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.

Lap-Shear Performance of Weld-Bonded Mg Alloy and Austenitic Stainless Steel in Three-Sheet Stack-Up

With the growing interest in utilizing Mg and austenitic stainless steel(ASS)in the automotive sector,joining them together in three-sheet configuration is inevitable.However,achieving this task presents considerable challenges due to the large differences in their physical,metallurgical and mechanical properties.To overcome these challenges,the feasibility of using weld-bonding to join Mg alloy/ASS/ASS was investigated.The nugget formation,interface characteristics,microstructure and mechanical properties of the joints were investigated.The results show that the connection between the Mg alloy and upper ASS was achieved through the combined effect of the cured adhesive and weld-brazing in the weld zone.On the other hand,a metallurgical bond was formed at the ASS/ASS interface.The Mg nugget microstructure exhibited fine columar grains composed predominantly of primaryα-Mg grains along with a eutectic mixture ofα-Mg andβ-Mg17Al12.The nugget formed at the ASS/ASS interface consisted largely of columnar grains of austenite,with some equiaxed dendritic grains formed at the centerline of the joint.The weld-bonded joints exhibited an average peak load and energy absorption of about 8.5 kN and 17 J,respectively(the conventional RSW joints failed with minimal or no load application).The failure mode of the joints changed with increasing welding current from interfacial failure via the Mg nugget/upper ASS interface to partial interfacial failure(part of the Mg nugget was pulled out of the Mg sheet).Both failure modes were accompanied by cohesive failure in the adhesive zone.

Weldability of steel used in an aged bridge

A series of experiments was performed to investigate the weldability of steel used in an aged bridge.A steel material used in an aged railway bridge constructed in 1912 was extracted for this investigation.The chemical compositions of this steel were suitable for welding.However,the aged steel contained much sulfur.Cruciform welded joints were fabricated with this aged steel.Welding defects or cracks were not observed in the joints.The Vickers hardness test on the welded part did not confirm extreme hardening or softening.After yielding by the static tensile test,the cruciform joints were fractured at the welded parts.One of the specimens was fractured in the middle of the thickness of the aged steel.The Sulfur contained in the aged steel might cause this type of fracture.The results show that there may be a risk of brittle fracture not only from the welded part but also from the base metal.The chemical compositions of aged steel must be analyzed when repair welding is applied to the steel.

Design of Fully Automatic Specification Selection System for Resistance Welding Equipment

A system for fully automatic selection of welding specifications in resistance welding equipment has been developed to address the problem of workers frequently choosing the wrong specifications during manual welding of multiple parts on a single machine in automobile factories. The system incorporates an automatic recognition system for different workpiece materials using the added machine fixture,visual detection system for nuts and bolts,and secondary graphical confirmation to ensure the correctness of specification calling. This system achieves reliable,fully automatic selection of welding specifications in resistance welding equipment and has shown significant effects in improving welding quality for massproduced workpieces,while solving the problem of specification calling errors that can occur with traditional methods involving process charts and code adjustments. This system is particularly suitable for promoting applications in manual welding of multiple parts on a single machine in automobile factories,ensuring correct specification calling and welding quality.

Effect of process parameters on the morphology of aluminum/copper alloy lap joints by red and blue hybrid laser welding

