Microstructures and properties analysis of dissimilar metal joint in the friction stir welded copper to aluminum alloy

This paper mainly concentrated on the feasibility of friction stir welding of dissimilar metal of aluminum alloy to copper （I2） and a preliminary analysis of welding parameters influencing on the microstructures and properties of joint was carried out. The results indicated that the thickness of workpiece played an important role in the welding parameters which could succeed in the friction stir welding of dissimilar metal of copper to aluminum alloy, and the parameters were proved to be a narrow choice. The interfacial region between copper and aluminum in the dissimilar joint was not uniformly mixed, constituted with part of incomplete mixing zone, complete mixing zone, dispersion zone and the most region＇ s boundary was obvious. Meantime a kind banded structure with inhomogeneous width was formed. The intermetallic compounds generated during friction stir welding in the interfacial region were mainly CugAl4, Al2Cu etc, and their hardness was higher than oihers.

Research on the creep damage and interfacial failure of dissimilar metal welded joint between 10Cr9Mo1VNbN and 12Cr1MoV steel

The mechanical properties, creep damage, creep rupture strength and features of interfacial failures of welded joints between martensite (SA213T91) and pearlite steel (12Cr1MoV) have been investigated by means of argon tungsten pulsed arc welding, high temperature accelerated simulation, creep rupture, mechanical property tests and scanning electronic microscope (SEM). The research results indicate that the mechanical properties of overmatched and medium matched joint deteriorate obviously, and they are susceptible to creep damage and failure after accelerated simulation operation 500 h, in the condition of preheat 250℃, and post welding heat treatment 750℃×1 h. However, the mechanical properties of undermatched joint are the best, the interfacial failure tendency of undermatched welded joint is less than those of medium and overmatched welded joint. Therefore, it is reasonable that low alloy material TR31 is used as the filler metal of weld between SA213T91and 12Cr1MoV steel.

Corrosion behavior of aluminum/steel dissimilar metals friction stir welding joint

The corrosion performance of aluminum/steel contact and aluminum/steel FSW joint in 3.5 wt.%NaCl solution were analyzed using potentiostatic tests.The post-corrosion microstructure of the welding joint was characterized by optical microscope(OM),scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS).The results showed that the localized corrosion of FSW joint of Al/steel dissimilar metals mainly initiated at the interface transition zone(ITZ).Precipitation of intermetallic compounds(IMCs)and Fe-rich phase particles in ITZ accelerated the corrosion of the FSW joint.This phenomenon has been attributed to distinct corrosion potentials between IMCs and steel,aluminum base metal.The corrosion resistance sequence of IMCs in ITZ is Fe_(3)Al>FeAl>Fe_(2)Al_(5).

Ultrasonic vibration assisted tungsten inert gas welding of dissimilar metals 316L and L415

Ultrasonic vibration assisted tungsten inert gas welding was applied to joining stainless steel 316 L and low alloy high strength steel L415.The effect of ultrasonic vibration on the microstructure and mechanical properties of a dissimilar metal welded joint of 316 L and L415 was systematically investigated.The microstructures of both heat affected zones of L415 and weld metal were substantially refined,and the clusters ofδferrite in traditional tungsten inert gas(TIG)weld were changed to a dispersive distribution via the ultrasonic vibration.The ultrasonic vibration promoted the uniform distribution of elements and decreased the micro-segregation tendency in the weld.With the application of ultrasonic vibration,the average tensile strength and elongation of the joint was improved from 613 to 650 MPa and from 16.15%to31.54%,respectively.The content ofΣ3 grain boundaries around the fusion line zone is higher and the distribution is more uniform in the ultrasonic vibration assisted welded joint compared with the traditional one,indicating an excellent weld metal crack resistance.

Numerical Simulation on Interfacial Creep Failure of Dissimilar Metal Welded Joint between HR3C and T91 Heat-Resistant Steel

The maximum principal stress, von Mises equivalent stress, equivalent creep strain, stress triaxiality in dissimilar metal welded joints between austenitic（HR3C） and martensitic heat-resistant steel（T91） are simulated by FEM at 873 K and under inner pressure of 42.26 MPa. The results show that the maximum principal stress and von Mises equivalent stress are quite high in the vicinity of weld/T91 interface, creep cavities are easy to form and expand in the weld/T91 interface. There are two peaks of equivalent creep strains in welded joint, and the maximum equivalent creep strain is in the place 27-32 mm away from the weld/T91 interface, and there exists creep constrain region in the vicinity of weld/T91 interface. The high stress triaxiality peak is located exactly at the weld/T91 interface. Accordingly, the weld/T91 interface is the weakest site of welded joint. Therefore, using stress triaxiality to describe creep cavity nucleation and expansion and crack development is reasonable for the dissimilar metal welded joint between austenitic and martensitic steel.

