Tensile Shear Load Research Articles

Effect of Double Pulse Resistance Spot Welding Process on 15B22 Hot Stamped Boron Steel

Double pulse resistance spot welding process by applying a second step welding current is a new pathway to alter the mechanical properties for advanced high strength steels. Herein, the resistance spot welding (RSW) of hot stamped boron steel 15B22 by one-step and two-step welding with different welding currents is investigated. The results of the tensile–shear test, size of the weld nugget, hardness distribution, microstructure, and failure mode of different welding parameters are analyzed. The weldment of the two-step RSW with a higher heat input exhibits a lower tensile–shear load and lower fracture energy when the size of the weld nugget is large. The microstructural study reveals the appearance of a partially melted zone and sub-critical heat affected zone in the weldment where the fracture readily occurred. Thus, the two-step RSW process weakens the strength of the sample, which is attributed to the partial softening in the weldment due to the higher heat input.

Pinless friction stir spot welding of aluminium alloy with copper interlayer

Abstract Spot welding joints of Al-Mg-Si alloy (AA6061-T6) were produced with and without the addition of copper interlayer using pinless friction stir spot welding (P-FSSW). To investigate the effects of welding parameters on the metallurgical and mechanical properties of the weldment, various tool plunge depth and dwell time were used. Optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy (EDS) have been used for microstructural characterisation.Meanwhile, the mechanical characterisation of the welded joints was evaluated by tensile-shear test. The experimental results showed that a larger bonding area and sound joint were achieved with the addition of Cu interlayer due to the improvement in thermal distribution. Also, an alloying reaction took place between the aluminium substrate and Cu interlayer during P-FSSW, forming intermetallic compounds layer in the interface through the diffusion process. The increasing of dwell time and plunge depth to some extent were beneficial to the formation of the joint and diffusion process, and thus increasing the tensile-shear load of the joints. The observed fracture mode of the joint was either completely shear off in the interface or complete nugget pullout.

Microstructure and mechanical properties of copper to nickel ultrasonic spot welds

Ultrasonic spot welding has been used for joining copper and nickel sheets. With increasing the welding time, the lap shear tensile load of Cu/Ni joints first increases up to the maximum (1879.1 ± 39.3 N), and then decreases. The hardness of the Cu side along the weld interface increases due to enhanced plastic deformation and element diffusion at the Cu/Ni interface. The microjoints expand from the weld interface below the peak of the sonotrode tip to the weld interface below the valley of the sonotrode tip. The joining mechanism is element diffusion, the generation and progressive expanding of microjoints, and mechanical interlocking.

A Quality Study of a Self-Piercing Riveted Joint between Vibration-Damping Aluminum Alloy and Dissimilar Materials

This study investigates the quality of self-piercing riveted joints between vibration-damping aluminum (Al) and other dissimilar materials, namely aluminum alloy (AL5052-H32), steel alloy (GA590DP), and carbon-reinforced plastic (CFRP). The effects of die types (flat, cone, and nipple) on the geometrical characteristics and mechanical performance of the joints are studied using a cross-section examination and tensile shear load testing. The failure modes of each joint are also presented, showing the nature of the forces leading to the joint failures. The results indicate that, for all configurations, adequate joining between vibration-damping Al with AL5052-H32 is expected with a maximum shear load up to 3.28 kN. A shear load up to 3.6 kN was measured for the joints with GA590DP panels with acceptable top and bottom seal characteristics. A vibration-damping Al panel can only be positioned at the bottom when riveting with CFRP due to the brittle nature of CFRP. A tensile shear load up to 2.26 kN was found, which is the lowest amongst the materials tested in this study.

Clarifying effect of welding conditions on microstructure and mechanical properties of friction stir spot-welded DH590 automotive high-strength steel plates

Friction stir spot welding was successfully applied to the 1.2-mm-thick DH590 dual-phase steel plates by using a polycrystalline cubic boron nitride rotating tool. During welding, the rotation speed ranged from 600 to 1000 r/min and the penetration depth ranged from 0.1 to 0.3 mm. In the spot joints, the size of the stir zone increased with the increase in rotation speed as well as the penetration depth of the tool. Comparing with the banded ferrite and martensite structure of the base metal, a mixed microstructure of ferrite and tempered martensite, refined bainite structure and coarse martensite structure were found in the heat-affected zone, thermomechanically affected zone and stir zone of the joints, respectively. Two kinds of interfacial shapes were formed between the upper and lower steel plates, and the formation of the bonded interface was dominated mainly by the penetration depth of the rotating tool rather than the rotation speed. It was revealed that the joints with straight interfaces showed higher shear tensile loads comparing with those with hook-like interfaces. Shear tensile tests showed that the maximum shear tensile load reached about 15.56 kN when the rotation speed and penetration depth were set as 800 r/min and 0.3 mm, respectively. The specimen was fractured through plug failure mode with a total elongation of about 5.6 mm.