In order to overcome the problems of many pores,large deformation and unstable weld quality of traditional laser welded aluminumcopper alloy joints,a red-blue dual-beam laser source and a swinging laser were introduced for welding.T2 copper and 6063 aluminum thin plates were lap welded by coaxial dual-beam laser welding.The morphology of weld cross section was compared to explore the influence of process parameters on the formation of lap joints.The microstructure characteristics of the weld zone were observed and compared by optical microscope.The results show that the addition of laser beam swing can eliminate the internal pores of the weld.With the increase of the swing width,the weld depth decreases,and the weld width increases first and then decreases.The influence of welding speed on the weld cross section morphology is similar to that of swing width.With the increase of welding speed,the weld width increases first and then decreases,while the weld depth decreases all the time.This is because that the red laser is used as the main heat source to melt the base metals,with the increase of red laser power,the weld depth increases.As an auxiliary laser source,blue laser reduces the total energy consumption,consequently,the effective heat input increases and the spatter is restrained effectively.As a result,the increase of red laser power has an enhancement effect on the weld width and weld depth.When the swing width is 1.2 mm,the red laser power is 550 W,the blue laser power is 500 W,and the welding speed is 35 mm/s,the weld forming is the best.The lap joint of T2 copper and 6063 aluminum alloy thin plate can be connected stably with the hybrid of blue laser.The effect rules of laser beam swing on the weld formation were obtained,which improved the quality of the joints.

基于SYSWELD的等高齿弧锥齿轮铣刀盘多层焊接数值模拟

本文借助Visual-Environment焊接模拟仿真软件,以SYSWELD为求解器对等高齿弧锥齿轮铣刀盘V形坡口多层焊接的焊接温度场进行数值模拟,得到了焊接温度场分布云图,分析并对比了每层焊缝中心、焊趾及距焊缝中心5mm处的温度循环曲线,推导出三层焊接温度场公式用于计算验证。结果表明,熔池中心温度最高,距离焊缝中心越近,温度梯度变化越大。

Analysis of Aluminum Alloy Double-Pulse MIG Welding Arc Signal Characteristics Based on Broadband Mode Decomposition

Welding voltage and current in arc signals are directly related to arc stability and welding quality.Process experiments with different parameters were organized according to the orthogonal experimental design method by constructing an aluminum alloy double-pulse metal inert gas(MIG)welding arc electric signal test platform.The data acquisition system of the aluminum alloy MIG welding process was established to obtain real-time arc signal information reflecting the welding process.The aluminum alloy’s collected double-pulse arc current signals are decomposed adaptively by broadband mode decomposition(BMD).The direct current(DC)signal,pulse signal,distortion signal,ripple signal,and noise signal are separated and extracted,and the composite multiscale fuzzy entropy(CMFE)is calculated for the component set of the electrical signal.The experimental results show that the current waveform obtained by the double-pulse MIG welding current signal is consistent with the corresponding weld forming diagram.Simultaneously,the composite multiscale fuzzy entropy is calculated for the arc characteristic parameters.The rationality of matching process parameters and arc stability of aluminum alloy’s double-pulse MIG welding were evaluated.

Recent research progress in the mechanism and suppression of fusion welding-induced liquation cracking of nickel based superalloys

Nickel-based superalloys are extensively used in the crucial hot-section components of industrial gas turbines,aeronautics,and astronautics because of their excellent mechanical properties and corrosion resistance at high temperatures.Fusion welding serves as an effective means for joining and repairing these alloys;however,fusion welding-induced liquation cracking has been a challenging issue.This paper comprehensively reviewed recent liquation cracking,discussing the formation mechanisms,cracking criteria,and remedies.In recent investigations,regulating material composition,changing the preweld heat treatment of the base metal,optimizing the welding process parameters,and applying auxiliary control methods are effective strategies for mitigating cracks.To promote the application of nickel-based superalloys,further research on the combination impact of multiple elements on cracking prevention and specific quantitative criteria for liquation cracking is necessary.

Frequency Domain Fatigue Evaluation on SCR Girth-Weld Based on Structural Stress

The Steel Catenary Riser(SCR)is a vital component for transporting oil and gas from the seabed to the floating platform.The harsh environmental conditions and complex platform motion make the SCR’s girth-weld prone to fatigue failure.The structural stress fatigue theory and Master S-N curve method provide accurate predictions for the fatigue damage on the welded joints,which demonstrate significant potential and compatibility in multi-axial and random fatigue evaluation.Here,we propose a new frequency fatigue model subjected to welded joints of SCR under multiaxial stress,which fully integrates the mesh-insensitive structural stress and frequency domain random process and transforms the conventional welding fatigue technique of SCR into a spectrum analysis technique utilizing structural stress.Besides,a full-scale FE model of SCR with welds is established to obtain the modal structural stress of the girth weld and the frequency response function(FRF)of modal coordinate,and a biaxial fatigue evaluation about the girth weld of the SCR can be achieved by taking the effects of multi-load correlation and pipe-soil interaction into account.The research results indicate that the frequency-domain fatigue results are aligned with the time-domain results,meeting the fatigue evaluation requirements of the SCR.