Microstructure characteristics of dissimilar metal weld between aluminum alloy and brass

Dissimilar metal joining between 5A02 aluminum alloy and H62 brass sheets was conducted by gas tungsten arc welding with Zn-15% Al and Al-12% Si flux-cored filler wires. The microstructure in the weld and distribution of major alloying elements in the intelfacial layer were examined, and the tensile strength of the resultant joints was measured. Pores appeared in the weld made with Zn-15% Al flax-cored filler wire, the interracial layer mainly consisted of AlCu phase, and the specimens fractured through the weld with tensile strength of 129 MPa. When Al-12% Si flux-cored filler wire was used, Cu diffused into the weld and Al2 Cu phase formed, and the specimens fractured along the interfacial layer with tensile strength of 122 MPa.

Microstructural Evolution on the T91 Dissimilar Metal Joints during Creep Rupture Tests

T91 steel is one of the new materials presently employed in power plant pipe components. The creep rupture strength and microstructure of the T91+10CrMo910 and T91+13CrMo44 welded joints were analyzed during creep rupture tests. Creep transgranular ductile rupture occurred at the 10CrMo910 matrix in the T91+10CrMo910 welded joints and creep intergranular brittle rupture occurred at the 13CrMo44 HAZ in the T91+13CrMo44 joints. Microhardness measurements showed high hardness at the heat affected zone (HAZ) of T91 and a sharply drop at the 13CrMo44 HAZ during creep rupture. The metallographic tests showed that no obvious microstructure degradation was observed in the 10CrMo910 HAZ and matrix, while creep cracks appeared at the 13CrMo44 HAZ. T91 steel had relatively high creep resistant strength in the welded joints tested. Recovery occurred in the T91 HAZ with the growth of subgrain size and the decrease of dislocation density during creep. It was concluded that the dissimilar joints of T91 and low alloy heat-resistant steel should have close creep strength matching to increase the service life of the overall joints at elevated temperature.

Analysis of Laser Micro Welding of Copper-Aluminum Dissimilar Metals and Its Mechanism

Micro welding of dissimilar metals can meet many performance requirements for modern engineering structures. In this experiment, laser micro welding of copper-aluminum dissimilar metals was conducted with an HWLW-300A energy negative feedback Nd:YAG pulse laser. By using the overlap welding method with copper on aluminum, with the laser energy being distributed unevenly, good weld joints were obtained. In this paper, the welding mechanism was analyzed from aspects such as welding temperature and the specific heat capacity of the solid metal. Existing defects were identified, and a feasible improvement scheme was proposed.

Effect of Interface Form on Creep Failure and Life of Dissimilar Metal Welds Involving Nickel-Based Weld Metal and Ferritic Base Metal

For dissimilar metal welds(DMWs)involving nickel-based weld metal(WM)and ferritic heat resistant steel base metal(BM)in power plants,there must be an interface between WM and BM,and this interface suffers mechanical and microstructure mismatches and is often the rupture location of premature failure.In this study,a new form of WM/BM interface form,namely double Y-type interface was designed for the DMWs.Creep behaviors and life of DMWs containing double Y-type interface and conventional I-type interface were compared by finite element analysis and creep tests,and creep failure mechanisms were investigated by stress-strain analysis and microstructure characterization.By applying double Y-type interface instead of conventional I-type interface,failure location of DMW could be shifted from the WM/ferritic heat-affected zone(HAZ)interface into the ferritic HAZ or even the ferritic BM,and the failure mode change improved the creep life of DMW.The interface premature failure of I-type interface DMW was related to the coupling effect of microstructure degradation,stress and strain concentrations,and oxide notch on the WM/HAZ interface.The creep failure of double Y-type interface DMW was the result of Type IV fracture due to the creep voids and micro-cracks on fine-grain boundaries in HAZ,which was a result of the matrix softening of HAZ and lack of precipitate pinning at fine-grain boundaries.The double Y-type interface form separated the stress and strain concentrations in DMW from the WM/HAZ interface,preventing the trigger effect of oxide notch on interface failure and inhibiting the interfacial microstructure cracking.It is a novel scheme to prolong creep life and enhance reliability of DMW,by means of optimizing the interface form,decoupling the damage factors from WM/HAZ interface,and then changing the failure mechanism and shifting the failure location.

