Fusion Zone Hardness Research Articles

Failure of dissimilar QP980/DP600 advanced high strength steels resistance spot welds

This study investigates the microstructure, failure characteristics, and mechanical properties of dissimilar resistance spot welds between QP980 quenching and partitioning steel and DP600 dual phase steel. The dissimilar spot welds exhibited a heterogeneous microstructure, with a fusion zone predominantly composed of martensite, significantly overmatching both base materials. A fully martensitic structure with grain size gradient was observed in the upper-critical heat affected zone (UCHAZ) on both sides of the dissimilar joint. In the sub-critical heat affected zone (SCHAZ), no significant phase transformation occurred on both sides. However, at high welding currents, a slight tempering of pre-existing martensite was detected on the QP980 side resulting in a minor hardness drop in this zone. There was a critical welding current at which the failure mode was changed from interfacial mode to pullout mode. The dissimilar QP/DP joint showed a lower tendency to fail via interfacial failure mode compared to similar QP/QP and DP/DP joints, attributed to a higher fusion zone hardness overmatching factor and greater nugget rotation. Pullout failure of dissimilar QP/DP combination was assessed in detail through interrupted tensile shear test. It was found that failure at stronger QP980 side occurred by ductile cracking mechanism initiated from the notch tip, while failure at softer DP600 side occurred by through-thickness localized necking. The former was found to be the prior failure mechanism controlling the load bearing capacity of the dissimilar joint. The mechanical properties of dissimilar QP/DP joints were discussed in terms of peak load and energy absorption, and then were compared to those of similar DP/DP and QP/QP joints in the light of weld microstructure and failure mechanisms.

A comparison of different experimental approaches in the determination of dynamic fracture toughness (J1d) behavior for RHA steel cost sustainable MMAW weldments

In the present investigation, the low hydrogen ferrite consumable weldment (weldment-1) and austenitic stainless steel consumable weldment (weldment-2) of rolled and homogenous armor steel (RHA) have been prepared with manual metal arc welding (MMAW). Further, the influence of weldments (weldment-1 and weldment-2) microstructure on the dynamic fracture toughness (J1d), strength, ductility and hardness has been evaluated and compared. The three different approaches have been utilized to estimate J1d for RHA weldments. Further, all three approaches’ outcomes have been compared with the elastic-plastic fracture mechanics (EPFM) approach through Jp-R curve. The results revealed that weldment-1 (yield strength 504.24 MPa and ultimate tensile strength 763.03 MPa) exhibited 26.93 and 25.07% more yield and tensile strength respectively than weldment-2 (yield strength 384.55 MPa and ultimate tensile strength 593.05 MPa). Further, the elongation of weldment-2 (elongation 42%) is 26.72% more than weldment-1 (elongation 32.10%). The joint efficiency and fusion zone hardness of weldment-1 are 5.85 and 57.56%, respectively more than weldment-2. The EPFM approach has good agreement with the correction factor approach (approach 2) and the compliance change rate approach (approach 3) with less than 7% difference in the estimation of dynamic fracture toughness of base metal and weldments (weldment-1 and weldment-2). Further, the percentage difference between maximum load point approach (approach 1) and the EPFM approach is more than 15% difference. The fractographic investigation confirms the ductile fracture in all under-investigated materials.

Microwave joining of SS-430 with a nano SS-316L powder

In this study, a hybrid microwave heating (MH) approach was employed to fabricate SS-430 to SS-430 joints using a nano-based SS-316L powder as a binding agent. The microstructural aspects of the developed fusion zone (FZ) revealed the perfect diffusion bonding between the faying surfaces through the fully melting of the filler material. The formation of martensite was observed at the diffused zone/heat-affected zone along both interfacing surfaces of the SS-430 base metals to be butt welded. The phase analysis of the developed reaction layer in the FZ showed the presence of small amounts of carbide particles (mainly chromium carbides) along with the presence of the main Ni-Fe-Cr enriched solid solution matrix. The hardness of the diffused zone/heat-affected zone is significantly high (760 ± 15HV) due to the presence of martensite followed by FZ hardness (410 ± 8HV) and then followed by base metal hardness (190 ± 4HV). The tensile properties of the fabricated weldment were evaluated by employing a computer-controlled universal testing machine. The tested weldment exhibits 475 MPa with an elongation of about 35%.