Improvement of Microstructure and Mechanical Properties of Rapid Cooling Friction Stir-welded A1050 Pure Aluminum

Two-mm thick A1050 pure aluminum plates were successfully joined by conventional and rapid cooling friction stir welding(FSW), respectively. The microstructure and mechanical properties of the welded joints were investigated by electron backscatter diffraction characterization, Vickers hardness measurements, and tensile testing. The results showed that liquid CO_(2) coolant significantly reduced the peak temperature and increased the cooling rate, so the rapidly cooled FSW joint exhibited fine grains with a large number of dislocations. The grain refinement mechanism of the FSW A1050 pure aluminum joint was primarily attributed to the combined effects of continuous dynamic recrystallization, grain subdivision, and geometric dynamic recrystallization. Compared with conventional FSW, the yield strength, ultimate tensile strength, and fracture elongation of rapidly cooled FSW joint were significantly enhanced, and the welding efficiency was increased from 80% to 93%. The enhanced mechanical properties and improved synergy of strength and ductility were obtained due to the increased dislocation density and remarkable grain refinement. The wear of the tool can produce several WC particles retained in the joint, and the contribution of second phase strengthening to the enhanced strength should not be ignored.

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.

A review of linear friction welding of Ni -based superalloys

Ni-based superalloys are one of the most important materials employed in high-temperature applications within the aerospace and nuclear energy industries and in gas turbines due to their excellent corrosion,radiation,fatigue resistance,and high-temperature strength.Linear friction welding(LFW)is a new joining technology with near-net-forming characteristics that can be used for the manu-facture and repair of a wide range of aerospace components.This paper reviews published works on LFW of Ni-based superalloys with the aim of understanding the characteristics of frictional heat generation and extrusion deformation,microstructures,mechanical proper-ties,flash morphology,residual stresses,creep,and fatigue of Ni-based superalloy weldments produced with LFW to enable future optim-um utilization of the LFW process.

Beam oscillating parameters on pore inhibition,recrystallization and grain boundary characteristics of laser-arc hybrid welded AZ31 magnesium alloy

Oscillating laser-arc hybrid welding of AZ31B magnesium alloy was carried out,the effects of beam oscillation parameters on pore inhibition,microstructure,grain boundary characteristics and tensile properties were investigated.The results showed that the pore formation can be inhibited with oscillating frequency higher than 75 Hz and radius smaller than 0.5 mm.The columnar grains neighboring the fusion line can be broken by the beam oscillation behavior,while the grain growth was promoted with the increase of frequency or radius.It should be noted that the coincidence site lattice(CSL)boundaries were mainlyΣ13b andΣ29 boundaries,which were contributed by{10■2}tensile twins and{11■2}compression twins,respectively.The total fraction of CSL boundaries reached maximum at radius of 0.25 mm and frequency of 75 Hz,which was also confirmed as the optimized parameters.In this case,the elongation rate increased up to 13.2%,12.8%higher than that of the weld without beam oscillation.Finally,the pore formation and inhibition mechanisms were illustrated according to the state of melt flow and keyhole formation,the abnormal growth was discussed basing on secondary recrystallization,and the relationship among the pore formation,grain size,boundary characteristics and weld toughness were finally established.

