds are discussed. The peak load and energy absorption of stainless steel resistance spot welds are compared with advanced high strength steels.

Mathematical Modeling of Sensitization in Resistance Spot Welding of SS

Stainless steel (SS) is sensitized by thermal treatment in the range of 450°C to 800°C and inter-granular attack would occur upon subsequent exposure to certain media. This study is focused on modelling the effect of sensitization (by taking into consideration hardness as the weld property) in resistance spot welding (RSW) of 304SS, before and after cryogenic treatment on tungsten electrode. In the present work, outcome of Taguchi model has been used for developing a mathematical model for sensitization; using Buckingham’s π-theorem. The parameters of significance are coolant flow rate (CFR), voltage and time. This study will provide main effects of these variables on hardness of weld joint and will shed light on the sensitization mechanism in RSW of 304SS. The comparison with experimental results will also serve as further validation of model.

Microstructure–properties relationships in martensitic stainless steel resistance spot welds

The paper addresses the phase transformations and mechanical response of martensitic stainless steel resistance spot welds. The fusion zone microstructure consists of carbon-rich martensite together with a relatively high amount of retained delta ferrite along the grain boundaries with a transition in solidification mode from equiaxed to columnar dendritic grains across the fusion zone. The heat affected zone microstructure is featured by martensitic matrix together with carbide precipitation. The very high hardness of the fusion zone and the heat affected zone, the sharpness of the notch at sheet/sheet interface, which is located in the hard microstructural zone, and the presence of delta ferrite in the weld nugget play important roles in failure characteristics and mechanical performance of the joint.

Relationship between failure behaviour and weld fusion zone attributes of austenitic stainless steel resistance spot welds

Resistance spot welding was used to join austenitic stainless steel sheets. Mechanical properties of the spot welds were evaluated using tensile shear test. Mechanical behaviour was described by peak load, failure energy and failure mode. The relationship between weld fusion zone attributes and failure behaviour was studied. Generally, it was observed that increasing fusion zone size is accompanied by an increase in load carrying capacity and energy absorption capability. However, when expulsion occurs, despite almost constant weld fusion zone size, energy absorption capability reduces significantly due to increase in electrode indentation depth. Considering the failure location and failure mechanism in the tensile shear test, minimum required fusion zone size to ensure the pull-out failure mode was estimated using an analytical model. According to this model, in addition to sheet thickness, ratio of fusion zone hardness to failure location hardness is the key metallurgical factor governing failure mode of spot welds during the tensile shear test.