Understanding fusion zone hardness in resistance spot welds for advanced high strength steels: Strengthening mechanisms and data-driven modeling

The fusion zone (FZ) hardness of resistance spot welds is a crucial factor affecting the performance and durability of the welds. Failure mode transition, tendency to fail in interfacial mode, interfacial failure load, and the impact of liquid metal embrittlement cracks on the weld strength are influenced by the FZ hardness. Therefore, accurately predicting the FZ hardness in resistance spot welds made on automotive steels is essential. A simple thermal model is used to calculate the time required for the temperature to drop from 800 °C to 500 °C (Δt85). With the aid of continuous cooling transformation (CCT) diagrams and experimental confirmation, it is shown that in most automotive steels, the FZ exhibits an almost full martensitic microstructure. Generally, the FZ hardness tends to increase in the order of interstitial-free (IF), drawing quality specially killed (DQSK), high strength low alloy (HSLA), ferrite-martensite dual-phase steels, transformation induced plasticity (TRIP), and quench & partitioning (Q&P) steels. To predict the FZ hardness, data-driven regression-based models have been developed based on carbon content, carbon equivalent concept, and the strengthening mechanisms of the martensite. Among these models, the model based on martensite strengthening mechanisms showed the best performance and robustness in estimating the FZ hardness. The assessment of the strengthening mechanism of the FZ showed that most important factors determining the FZ hardness are dislocation hardening due to carbon atoms, interface hardening due to block boundaries, and solid solution hardening due to substitutional alloying elements, respectively. A simple relation is proposed to estimate the load-bearing capacity of automotive steel spot welds during interfacial failure based on the FZ hardness.

Study on Al migration, microstructure and hardness in fusion zone of Al-Si coated press-hardened steel by various laser powers

In this paper, the influence of Al distribution in the fusion zone (FZ) on microstructures and properties of welded joints was studied under 13 different laser powers (300 W→3000 W). The results show that Al can quickly accumulate at the upper surface of the FZ and the fusion line. Due to the stirring action of the beam and the dilution of the molten pool metal, Al becomes gradually uniform with the increase of laser power. This work guides the welding process parameters of lap welding the second layer steel plate to obtain uniform weld. In addition, with the decrease of Al content, the microstructure and hardness of the FZ change as follows: Al-based solid solution (Al > 60.2 wt%, 80 ∼ 120 HV) →Fe-Al intermetallic compound (IMC) (10 wt% < Al < 60.2 wt%, 340 ∼ 980 HV) →Fe-based solid solution (Al < 10 wt%, 350 ∼ 530 HV). Therefore, this work revealed the relationship between the distribution-microstructure-hardness of Al and gave the possible microstructures and properties database of Fe/Al welding joints.

Minimizing intermetallic Laves phase evolution and enhancing precipitation strengthening of Superalloy-718 joints using InterPulse magnetic arc constriction and high frequency pulsation

The techniques of InterPulse magnetic constriction and high frequency pulsation of arc in gas tungsten arc welding (GTAW) were deployed to minimize the detrimental intermetallic laves phase evolution in fusion zone (FZ) of Superalloy-718 welds and enhance the precipitation strengthening of joints. The Superalloy-718 joints were developed by using an advanced variant of GTAW popularly known as InterPulse gas tungsten constricted arc welding (IP-GTCAW). The joints were subjected to the post weld heat treatment (PWHT) cycles of direct aging (DA), solution annealing at 980 °C and 1065 °C followed by aging (980STA and 1065STA) respectively. The tensile properties and hardness of joints were evaluated. The microstructures of joints were analyzed using optical (OM) and scanning electron microscopy (SEM). The elemental analysis of Laves phase and dendrite core of FZ of joints was performed by using energy dispersive spectroscopy (EDS). The tensile fractured surfaces were analyzed using SEM. Results showed that the Superalloy-718 joints developed using IP-GTCAW showed better response to PWHT than GTAW joints because of the greater refinement of FZ microstructure and evolution of finer and discrete Laves phases in FZ. The 980STA joints exhibited superior tensile properties than DA and 1065STA joints. It is correlated to the greater dissolution of Laves phases in nickel austenitic (γ) matrix of FZ resulting in more niobium (Nb) accessible for the precipitation strengthening of FZ. The 1065STA joints showed slightly higher hardness of FZ than 980STA joints because of the almost complete dissolution of Laves phases in FZ. However, the tensile properties of 1065STA joints are lower than 980STA joints because of severe grain growth in FZ and BM. The evolution of microvoids at the interface of Laves phase/weld matrix leading to the development and coalescence of microcracks in FZ on tensile loading is the main mechanism responsible for the premature fracture of joints.