Strengthening strategy for high-performance friction stir lap welded joints based on 5083 Al alloy

During aircraft,ship,and automobile manufacturing,lap structures are frequently produced among Al alloy skins,wall panels,and stiffeners.The occurrence of welding defects severely decreases mechanical properties during friction stir lap welding(FSLW).This study focuses on investigating the effects of rotation rate,multipass welding,and cooling methods on lap defect formation,microstructural evolution,and mechanical properties.Hook defects were eliminated by decreasing welding speed,applying two-pass FLSW with a small welding tool,and introducing additional water cooling,thus leading to a remarkable increase in effective sheet thickness and lap width.This above strategy yielded defect-free joints with an ultrafine-grained microstructure and increased tensile shear force from 298 to 551 N/mm.The fracture behavior of FSLW joints was systematically studied,and a fracture factor of lap joints was proposed to predict their fracture mode.By reducing the rotation rate,using two-pass welding,and employing additional water cooling strategies,an enlarged,strengthened,and defect-free lap zone with refined ultrafine grains was achieved with a quality comparable to that of lap welds based on 7xxx Al alloys.Importantly,this study provides a valuable FSLW method for eliminating hook defects and improving joint performance.

Theoretical analysis of the elastic Kelvin-Helmholtz instability in explosive weldings

By considering the joint effects of the Kelvin-Helmholtz(KH) and Rayleigh-Taylor(RT) instabilities, this paper presents an interpretation of the wavy patterns that occur in explosive welding. It is assumed that the elasticity of the material at the interface effectively determines the wavelength, because explosive welding is basically a solid-state welding process. To this end, an analytical model of elastic hydrodynamic instabilities is proposed, and the most unstable mode is selected in the solid phase. Similar approaches have been widely used to study the interfacial behavior of solid metals in high-energy-density physics. By comparing the experimental and theoretical results, it is concluded that thermal softening,which significantly reduces the shear modulus, is necessary and sufficient for successful welding. The thermal softening is verified by theoretical analysis of the increase in temperature due to the impacting and sliding of the flyer and base plates, and some experimental observations are qualitatively validated.In summary, the combined effect of the KH and RT instabilities in solids determines the wavy morphology, and our theoretical results are in good qualitative agreement with experimental and numerical observations.

3D modeling for effect of tool eccentricity on coupled thermal and material flow characteristics during friction stir welding

A novel three-dimensional numerical model is proposed to investigate the effect of tool eccentricity on the coupled thermal and material flow characteristics in friction stir welding(FSW) process.An asymmetrical boundary condition at the tool-workpiece interface,and the dynamic mesh technique are both employed for the consideration of the tool eccentricity during tool rotating.It is found that tool eccentricity induces the periodical variation of the heat densities both at the tool-workpiece interface and inside the shear layer,but the fluctuation amplitudes of the heat density variations are limited.However,it is demonstrated that tool eccentricity results in significant variation of the material flow behavior in one tool rotating period.Moreover,the material velocity variation at the retreating side is particularly important for the formation of the periodic characteristics in FSW.The modeling result is found to be in good agreement with the experimental one.

Microstructure homogeneity and mechanical properties of laser-arc hybrid welded AZ31B magnesium alloy

Laser-arc hybrid welding of AZ31B magnesium alloy was carried out,the effects of welding parameters on weld formation,microstructure homogeneity and mechanical properties were investigated.The results showed that laser-arc hybrid welding was beneficial to improve the weld formation of magnesium alloy by inhibiting the defect of undercut and pores.The weld microstructure was mainly columnar grains neighboring the fusion line and equiaxed grains at the weld center.It was interesting that the grain size at the upper arc zone was smaller than that at the lower laser zone,with the difference mainly affected by laser power rather than welding current and welding speed.The welding parameters were optimized as laser power of 3.5 kW,welding current of 100 A and welding speed of 1.5 m/min.In this case,the weld was free of undercut and pores,and the tensile strength and elongation rate reached 252 MPa and 11.2%,respectively.Finally,the microstructure homogeneity was illustrated according to the heat distribution,and the evolution law of tensile properties was discussed basing on the weld formation and microstructure characteristics.