Wire-feed laser additive manufacturing of dissimilar metals via dual molten pool interface interlocking mechanism

Intermetallic compounds produced in laser additive manufacturing are the main factors restricting the joint performance of dissimilar metals.To solve this problem,a dual molten pool interface interlocking mechanism was proposed in this study.Based on a dual molten pool interface interlocking mechanism,the dissimilar metals,aluminum alloy and stainless steel,were produced as single-layer and multilayer samples,using the wire-feed laser additive manufacturing directed energy deposition technology.The preferred parameters for the dual molten pool interface interlocking mechanism process of the dissimilar metals,aluminum alloy and stainless steel,were obtained.The matching relationship between the interface connection of dissimilar metals and the process parameters was established.The results demonstrated excellent mechanical occlusion at the connection interface and no apparent intermetallic compound layer.Good feature size and high microhardness were observed under a laser power of 660 W,a wire feeding speed of 55 mm/s,and a platform moving speed of 10 mm/s.Molecular dynamics simulations demonstrated a faster rate of aluminum diffusion in the aluminum alloy substrate to stainless steel under the action of the initial contact force than without the initial contact force.Thus,the dual molten pool interface interlocking mechanism can effectively reduce the intermetallic compound layer when dissimilar metals are connected in the aerospace field.

Achieving High‑Quality Aluminum to Copper Dissimilar Metals Joint via Friction Stir Double‑Riveting Welding

In order to achieve a high-quality joining of aluminum(Al)and copper(Cu)dissimilar metals,a new friction stir doubleriveting welding(FSDRW)with a Cu rod as the rivet was proposed,and the rotating tool with a large concave angle shoulder was specially designed.The results showed that under the thermal–mechanical effect of rotating tool,the Cu rod was deformed to be a double riveting heads structure with a Cu anchor at the upper surface of Al plate and an Al anchor above the lap interface of joint,and these two anchors greatly enhanced the mechanical interlocking of Al/Cu joint.The effective bonding interfaces were formed among the double riveting heads structure,the upper Al plate and the lower Cu plate,which contained the Cu/Cu interface and the Al/Cu interface.The Cu/Cu interface without the kissing bond and the Al/Cu interface with the rationally thin AlCu and Al_(2)Cu intermetallic compounds(IMCs)layers were beneficial to heightening the joint tensile shear strength.The maximum tensile shear load of the FSDRW joint achieved 5.52 kN,and the joint under different plunging depths of rotating tool presented a mixed mode of ductile fracture and brittle fracture.This novel FSDRW technique owns the advantages of strong mechanical interlocking and superb metallurgical bonding,and provides a new approach to acquiring a high-quality Al/Cu dissimilar metals joint.

Microstructure and thermal properties of dissimilar M300–CuCr1Zr alloys by multi-material laser-based powder bed fusion

Multi-material laser-based powder bed fusion (PBF-LB) allows manufacturing of parts with 3-dimensional gradient and additional functionality in a single step. This research focuses on the combination of thermally-conductive CuCr1Zr with hard M300 tool steel.Two interface configurations of M300 on CuCr1Zr and CuCr1Zr on M300 were investigated. Ultra-fine grains form at the interface due to the low mutual solubility of Cu and steel. The material mixing zone size is dependent on the configurations and tunable in the range of0.1–0.3 mm by introducing a separate set of parameters for the interface layers. Microcracks and pores mainly occur in the transition zone.Regardless of these defects, the thermal diffusivity of bimetallic parts with 50vol% of CuCr1Zr significantly increases by 70%–150%compared to pure M300. The thermal diffusivity of CuCr1Zr and the hardness of M300 steel can be enhanced simultaneously by applying the aging heat treatment.

Microstructure and intergranular stress corrosion cracking susceptibility of a SA508-52M-316L dissimilar metal weld joint in primary water

Correlation of microstructure and intergranular stress corrosion cracking （IGSCC） susceptibility for the SA508-52M-316L dissimilar metal weld joint in primary water was investigated by the interrupted slow strain rate tension test following a microstructure characterization. The susceptibility to IGSCC in var- ious regions of the dissimilar metal weld joint was observed to follow the order of Alloy 52 Mb〉 the heat affected zone of 316L〉 the dilution zone of Alloy 52 Mw〉 Alloy 52 Mw weld metal. The chromium- depletion at the grain boundary is the dominant factor causing the high IGSCC susceptibility of Alloy 52 Mh. However, IGSCC initiation in the heat affected zone of 316L is attributed to the increase of resid- ual strain adjacent to the grain boundary. In addition, the decrease of chromium content and increase of residual strain adjacent to the grain boundary increase the IGSCC susceptibility of the dilution zone of Alloy 52 Mw.