Investigation on post-weld heat treatment (PWHT) inputs for controlling average fusion zone hardness in fibre laser-welded modified 9Cr–1Mo steel

Post-weld heat treatment (PWHT) generally relieves stress and enhances weldments’ mechanical properties. Optimisation of PWHT parameters is very much needed to control the process parameters for achieving the desired property requirements. In this research, the PWHT parameters for laser-welded P91 steel were optimised using the response surface methodology-based Box–Behnken methodology. Tempering temperature, heating rate and holding time are considered the input parameters and using analysis of variance (ANOVA), their impact on average fusion zone hardness has been analysed. The results indicate that the hardness values are highly influenced by temperature followed by holding time. A parametric combination of 780°C tempering temperature, 6 hours holding time, and 200°C/h heating rate have evolved as the optimal solution. The autogenous nature of the welding process resulted in a heterogeneous microstructure with untempered lath martensite structure showing precipitate dissolution in the as-welded fusion zone, ultimately resulting in a high average fusion zone hardness of 502 HV0.3, whereas PWHT with the optimal solution resulted in an average fusion zone hardness value of 252 HV0.3. The decrease in hardness value followed by heat treatment is primarily due to the tempering process, which resulted in precipitation within and over the grain boundaries. The validation experiment result using the optimum combination of parameters has been observed to be aligned significantly with the expected values, indicating the adequacy of the developed model.

Transition in Interfacial Failure Mechanism of Resistance Spot Welds during Tensile–Shear Loading: Role of Fusion Zone Hardness

The failure of resistance spot welds through the fusion zone along the sheet/sheet interface (i.e., interfacial failure) is critical for automotive crashworthiness. This paper investigates the effect of fusion zone hardness on the interfacial failure behavior of resistance spot welds during the tensile–shear test. AISI 1040 medium carbon steel, producing a high level of hardness mismatch during resistance spot welding, was selected as the base metal. By ex situ tempering heat treatment, various levels of fusion zone hardness are achieved in the welds with constant fusion zone size. It is shown that the interfacial failure of the spot welds is a competition between ductile shear failure and rapid crack propagation. It is found that there is a critical fusion zone hardness beyond which the interfacial failure mechanism transitions from ductile shear failure to rapid crack propagation. In welds with high fusion zone hardness, the mechanism of interfacial failure is rapid crack growth, and fusion zone fracture toughness is the governing factor for the interfacial failure load. Conversely, in welds with low FZ hardness, the mechanism of interfacial failure is a ductile shear failure, and fusion zone hardness would be the governing factor for the interfacial failure load.

Investigation of the microstructure and mechanical behaviour of resistance spot-welded CR210 steel joints using graphene as an interlayer

The present work investigated the resistance spot weldability of 0.85 mm thick galvanised CR210 steel sheets employing graphene nanoplatelets (GNPs) as an interlayer. The GNPs were drop-casted on the steel surface and resistance spot welding (RSW) was carried out at optimum welding current range of 6–9 kA with a constant weld time of 0.7 s. The lap-shear and cross-tensile tests were conducted to assess the mechanical behaviour of the weldments obtained with GNPs as an interlayer. An enhancement of ∼124% in the lap shear strength was observed in the specimen welded at 9 kA. However, the incorporation of GNPs led to the deterioration of the cross-tensile strength of the welded joints. The analysis of the different zones formed after welding was done using SEM and EBSD techniques which revealed the martensitic structure and finer grains obtained in the fusion zone. The presence of dislocation pileups, nano-precipitates, and interfacial shear stress transfer at the GNP-Fe interface was observed by TEM. Raman spectroscopy was carried out to get an insight into the stresses generated in the GNPs owing to RSW at different welding currents. Fracture surface analysis of lap shear specimens revealed the presence of shear dimples at the nugget zone welded at 7 kA and a mixed mode of fracture was observed in the specimen welded at 8 kA. However, a complete nugget pull-out was consistently found during the lap shear test of the specimen welded at 9 kA. Microhardness study revealed an increase in the fusion zone hardness at different welding parameters owing to the existence of GNPs and martensitic structure.