A critical review on solid-state welding of high entropy alloys-processing,microstructural characteristics and mechanical properties of joints

The high entropy alloys(HEAs)are the newly developed high-performance materials that have gained significant importance in defence,nuclear and aerospace sector due to their superior mechanical properties,heat resistance,high temperature strength and corrosion resistance.These alloys are manufactured by the equal mixing or larger proportions of five or more alloying elements.HEAs exhibit superior mechanical performance compared to traditional engineering alloys because of the extensive alloying composition and higher entropy of mixing.Solid state welding(SSW)techniques such as friction stir welding(FSW),rotary friction welding(RFW),diffusion bonding(DB)and explosive welding(EW)have been efficiently deployed for improving the microstructural integrity and mechanical properties of welded HEA joints.The HEA interlayers revealed greater potential in supressing the formation of deleterious intermetallic phases and maximizing the mechanical properties of HEAs joints.The similar and dissimilar joining of HEAs has been manifested to be viable for HEA systems which further expands their industrial applications.Thus,the main objective of this review paper is to present a critical review of current state of research,challenges and opportunities and main directions in SSW of HEAs mainly CoCrFeNiMn and Al_xCoCrFeNi alloys.The state of the art of problems,progress and future outlook in SSW of HEAs are critically reviewed by considering the formation of phases,microstructural evolution and mechanical properties of HEAs joints.

Bulging Distortion of Austenitic Stainless Steel Sheet on the Partially Penetrated Side of Non-Penetration Lap Laser Welding Joint

Non-penetration laser welding of lap joints in austenitic stainless steel sheets is commonly preferred in fields where the surface quality is of utmost importance.However,the application of non-penetration welded austenitic stainless steel parts is limited owing to the micro bulging distortion that occurs on the back surface of the partial penetration side.In this paper,non-penetration lap laser welding experiments,were conducted on galvanized and SUS304 austenitic stainless steel plates using a fiber laser,to investigate the mechanism of bulging distortion.A comparative experiment of DC01 galvanized steel-Q235 carbon steel lap laser welding was carried out,and the deflection and distortion profile of partially penetrated side of the sheets were measured using a noncontact laser interferometer.In addition,the cold-rolled SUS304 was subjected to heat holding at different temperatures and water quenching after bending to characterize its microstructure under tensile and compressive stress.The results show that,during the heating stage of the thermal cycle of laser lap welding,the partial penetration side of the SUS304 steel sheet generates compressive stress,which extrudes the material in the heat-affected zone to the outside of the back of the SUS304 steel sheet,thereby forming a bulge.The findings of these experiments can be of great value for controlling the distortion of the partial penetrated side of austenitic stainless steel sheet during laser non-penetration lap welding.

Microstructure and mechanical properties of continuous drive friction welded Ti_(2)AlNb alloy under different rotational rates

Ti_(2)AlNb-based alloy was joined in a continuous drive friction welding machine under different rotational rates(500,1000 and 1500 r/min).The microstructure and mechanical properties of the joints were investigated.It is shown that the weld zone(WZ) is fully composed of recrystallized B2 phase,and the grain size decreases with increasing rotational rate.The thermo-mechanically affected zone(TMAZ) suffers severe deformation during welding,due to which most of original precipitation phase is dissolved and streamlines are present.In the heat affected zone(HAZ),only the fine O phase is dissolved.The as-welded joint produced using 1000 r/min has the best mechanical properties,whose strength and elongation are both close to those of the base metal,while the as-welded joint obtained using 500 r/min exhibits the worst mechanical properties.Post-weld annealing treatment annihilates the deformation microstructure and fine O phase precipitates in the joints,consequently improving the mechanical properties significantly.Decomposed α_(2) phase is a weakness for the mechanical performance of the joint since microcracks are apt to form in it in the tensile test.