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.

Failure Analysis of Dissimilar Metal Welds between Ferritic Heat Resistant Steels and Austenitic Stainless Steels in Power Plant

This study analysed the failure of dissimilar metal welds(DMWs)between ferritic heat resistant steels and austenitic stainless steels and investigated its influencing factors by means of numerical simulation,microstructure characterization and mechanical property test.Under the long-term high-temperature service condition in practical power plant,the DMW failure mode was along the interface between nickel-based weld metal(WM)and ferritic heat resistant steel,and the failure mechanism was stress/strain concentration,microstructure degradation and oxidation coupling acting on the interface.The numerical simulation results show that interface stress/strain concentration was due to the differences in coefficient of thermal expansion and creep strength,and the degree of stress/strain concentration was related to service time.The ferrite band formed at the WM/ferritic steel interface was prone to cracking,attracting the fracture along the interface.The interface crack allowed oxidation to develop along the WM/ferritic steel interface.During long-term service,the interface stress/strain concentration,microstructure and oxidation all evolved,which synergistically promoted interface failure of DMW.However,only under the long-term service of low stress conditions could trigger the interface failure of DMW.Meanwhile,long-term service would reduce the mechanical strength and plasticity of DMW.

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.

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.

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.

Microstructures and mechanical properties of metal inert-gas arc welded Mg-steel dissimilar joints

The joining of Mg alloy to steel was realized by metal inert-gas arc welding, and the weld thermal cycle characteristics and Mg-steel joints were investigated. The results show that the temperature distribution in the joints is uneven. Mg alloy welds present a fine equiaxed grain structure. There exists a transition layer consisting mainly of AlFe, AlFe3 and Mg（Fe, Al）2O4 phases at Mg/steel interface, and it is the weakest link in Mg？steel joints. The welding heat input and weld Al content have the significant effect on the joint strength. The joint strength increases with increasing the heat input from 1680 J/cm to 2093 J/cm, due to promoting Mg/steel interface reaction. When weld Al content is increased to 6.20%, the joint strength reaches 192 MPa, 80% of Mg alloy base metal strength. It is favorable to select the suitable welding heat input and weld Al content for improving joint strength.

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.

New methods of predicting dissimilar steel weld metal microstructures by Schaeffler Diagram

On the base of a number of analyses and researches, some new methods of predicting and expressing the microstructure kinds, of the dissimilar steel welded joint of austenite/pearlite(ferrite) have been presented Those new methods can ascertain the microstructure kind not only in the different characteristic zones of weld metal but also in the different morphologies in the heterogeneous mixture zone of weld metal. Those new methods. enrich and develop the traditional methods of predicting the microstructure of weld metal by Schaeffler Diagram, and are more concise and practical.

Microstructure in the Weld Metal of Austenitic-Pearlitic Dissimilar Steels and Diffusion of Element in the Fusion Zone

Microstructure and alloy element distribution in the welded joint between austenitic stainless steel (1Cr18Ni9Ti) and pearlitic heat-resistant steel (1Cr5Mo) were researched by means of light microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Microstructure, divisions of the fusion zone and elemental diffusion distributions in the welded joints were investigated. Furthermore, solidification microstructure and S-ferrite distribution in the weld metal of these steels are also discussed.

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.

New methods of predicting dissimilar steel weld metal microstructures by Schaeffler Diagram(Part 2)

On the base of the methods of predicting weld metal microstructures of pearlitic dissimilar steel welded joints using austenitic type filler materials by Schaeffler Diagram[1], the other new methods of predicting and expressing weld metal microstnictiires of two kinds of dissimilar steel welded joints (pearlite/pearlite and austenite/pearlite) using austenitic filler materials by Schaeffler Diagram are suggested. Those new methods resolve some difficult problems which the microstructure kinds in two heterogeneous mixture zones of weld metal neighbouring two kinds of welded base metals are difficult to be accurately ascertained and the fluctuations of weld metal microstnictiires across fusion line are difficult to be conveniently expressed according to the traditional predicting method. The new predicting methods are more concise and practical.

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.