Effect of Beam Oscillation on Microstructure and Mechanical Properties of Electron Beam Welded EN25 Steel.

EN25 steels have been found to be applicable in shafts, gears, etc., but welding of EN25 steel was performed using electron beam welding with different oscillation beam diameters varying from 2 mm to 0.5 mm. The present study reports the effect of beam oscillation on the evolution of nonmetallic inclusions, microstructures, and mechanical properties of EN25 steel. Heat input calculations showed that the application of beam oscillations resulted in significantly lower heat inputs compared to their non-oscillating counterparts. The highest fraction of the retained austenite (9.35%) was observed in a weld prepared with beam oscillation at a 2-mm oscillation diameter, and it decreased to 3.27% at an oscillating diameter of 0.5 mm, and it further reduced to 0.36% for non-oscillating beam cases. Residual stresses were compressive in the fusion zone, irrespective of beam oscillation. Beam oscillation resulted in equiaxed grain in the recenter region of the fusion zone, attributed to heat mixing and the evolution of random texture. The application of beam oscillations resulted in a significant decrease in the size of the nonmetallic inclusions to 0.1-0.5 compared to 5-20 mm in base metal. All tensile samples failed in the base metal, indicating good strength of the weld. Fusion zone hardness (250-670 HNV) and wear properties (COF 0.7 to COF 0.45) improved irrespective of with and without beam oscillation.

Characterizing local distribution of microstructural features and its correlation with microhardness in resistance spot welded ultra-low-carbon steel: Experimental and finite element characterization

This study investigated the influence of microstructural features on the heterogeneous distribution of hardness in resistance spot welded (RSWed) 340BH steel. The microstructure and texture across the weld region was investigated using optical microscopic and electron backscattered diffraction (EBSD) techniques. Finite element analysis (FEA) was used to simulate the RSW process, which helped in predicting the nugget dimensions as well as the cooling rate in different weld regions. Rapid cooling in the fusion zone (FZ) led to the formation of martensite with typical packet and block substructures. Numerical reconstruction of the prior austenite grain (PAG) was employed to understand the orientation relationship between martensite and austenite. The results showed that child laths obey a Kurdjumov-Sachs orientation relationship (K-S OR) with respect to the austenite matrix with a slight deviation (~3° angular deviation) from a conventional K-S OR. Moreover, microhardness was measured across the weld zone. An increase in the hardness was observed throughout the weld zones. The hardness increase in the heat-affected zone (HAZ) was due to a finer (smaller) grain size and an increase in the dislocation density, whereas in the FZ, the improvement in hardness was attributed to the phase change during the solidification process. The microhardness of martensite formed during RSW was strongly dependent on the block size according to the Hall-Petch relationship. • A heterogeneous distribution of microstructure and hardness in RSWed 340BH steel was characterized using EBSD and FEA. • Lower ZHP values and fast kinetics of dynamic recrystallization resulted in fine structure of ferrite grains in ICHAZ. • FZ hardness was mainly driven by martensite formation, while grain size distribution and dislocation density were mainly responsible for higher hardness in HAZ. • The block size in the martensite substructure followed Hall-Petch type relationship with microhardness of lath martensite.