Micromechanical Analysis of In-Plane Constraint Effect on Local Fracture Behavior of Cracks in the Weakest Locations of Dissimilar Metal Welded Joint

In this work, a set of GTN （Gurson-Tvergaard-Needleman） parameters of the Alloy52M dissimilar metal welded joint （DMWJ） have been calibrated, and a micromechanical analysis of in-plane constraint effects on the local fracture behavior of two cracks, which located in the weakest regions of the DMWJ, has been investigated by the local approach based on the GTN damage model. The results show that the partition of the material and the variation of the q2 parameter make the J-resistance curves obtained by numerical simulations close to the experimental values. The numerical J-resistance curves and crack growth paths are consistent with the experiment results, which show that the GTN damage model can incorporate the in-plane constraint effect. Furthermore, after the stress, strain and damage fields at the crack tip during the crack propagation process have been calculated, and the change of the J-resistance curves, crack growth paths and fracture mechanism with in-plane constraint have been analyzed.

Multi-scale study of ductility-dip cracking in nickel-based alloy dissimilar metal weld

A ductility-dip-cracking(DDC)-concentrated zone(DCZ) in a width of about 3 mm was observed adjacent to the AISI 316 L/52 Mw fusion boundary(FB) in 52 Mw. The morphology, microstructure, mechanical and thermal properties and corrosion behavior in simulated primary water of DDC/DCZ were investigated by scanning electron microscopy(SEM), transmission electron microscopy(TEM), 3 D X-ray tomography(XRT), 3 D atom probe(3 DAP), slow strain rate tensile(SSRT) testing and thermal dilatometry. The results indicate that DDCs are random-shaped and disc-like cavities with corrugated structure of inner surface and are parallel in groups along straight high-angle boundaries of columnar grains, ranging from micrometers to millimeters in size. Large-size M_(23)C_6 carbides dominate on the grain boundaries rather than MC(M=Nb, Ti), and thus the bonding effect of carbides is absent for the straight grain boundaries.The impurity segregation of O is confirmed for the inner surfaces of DDC. The oxide film formed on the inner surface of DDC(about 50 nm) is approximately twice as thick as that on the matrix(about 25 nm)in simulated primary water. The yield strength, tensile strength and elongation to fracture of 52 MwDCZ(400 MPa, 450 MPa and 20 %, respectively) are lower than those of 52 Mw-MZ(460 MPa, 550 MPa and 28 %, respectively). The intrinsic high-restraint weld structure, the additional stress/strain caused by the thermal expansion difference between AISI 316 L and 52 Mw as well as the detrimental carbide precipitation and the resulting grain boundary structure all add up to cause the occurrence of DCZ in the dissimilar metal weld.

Influence of Rotational Speed on Microstructure and Mechanical Properties of Dissimilar Metal AISI 304-AISI 4140 Continuous Drive Friction Welds

Fundamental investigation of continuous drive friction welding of austenitic stainless steel (AISI 304) and low alloy steel (AISI 4140) is described. The emphasis is made on the influence of rotational speed on the microstructure and mechanical properties such as hardness, tensile strength, notch tensile strength and impact toughness of the dissimilar joints. Hardness profiles across the weld show the interface is harder than the respective parent metals. In general, maximum peak hardness is observed on the stainless steel side, while other peak hardness is on the low alloy steel side. A trough in hardness distribution in between the peaks is located on the low alloy steel side. Peak hardness on the stainless steel and low alloy steel side close to the interface increases with a decrease in rotational speed. All transverse tensile joints fractured on stainless steel side near the interface. Notch tensile strength and impact toughness increase with increase in rotational speed up to 1 500 r/min and decrease thereafter. The mechanism of influence of rotational speed for the observed trends is discussed in the torque, displacement characteristics, heat generation, microstructure, fractography and mechanical properties.