Dissimilar welding of high nitrogen stainless steel and low alloy high strength steel under different shielding gas composition:Process,microstructure and mechanical properties

Ar-N_(2)-O_(2)ternary shielding gas is employed in dissimilar welding between high nitrogen steel and low alloy steel.The effect of O_(2)and N_(2)is investigated based on the systematical analysis of the metal transfer,nitrogen escape phenomenon,weld appearance,nondestructive detection,nitrogen content distribution,microstructure and mechanical properties.There are two nitrogen sources of the nitrogen in the weld:high nitrogen base material and shielding gas.The effect of shielding gas is mainly reflected in these two aspects.The change of the droplet transfer mode affects the fusion ratio,N2in the shielding gas can increase nitrogen content and promote the nitrogen uniform distribution.The addition of 2%O_(2)to Ar matrix can change the metal transfer from globular transfer to spray transfer,high nitrogen base material is thereby dissolved more to the molten pool,making nitrogen content increase,ferrite decrease and the mechanical properties improve.When applying N2-containing shielding gas,arc stability becomes poor and short-circuiting transfer frequency increases due to the nitrogen escape from droplets and the molten pool.Performance of the joints is improved with N_(2)increasing,but internal gas pores are easier to appear because of the poor capacity of low alloy steel to dissolve nitrogen,The generation of pores will greatly reduce the impact resistance.4-8%N2content in shielding gas is recommended in this study considering the integrated properties of the dissimilar welded joint.

Cu/V复合中间层对TC4/IN718激光焊接接头组织及性能的影响

为了实现TC4钬合金与GH4169镍基高温合金的有效连接.采用Cu/V复合中间层对TC4和IN718合金进行连续激光焊接,并分析添加中间层对接头裂纹、组织及力学性能的影响机制.结果表明,常规激光焊接时,TC4/IN718接头焊缝区产生大量Ti-Ni脆性金属间化合物,导致接头形成大量纵向裂纹,焊缝组织为Ti(s,s)+Ti_(2)Ni+Ti-Cr+NiTi+Ni_(3)Ti+Cr(s,s).当采用Cu/V复合中间层后,实现了TC4与IN718合金的有效连接,接头抗拉强度达到271MPa,焊缝组织转变为Ti(s,s)+V(s,s)+NiV_(3)+Cr(s,s)+Cu(s,s)+未熔铜。

铝/钢异种金属旋转摩擦焊接技术研究进展

铝/钢异种金属结构以其质量轻、设计柔性大的特点在航空航天领域尤其是大推力液体运载火箭输送管路中有着重要应用。相比于熔焊、钎焊等其他焊接方法,旋转摩擦焊可以实现铝/钢异种金属高质、高强、高可靠连接。本文基于对铝/钢异种金属焊接性分析,从工艺参数优化、界面显微组织分析、界面不均匀性调控和添加中间层金属冶金调控等方面综述了铝/钢异种金属旋转摩擦焊接技术研究进展,并对铝/钢异种金属旋转摩擦焊接技术发展趋势及亟须解决的科学问题进行了总结。

基于分区表征的异质金属搅拌摩擦焊接残余变形数值模拟

为了研究焊缝材料屈服强度变化与焊接残余变形之间的关联机制,提出了基于力学性能分区表征的异质金属搅拌摩擦焊接残余变形计算方法。通过搅拌摩擦焊接实验研究了搅拌头转动方向对搅拌摩擦焊接搅拌区的力学性能曲线影响,并以实验测定的材料本构关系定义搅拌区材料的屈服强度,通过分区表征不同的屈服强度,建立了搅拌摩擦焊接的顺序热力耦合模型,结合分区表征模型研究了搅拌区屈服强度变化对异质金属搅拌摩擦焊接残余变形的影响。结果表明,前进侧材料为2024-T3,后退侧材料为6061-T6时,搅拌区屈服强度为171 MPa、抗拉强度为194 MPa,前进侧和后退侧材料互换,搅拌区屈服强度为166 MPa、抗拉强度为186 MPa。前进侧材料和后退侧材料互换后屈服强度和抗拉强度产生差异的原因是前进侧和后退侧材料的流动行为差异。以实测的力学性能曲线为依据,建立了搅拌摩擦焊接异质合金的顺序热力耦合模型,前进侧材料为2024-T3,后退侧材料为6061-T6时,搅拌摩擦焊接构件最大残余变形为1.2 mm,前进侧材料为6061-T6,后退侧材料为2024-T3时,搅拌摩擦焊接构件最大残余变形为1.3 mm。随着搅拌区材料屈服强度提升,搅拌区弯曲刚度增加,导致残余变形减小。