On the mechanical, microstructural, and corrosion properties of pulsed gas tungsten arc and friction stir welded RZ5 rare earth grade magnesium alloy

Experimental studies have been conducted on the microstructure, mechanical, and corrosion characteristics of magnesium alloy RZ5 that has been butt welded. Pulsed tungsten inert gas (PTIG) and friction stir welding (FSW) are two distinct welding techniques that have been considered. The weld metal of the PTIG joint exhibited finer grain cast structures of 10 μm with coarser intergranular eutectic τ-Phase particles as compared to the coarse-grained cast base metal microstructure of 104 μm. The FSW joints microstructural investigation revealed that the precipitates with globular morphology had spread out throughout the wrought ultrafine α-Mg grains of 2 μm. X-ray elemental distribution and phase analysis indicated that in base metal and weld joints, the grain boundaries and interior zones were enriched with Zn and Zr elements with corresponding secondary phases. Microhardness measurements showed the softening is reduced in the heat-affected zone of FSW compared to PTIG joints. The stir zone exhibits the highest hardness of 120.4 HV0.2 which is 40% higher than the fusion zone hardness of PTIG welds. Electrochemical polarization scans and immersion testing indicated that the weld zone of the FSW joint exhibits higher corrosion resistance than the RZ5 base alloy and PTIG welds. The corrosion data (i.e., higher corrosion potential, lower current density, and higher breakdown potential) obtained from the polarisation scans are correlated with the microstructural features after immersion testing.

Heat-mass transfer and its effects on the in-situ Al/SiC reactions in aluminum welds during wobbling laser melting injection of SiC particles

The mechanical properties of aluminum welds can be enhanced by the addition of SiC particles via laser melting injection (LMI). During LMI, in-situ reactions will happen in aluminum welds with SiC particles (SiC p ) and how to optimize it is challenging. In this paper, a novel process named wobbling laser melting injection (WLMI) is employed to successfully control in-situ reactions of Al/SiC, then improve hardness and wear performance of fusion zone (FZ) on the basis of LMI. Moreover, a multiphase flow model by bidirectionally coupling the computational fluid dynamics and discrete element method (CFD-DEM) is developed to reproduce this process. The temperature and velocity fields of both Al fluid and SiC p during WLMI are characterized by numerical simulation. Firstly, the area greater than 1670 K can go through all parts of molten pool under WLMI compared with LMI that area just stays in the melt middle. Secondly, the temperature of particles prepared with WLMI can reach 1670 K and the duration above 940 K is longer. These special kinetic-thermodynamic behaviors during WLMI will cause in-situ products Al 4 SiC 4 to be distributed evenly in Al substrate, and on SiC/Al interface. Finally, the evenly distributed strengthening phase in Al matrix and the strong bonding force of SiC/Al interface under WLMI result in better microhardness and wear performance of FZ respectively compared with LMI. • In-situ reactions in Al welds with SiC during laser melting injection is optimized by wobbling laser for the first time. • Temperature-velocity field during wobbling laser melting injection is analyzed by CFD-DEM model innovatively. • Influencing mechanism of wobbling and non-wobbling laser on in-situ reactions in Al/SiC system is illustrated. • Microhardness and wear resistance of welds with SiC under laser melting injection, can be improved by wobbling laser.

Microstructure and mechanical properties of gas metal arc welded CoCrFeMnNi joints using a 410 stainless steel filler metal

The use of filler materials during fusion-based welding processes is widely used to regulate and modify the composition of the welded joints aiming at producing a desired microstructure and/or achieving an improvement in its mechanical performance. Welding of high entropy alloys is still a new topic and the impact of different filler materials on the microstructure and mechanical properties is yet unknown. In this work, gas metal arc welding of the CoCrFeMnNi high entropy alloy using 410 stainless steel as a filler wire was performed. The microstructural evolution of the welded joints was evaluated by optical microscopy, scanning electron microscopy aided by electron backscattered diffraction, high energy synchrotron X-ray diffraction and thermodynamic calculations. Meanwhile, the mechanical behavior of the welded joint, as well as the local mechanical response were investigated with microhardness mapping measurements and with non-contact digital image correlation during tensile loading to failure. The weld thermal cycle promoted solid state reactions in the heat affected zone (recovery, recrystallization and grain growth), which impacted the microhardness across the joint. The role of the 410 stainless steel filler material in the solidification path experienced by the fusion zone was evaluated using Scheil-Gulliver calculations, and a good agreement with the experimentally observed phases was observed. Despite the addition of the 410 stainless steel filler was not conducive to an increase in the fusion zone hardness, the associated bead reinforcement promoted an improvement in both the yield and tensile strengths of the joint compared to a similar weld obtained without filler material (355 vs 284 MPa and 641 vs 519 MPa, respectively). This allows to infer that the addition of filler materials for welding high entropy alloys is a viable method for the widespread use of these novel materials. In this work, by coupling microstructure and mechanical property characterization, a correlation between the processing conditions, microstructure and mechanical properties was obtained providing a wider basis for promoting the application of gas metal arc welding of high entropy alloys for industrial applications.