Influence of Welding Speed on Characteristics of Non-Axisymmetric Laser-Tungsten Inert Gas Hybrid Welded Mg/Al Lap Joints with Zn Filler.

A non-axisymmetric laser-tungsten inert gas (TIG) heat source was designed to join Mg–Al dissimilar metals with pure Zn filler at a series of welding speeds (500–900 mm/min). Laser and TIG heat sources respectively acted on Al and Mg base metals to precisely control their dissolution into the welding pool. The solidification rate of liquid metal was controlled by adjusting the welding speed, then the reaction process of Mg, Al and Zn could be accurately regulated. The results indicated that various microstructures including Al solid solution, Zn solid solution, Mg–Zn intermetallic compounds (IMCs) and eutectic structure formed in the joint produced at different speeds. Lower welding speed (500 mm/min) caused the microstructure coarsening and higher welding speed (900 mm/min) would lead to the enrichment of MgZn2 intermetallic compounds. At the optimal welding speed of 800 mm/min in particular, fine MgZn2 IMCs grains uniformly distributed in the Al and Zn solid solution. The tensile-shear load reached a maximum of 1052.5 N/cm and the joint fractured at the fusion zone near the Al base metal.

Effects of welding parameters on micro-junction structure and fracture behavior of refill friction stir spot welded joints for 2060 aluminum alloys

Due to different filling modes of the material in sleeve-affected zone (SAZ), probe-affected zone (PAZ), and thermo-mechanically affected zone (TMAZ), the micro-junction structure is formed at the bottom of the SAZ of the refill FSSW joint for 3.2-mm-thick 2060 aluminum alloys. The plasticized degree of the filling material and refilling time greatly affect the morphologies of micro-junction structure. Thus, the welding defects such as void with different sizes, kissing bond, and crack are easily formed in the micro-junction structure. Besides, the tensile-shear properties are related to the micro-junction structure and bonding strength of the lap interface. The increase of the plunge depth or rotation speed within a certain range enhances the flow of material in the stir zone and increases the atomic diffusion capacity of the lap interface, which is beneficial to increasing the tensile-shear load. The maximum tensile-shear load was obtained when the rotation speed of 2500 rpm and the plunge depth of 4 mm are used, and the tensile-shear specimen fractures along the bottom of the SAZ. Other fracture paths are also obtained under different welding parameters combinations, first along the lap interface, and second, not only along the lap interface but along the SAZ bottom.

The investigation of microstructure and mechanical properties of resistance spot welded AISI 316L austenitic stainless steel

Today, the resistance spot welding (RSW) has become one of the most deployed welding techniques in different industries, with the bulk of automobile industries deploying it due to the simplicity, automation, and higher process speed. Past studies are a deficit of findings involving the use of 3 mm thick sheets. The bulk of the studies focused on the spot welding of 316L with a thickness below 2 mm. The lack of studies on resistance spot welding of 316L stainless steel with a thickness of more than 2 mm is evidenced. This paper aims to investigate the mechanical and microstructure properties of resistance spot welded 316L stainless steel with 3 mm thickness. The effect of constant welding parameters on the weld properties such as weld nugget size, tensile–shear load of welded materials, failure modes, Microhardness, and microstructure of weld nuggets were investigated. The results show that the maximum tensile- shear load was 21.549 KN. The Microhardness of the Fusion Zone (FZ) was 230 HV which was higher than that of both Base Metal (BM) (198.8 HV) and Heat Affect Zone (HAZ) (184 HV) respectively. The optical microscopic images showed phase transformation in the FZ.

The effect of holding time on dissimilar resistance spot welding of stainless steel 316L and Ti6Al4V titanium alloy with aluminum interlayer

A combination of stainless steel and titanium alloy is obviously a good choice for the industrial application using resistance spot welding process. However, to provide good quality of the welded joint was difficult due to the different types of materials. To enhance the weld quality, aluminum was placed in between stainless steel and titanium alloy as an interlayer. The mechanical and physical properties of the welded joint were investigated. The results of the SS316L/Ti6Al4V joint show that the tensile-shear load was 895 N and the maximum hardness value was 367.3 HV under the weld current of 12 kA, welding time and holding time of 10 cycles and electrode force of 4 kN. The microstructure result shows that a very thin reaction layer was formed on the intermetallic compound of the SS316L/AA5754 interface. Meanwhile, a diffusion–reaction has occurred on the interface of AA5754/Ti6Al4V during resistance spot welding. The effect of holding time was investigated due to the void appears in the middle of the weld nugget. The existence of the void will significantly reduce weld strength. The increase of holding time successfully remove the void and increase the weld strength of the joint under the tensile-shear test.