Oxidation damage and interfacial failure of dissimilar metal welds containing ferritic heat resistant steels

The ex-service steam tubes containing dissimilar metal weld(DMW)between high Cr ferritic steel T91 and austenitic stainless steel TP347H and the ex-service steam tubes containing DMW between low Cr ferritic steel G102 and austenitic stainless steel TP347H were obtained from coal-fired thermal power plants in China,and their microstructures at the nickel-based weld metal(WM)/ferritic steel interfaces and oxidation characteristics were investigated.After operating for 15,000 h at steam temperature of 541 C and steam pressure of 17.5 MPa,a G102/TP347H DMW failed along the WM/G102 steel interface,which was a dangerous premature failure mode without obvious plastic deformation.This interfacial failure was attributed to the interaction between oxidation and cracking along the interface,where fracture appeared to be related with the strain concentration at the interface.Oxide notch along the WM/G102 steel interface was the precursor of premature interfacial failure of DMW involving G102.For the DMW involving high Cr ferritic steel T91,ferritic steel side could form a Cr-rich passive film during service and thus would not be further oxidized after operating for 67,000 h at steam temperature of 541 C and steam pressure of 3.5 MPa.It was concluded that oxidation played a more important role in failure of these DMWs,and retarding the development of oxidation and avoiding the interfacial oxide notch would dramatically improve the service performance of steam tubes containing DMWs.

Joining dissimilar metal of Ti and CoCrMo using directed energy deposition

We report laser cladding of pure titanium on a CoCrMo alloy using directed energy deposition.Using electron microscopy,the microstructural evolution upon varying the process parameters,especially laser power and powder feed rate,was investigated in relation to crack formation.Cladding layers showing dilution rates of more than 5%contained cracks due to the formation of the brittle Co_(2)Ti intermetallic phase.The observed cracks could be ascribed to a mismatch in thermal expansion and a resulting stress of more than 440 MPa acting on the Co_(2)Ti phase,as determined by density functional theory and nanoindentation.Furthermore,an excess laser energy caused chemical inhomogeneity and unmelted Ti powder particles,while a deficient laser energy resulted in a lack of fusion.Neither cracks nor partially melted powders were observed for a powder feed rate of 3 g/min and a laser power of 225–300 W,for which the dilution rate was minimized to less than 5%.For such samples,the cladding layers comprised pureα-Ti and a uniform CoT i interface with Co_(2)Ti islands.

Research on microstructure and properties of boron/Q235 steel laser welded dissimilar joints under synchronous thermal field

The mechanical mismatch effect frequently occurs in the dissimilar materials welded joints, thus leading to plastic gradient at the interface between the weld and heat-affected zone(HAZ). In this work, the boron steel and Q235 steel were selected for laser tailor welding,which obtained boron/Q235 steel tailor-welded blanks(TWBs). The method of welding with synchronous thermal field(WSTF) was utilized to eliminate the mismatch effects in TWBs. The WSTF was employed to adjust cooling rates of welded joints, thereby intervening in the solidification behaviors and phase transition of the molten pool. Boron/Q235 steel was welded by laser under conventional and WSTF(300-600 ℃) conditions, respectively. The results show that the microstructure of weld and HAZ(boron) was adequately transitioned to ferrites and pearlites instead of abundant martensite by WSTF. Meanwhile, the discrepancy of microhardness and yield strength between various regions of welded joints was greatly reduced, and the overall plasticity of welded joints was enhanced by WSTF. It is indicated that WSTF can effectively contribute to reducing plastic gradient and achieving mechanical congruity in welded joints by restraining the generation of hardbrittle phase, which could significantly improve the formability of TWBs in subsequent hot stamping.

Numerical Investigation on Residual Stresses of the Safe-End/Nozzle Dissimilar Metal Welded Joint in CAP1400 Nuclear Power Plants

The residual stress evolution in a safe-end/nozzle dissimilar metal welded joint of CAP1400 nuclear power plants was investigated in the manufacturing process by finite element simulation. A finite element model, including cladding,buttering, post-weld heat treatment (PWHT) and dissimilar metal multi-pass welding, is developed based on SYSWELD software to investigate the evolution of residual stress in the aforementioned manufacturing process. The results reveal a large tensile axial residual stress, which exists at the weld zone on the inner surface, leads to a high sensitivity to stress corrosion cracking (SCC). PWHT process before dissimilar metal multi-pass welding process has a great in?uence on the magnitude and distribution of final axial residual stress. The risk of SCC on the inner surface of the pipe will increase if PWHT process is not taken into account. Therefore, such crucial thermal manufacturing process such as cladding, buttering and post-weld heat treatment, besides the multi-pass welding process, should be considered in the numerical model in order to accurately predict the distribution and the magnitude of the residual stress.