Comparative study on autogenous diode laser, CO2 laser-MIG hybrid and multi-pass TIG welding of 10-mm thick Inconel 617 superalloy

The present study investigated comparative welding performance of single-pass autogenous diode-laser (DLW) and CO2+ MIG laser-hybrid welding (LHW) processes with multi-pass conventional TIG welding in 10-mm thick Inconel 617 with comprehensive correlation of bead profile, microstructure with mechanical performance of weldments employing FESEM, XRD, high-speed Nano Indentation, uniaxial tensile and impact testing. Results indicated that welds produced by LHW and DLW constituted ‘wine-cup’ and ‘Y” shaped profiles respectively with reduced fusion zone (FZ) and heat-affected-zone (HAZ), high depth-to-width aspect ratio and low distortion as compared to that of ‘bath-tub’ shaped TIG weld on account of low heat input and high-coupling efficiency of laser. Indeed, inhomogeneous microstructures were observed in both LHW and TIG welds due to dual heat sources in LHW (laser + MIG) and multiple passes in TIG as compared to that of DLW, wherein, refined microstructure with fine secondary-dendritic arm spacing and low levels of micro-segregation in interdendritic regions of FZ. XRD confirms presence of higher amounts of M23C6 and M6C carbides in TIG FZ as compared to that of LHW and DLW counterparts and attributed to enhanced micro-segregation due to reduced cooling rate and repetitive thermal cycling effect of multiple passes. Micro and Nano-Indentation hardness profiling indicated that the hardness of FZ of TIG welds is comparatively lower to that of DLW and LHW on account of enhanced segregation and grain coarsening effect. Weld joint efficiency obtained in DLW and LHW are slightly higher than TIG weldments with highest elongation observed in DLW. Overall, joint efficiency of LHW and DLW was found adequate with vast reduction in distortion and improved adoptability for applications in thermal power plant.

Effect of beam wobbling on microstructure and hardness during laser welding of X70 pipeline steel

Laser welding with beam wobbling was employed to weld X70 pipeline steel. The influence of beam wobbling on weld surface morphology, fusion zone microstructure, and microhardness was investigated. The formation of spatter was suppressed by beam wobbling using a beam with higher energy density. At a welding speed of 1.0 m min−1, the weld metal produced with a circular laser wobble pattern contained more martensite but less ferrite, compared to that of static spot welded joint. The presence of more martensite led to the increase in fusion zone hardness. A softened region was found in the inter-critical heat-affected zone. The hardness of the softened region could be improved by increasing the welding speed (from 1.0 to 1.5 m min−1).

Effect of heat input on microstructure and mechanical properties of TIG-welded semisolid cast AXE622 Mg alloy