High-productivity and high-strength Fe/Al dissimilar metal joining by spot forge welding

To develop a high-productivity dissimilar joining technique, which is highly sought after in the automobile assembly process, I developed a new method based on spot forge-welding. In the lap joint of 1-mm-thick SPCC steel and A5083 aluminum plates, within the 10-mm forge diameter, a base metal fracture occurred at a reduction ratio R of 1.8. The maximum tensile-shear load reached 3.8 kN during the approximately 80-ms forge welding process. The newly formed surface at the bonded interface exhibited a mesoscopic reaction layer, in which the brittleness was minimal given the short diffusion time. Thus, this approach shows great potential to replace conventional resistance spot welding for next-generation solid-state spot welding of dissimilar joints, for high-throughput welding applications and high-strength joints.

Multi-objective optimization of the resistance spot welding process using a hybrid approach

This study proposed an approach to optimize the process parameters using the entropy weight method combining regression analysis in the resistance spot welding process. Based on the central composite experimental design, tests were carried out with three levels of process parameters for spot-welded titanium alloy sheets. Multiple quality characteristics, namely nugget diameter, maximum displacement, tensile shear load, and failure energy, were converted into a comprehensive welding quality index. The weight for each quality index to obtain the comprehensive welding quality index was determined based on the grey entropy method. The welding heat input for each welding joints was calculated based on the dynamic power signal in the welding process. The mathematical model correlating process parameters and the comprehensive welding quality index was established on the basis of regression analysis. The relationship between the welding process parameters and welding heat was also quantified using a regression model. The effects of welding process parameters on welding quality and welding heat were also discussed. To optimize multi-performance characteristics, the desirability function was employed. The verification test results proved that the method proposed in this paper effectively optimized the welding parameters and kept the welding heat input as low as possible at the same time. Welding current is the most significant parameter affecting the welding quality followed by welding time. This can be owing to its direct influence on the amount of heat supplied to the welding zone during the welding process. The method proposed in this study can serve as a guidance and recommendation for resistance spot welding welders to guarantee welding quality and meet the needs of high production and effective energy saving.

Refill Friction Stir Spot Welding of Dissimilar AA6014 Al Alloy and Carbon-Fiber-Reinforced Polymer Composite

Both Al and carbon-fiber-reinforced polymer (CFRP) are being investigated as lightweight materials for automobile applications. To form effective hybrid materials from Al and CFRP, strong bonding between these two dissimilar materials is critical. Here, we analyzed the microstructure and tensile shear strength of joints between an Al alloy (AA 6014) and CFRP fabricated using refill friction stir spot welding (RFSSW). A maximum tensile shear load of 1.6 kN was measured for untreated AA 6014/CFRP specimens. Microscopy images of the Al–CFRP joints exhibited tight bonding along the joint interface without any noticeable defects. Moreover, Al–O–C, C–C, and C=O chemical bonds were detected on the tensile–shear fracture surfaces of both the Al alloy and CFRP sheets using X-ray photoelectron spectroscopy. Micro-mechanical interlocking and chemical bond formation were the key factors contributing to the tensile shear strength of the RFSSW joint. This study demonstrated that the RFSSW technique is a suitable bonding method for manufacturing metal–polymer hybrid structures.

Analytical model for determining effective stiffness and mechanical behavior of polymer matrix composite laminates using continuum damage mechanics

The present paper proposes a new analytical model for predicting the effective stiffness of composite laminates with fiber breaks and transverse cracks. The model is based on continuum damage mechanics and the classical laminate theory. We derived damage variables describing stiffness reduction due to fiber breaks and its maximum value during ultimate tensile failure from the global load-sharing model. Furthermore, a simplified analytical model is presented for obtaining two damage variables for a cracked ply subjected to transverse tensile loading or in-plane shear loading. This model was developed assuming that the displacement field of the longitudinal direction can be expressed in the form of a quadric function by loosening the boundary condition for the governing differential equation. For verifying the developed model, the elastic constants of damaged composite laminates were predicted for cross-ply and angle-ply laminates and compared with the finite element analysis results. As for the appropriate expression of the effective elastic stiffness matrix of the damaged ply, we verified four types of effective compliance/stiffness matrices including the Murakami, Yoshimura, Li, and Maimí models. We found the Maimí model to be the most appropriate among these four models. Moreover, we successfully simplified the expressions for damage variables in the complicated infinite series obtained in our previous study. We also proved that this could contribute toward improving the accuracy of our analysis. After verifying the present model, the stress–strain response and failure strength of carbon- or glass-fiber-reinforced plastic cross-ply laminates were predicted using Maimí’s compliance model and the simplified damage variables.

Bonding mechanism of Al/steel interface formed by laser-TIG welding assisted riveting technology

Laser-tungsten inert gas (TIG) welding assisted riveting technology was used to join Q460 high strength steel and AA6061-T651 aluminum alloy. The effect of laser power on the mechanical properties of Al/steel joints was studied. In this study, the welding-riveting hybrid structure was used to weaken the adverse effect of thermal-physical properties differences between aluminum alloy and steel, and the distribution of Fe-Al intermetallic compounds was optimized by adjusting laser power to obtain the joint with the best mechanical properties. Due to the different laser power, there were two fracture modes of Al/steel joints, namely, button-type fracture and pull-out fracture. The failure processes of these two fracture modes were discussed. Under the appropriate welding parameters, the maximum tensile shear load of the joint could reach to 6.3 kN, which was 92.2 % of the Q460 base metal. The Al/steel interface of the joint was mainly composed of Al, Fe4Al13, Fe2Al5, and α-Fe phases. When the favorable orientation relationships of the Fe4Al13/Al system, Fe4Al13/Fe2Al5 system, and Fe2Al5/α-Fe system in the Al/steel interface were selected, there was a semi-coherent-coherent-coherent matching relationship between these phases. The Al/steel interface growth model of the Al/steel joint was discussed in this study.

Influence of resistance spot welding process parameters on dissimilar austenitic and duplex stainless steel welded joints

This paper examines the impact of welding parameters on tensile shear fracture load, nugget geometry and microstructure of resistance spot welds (RSW) of austenitic stainless steel AISI 316 L and duplex stainless steel 2205 under lap shear loading condition. The macroscopic examination resulted that many of the nugget lengths were nearer to and higher than the AWS recommended value 4√t and failed at higher tensile shear load. Nugget height for DSS 2205 side was higher in comparison with AISI 316 L due to higher thermal conductivity of duplex stainless steel. Three welding parameters mainly welding current of 9 kA, heating cycle of 9 and electrode tip diameter of 6 mm were discovered as most effectual parameters on the tensile shear load and microstructure of weldments. Heterogeneous hardness was observed in the fusion zone due to the transition of equiaxed to columnar grains takes place in the both sides of nugget edge. DSS HAZ nearby BM observed higher hardness and ASS HAZ nearby BM reported lower hardness. WMZ Microstructure confirmed that thickness of austenite layers increased with heat input. Also, an unmixed zone in the microstructure identified as HAZ which contains delta ferrite. Scanning Electron Microscope (SEM) images in the nugget zone for different welding parameters confirmed that Intra-Granular Austenite (IGA) highly developed at higher welding current. SEM fractrograph for the tensile sheared specimens at higher and lower heat input confirmed the ductile type fracture even failed at Inter-Facial (IF) mode. Nugget area and nugget hardness were positively correlated with Tensile Shear Fracture Load (TSFL).

Effects of rotation speed and dwell time on the mechanical properties and microstructure of dissimilar aluminum‐titanium alloys by friction stir spot welding (FSSW)

AbstractIn this study, the effects of rotation speed and dwell time on the mechanical properties and microstructure of friction stir spot welded joints of dissimilar aluminum and titanium alloys were investigated. Aluminum AA6061 and titanium Ti‐6Al‐4 V alloys were selected as the work piece. The joint quality, mechanical behavior, and microstructural evolution in the welded regions were considerably affected by the welding parameters. The results of scanning electron microscopy showed the formation of Ti3Al intermetallic compounds near the thermomechanical affected zone, which significantly affected the properties of the welding joint. Maximum tensile shear load was produced at 1000 min−1 and 10 s dwell time. Moreover, the welding joint microhardness was improved with increasing the rotation speed.