The present study investigats the effect of heat input on the microstructure and mechanical properties of the TIG-welded semisolid AXE622 alloy. Three heat inputs, 18 kJ/mm, 21 kJ/mm, and 24 kJ/mm are used, and the corresponding joints referred to as HI18, HI21, and HI24 joints. Results showed that the average grain size in the heat-affected zone (HAZ) (~38 μm, 50 μm, and 67 μm) was more considerable than that in the fusion zone (FZ) (~15 μm, 17 μm, and 18 μm) for the HI18, HI21, HI24 joint, respectively. The volume fraction of the second phases in FZ was significantly higher concerning the HAZ. The eutectic Mg 17 Al 12 phase formed a network structure in FZ of the HI18 joint and fragmented in the HI21 joint, while less fragmentation was observed in the HI24 joint. The volume fraction, the average length, and the width of secondary phases decreased in the HAZ with increasing the heat input. The HI21 joints exhibited the largest tensile strength (~147 MPa) and elongation (~3%) and ruptured in the BM. With increasing the heat input, the hardness decreased in the HAZ and FZ. Besides, the hardness in FZ (~70 HV, 63 HV, and 58 HV) was more significant than that in the HAZ (~ 50 HV, 47 HV, 43 HV) for the HI18, HI21, and HI24 joint, respectively. The beneficial effect of semisolid processing was degraded due to the significant heat input imposed in the HAZ of the HI24 joint. • Semi-solid Mg alloy specimens were successfully produced and TIG-welded in three heat inputs • With increasing the heat input, the grain size increased more significantly in the HAZ. • The least amount of Mg 17 Al 12 phase was formed in the FZ of the HI21 joint. • The HI21 joints exhibit the largest tensile strength, yield stress, and uniform elongation. • With increasing the heat input, the hardness decreased slightly in the HAZ and FZ.

Effects of notch position on the fatigue crack growth behavior of dissimilar laser welded DP980/QP980 joint

AbstractLaser welding of heterogeneous high‐strength steels (AHSSs) is increasingly used in the automotive industry, while the inhomogeneous microstructure of the joint could affect its fatigue crack growth (FCG) behavior. Therefore, fiber laser welding experiments were performed in this study to evaluate the microstructure, mechanical properties, and effects of notch position on FCG behavior of dissimilar laser welded DP980/QP980 joint. Results show that the hardness of fusion zone (FZ) near the QP980 side was higher and softening zones were observed in the heat affect zone (HAZ) on both sides of the joint. The stress–strain curves of all the samples show continuous yield and follow the two‐stage work hardening mechanism. The weld overmatch effect induced by laser welding could hinder the propagation of fatigue crack. The microstructural inhomogeneity, weld mismatch effect, and crack closure contribute to the variety of FCG rates. Ductile fracture mechanism became more dominant with the increase of ΔK.

The effect of Zr addition on the laser welding of SiCp/2A14Al composite

Zr foil was applied in the laser welding of SiCp/2A14Al composites. The Al4C3 phase and pores was decreased in the fusion zone with Zr addition. The area of fusion zone decreased with the amount of Zr addition. Two new phases (Al3Zr and ZrC) were formed in the fusion zone with Zr addition. Al3Zr was strip-like. ZrC was polygonal particle. ZrC particles were formed around the SiC. The size of Al3Zr and ZrC was smaller than that of Al4C3. Only a thin layer of Al4C3 was formed on surface the SiC particles with Zr addition. The pores and pits left by Al4C3 hydration significantly decreased the hardness of fusion zone without Zr addition. The hardness of fusion zone increased with Zr addition. The tensile strength of welded joint increased from 105.6 MPa (without Zr addition) to 234.7 MPa (with 0.1 mm Zr foil). The mechanical strength of welded joints was significantly improved by Zr addition.

Resistance Spot Welding of Dissimilar Alloys 1008 Low Carbon Steel- 1100 Aluminum Alloy

The resistance spot welding is adopted to joint dissimilar alloys such as aluminum alloy 1100 , and low carbon steel alloy 1008 by using cover plate. This study aims to optimization the best conditions of dissimilar welding of Aluminum with low carbon steel by RSW, and improving the properties of joints by many method. three different variables were used for the welding process: welding current (5, 6 ,7 and 8 KA), weld time (0.5, 1 and 1.5 sec) and electrode force (13.2 and 15.5 N).The welding joints are ―examined by a scanning electron microscope SEM and a X-ray diffraction‖ for the purpose of discussing the causes of the improved characteristics. The results revealed that the best welding conditions were under welding current 7 KA , weld time 1 sec and electrode force 13.2 KN, where the joint possessed the maximum shear force (4.8KN) and after improvement the tensile shear force become (6.02 KN), in addition to presence of the intermetallic compounds at optimum condition , such as AlFe3,Al5Fe4 and Al13Fe4, in the joint layer between dissimilar metal improves of the tensile shear forces and hardness in fusion zone.