Dissimilar resistance spot welding of 6061-T6 aluminum alloy/St-12 carbon steel using a high entropy alloy interlayer

The microstructure and mechanical properties of dissimilar resistance spot weldments between the sheets of 6061-T6 aluminum alloy and St-12 low carbon steel using Al0·5FeCoCrNi high entropy alloy (HEA) interlayers (with the thicknesses of 0.4, 0.7 and 1 mm) were studied and compared to interlayer-free weldment. Although HEA contained Al and Fe, its microstructure showed no evidence of Al–Fe intermetallic compounds (IMCs). Barely visible interfaces formed at the carbon steel/HEA side of all weldments, suggesting metallurgical consistency between both materials. Conversely, complex IMCs formed at the interfaces of Al/HEA, as a result of strong chemical tendency of HEA constituent elements towards the aluminum alloy. The application of HEA interlayer was found to change the type of the formed IMCs from AlxFey in the interlayer-free weldment to complex IMCs. The latter prevented cracking in the Al-side of the weldments. Meanwhile, it increased the tendency to hot tearing in the Al-side of the weldments. The thickness of HEA interlayer affected the morphology of IMCs. For instance, a 1 mm-thick interlayer produced needle-shaped complex IMCs owing to initial dissolution of IMCs into the molten aluminum and subsequent eutectic solidification. The weldments fabricated by HEA interlayer exhibited improvement of tensile shear load compared to the interlayer-free joint. Distinct fracture mechanisms were involved in the weldments produced with and without HEA interlayer. The pre-existing cracks and hot tearing at the Al-side of the joint were deduced to be responsible for the fracture initiation in the fabricated weldments without and with HEA interlayer, respectively.

Analysis of tensile-shear strength of single and multi-friction stir spot welding joints under fixed welding process conditions

The tensile-shear strength of AA 5052 single and multi-friction stir spot welding joints were analyzed using experimental, numerical, and analytical approaches. Benchmark specimens were designed and manufactured in a similar manner with respect to industrial practice. Under the fixed welding process condition, the failure mechanism of friction stir spot welded specimens under tensile-shear loading was first determined by using macro- and micro-structural analysis. It is shown that increasing the tool shoulder diameter and the number of friction stir spot weldings may nonproportionally increase the strength of the joints. In the linearly arranged multi-friction stir spot welding joints, the strength of these joints was discussed using analytical approach. It is demonstrated that in certain cases, increasing the nugget diameter is preferred than increasing the number of nuggets. This is only applicable to a certain friction stir spot welding failure mechanism. A finite element model prediction tool was developed to predict the tensile-shear strength of friction stir spot welded joints using the material properties obtained from the measurement of experimental hardness.

A Novel Friction Stir Spot Riveting of Al/Cu Dissimilar Materials

In order to eliminate the disadvantages of the keyhole in conventional friction stir spot welding joint and attain the high-strength lap joint of Al/Cu dissimilar metals, a novel welding technique, named as friction stir spot riveting (FSSR), was proposed. A pinless tool and an extra filling stud were employed. The Al/Cu spot joints without keyhole defect were achieved by the FSSR. A Cu anchor-like structure was formed, which greatly increased the mechanical interlocking between the upper Al sheet and lower Cu sheet. The thin intermetallic compounds containing CuAl2 and CuAl at the Al/Cu interface strengthened the joining interface between the Al sheet and the Cu stud. Increasing rotating velocity increased frictional heat and plastic deformation and then eliminated the interfacial joining defects. The FSSR joint with the maximum tensile shear load of 3.50 kN was achieved at a rotating velocity of 1800 rpm and a dwell time of 20 s, whose fracture path passed through the softened region of upper Al sheet. In summary, the novel FSSR technique has the advantages of strong mechanical interlocking and metallurgical bonding between dissimilar materials, thereby attaining the high-strength spot joint.

Fracture properties of reclaimed asphalt pavement mixtures with rejuvenator

Reclaimed asphalt pavement (RAP) technology has been extensively promoted to conserve depleting virgin materials for asphalt mixtures. High RAP content is desirable from economic and environmental standpoints. However, RAP mixtures become too stiff and require modification such as rejuvenator. This paper presents the evaluation on the fracture characteristics of mixtures prepared with 50% and 70% RAP, with and without rejuvenator that were subjected to indirect tensile strength (ITS) and notched semi-circular bending (SCB) tests. The fractured surfaces of the tested specimens were quantified using geospatial imaging technique to identify the proportion contribution to failure, namely cohesive, adhesive and broken aggregates. The results showed that the fractured rejuvenated mixtures were predominantly of the cohesive type when compared with the non-rejuvenated mixtures. On the other hand, the failure modes of non-rejuvenated mixtures were of the adhesive and aggregate failure types. The measured ITS at two temperatures corresponded with the expected damage trends. Similar behaviour was found in the derived fracture energy and pre-peak slope that were obtained from the SCB pure tensile and tensile-shear load–displacement curves. The findings showed that the fracture properties of rejuvenated mixtures performed comparably with virgin mixtures in terms of fracture toughness, tensile strength and proportion of damage contribution.