Dissimilar metal weld joints (DMWJ) widely exist in the nuclear power plants to join the different parts which are made of different structural materials. Among these DMWJs, safe-end DMWJ has attracted much attention of researchers and operating enterprises, as premature failures, mainly stress corrosion cracking failures, have occurred in these kinds of joints. However, DMWJ with 52M as filler metal in the nuclear power plants has no in-service experience. To ensure the structural integrity of the weld joint and the safe operation of the future plants, the microstructure and local properties of a domestic safe-end DMWJ by using hot-wire gas tungsten arc welding (GTAW) technology was studied in detail by OM, SEM, micro-hardness testing, local mechanical tensile testing and slow strain rate tests. The tensile tests were performed at room temperature with the tensile speed of 5 μm/s while the slow strain rate tests were conducted in simulated primary water containing 1500 mg/L B as H3BO3 and 2.3 mg/L Li as LiOH with 2 mg/L dissolved oxygen at 325 ℃. A large amount of type I boundaries and type II boundaries which are susceptible to stress corrosion cracking (SCC) exist in 52Mb near the SA508/52Mb interface and result in the highest SCC susceptibility of this interface. Microstructure transition was found in the SA508 heat affected zone (HAZ). In 316LN HAZ, increasing the distance from the fusion boundary, the number fraction of CSL boundaries increase while the residual strain decreases, resulting in the second-highest SCC susceptibility of 316LN HAZ. In 52M, residual strain distributes randomly but not uniformly, the residual strain is prone to accumulate at the grain boundaries. Dramatic changes of mechanical properties are observed across the joint, especially at the SA508/52M interface. The differences of the local microstructure and chemical composition lead to the differences of the local properties of the weld joint.
The Q235A mild steel and AISI304 austenite stainless steel were subjected to solid diffusion welding by vacuum diffusion bonding approach to investigate the influence of welding temperature on the interfacial morphology, microstructural constituents and mechanical properties. The results show that the single ferrite layer (zone II) and carbon-enriched layer (zone III) were formed nearby the bonding interface of Q235A mild steel and AISI304 austenite stainless steel, and heterogeneous microstructure on both sides of interface formed a common grain boundary by diffusion. The strength and toughness of the bonded joint reached the highest values, for welding temperature of approximately 850 ℃, welding pressure of beyond 10 MPa, and welding time of approximately 60 min, which was larger than those of the Q235A mild steel layer. Otherwise, the Cr23C6 carbide easily formed at a relatively lower temperature (≤800 ℃), whereas the secondary carbides and intermetallic compounds formed at a relatively higher temperature (≥900 ℃). Both cases would dramatically deteriorate the strength-toughness of the bonded joint. Therefore, it was proposed that the brittle precipitate phases can be effectively avoided by controlling the welding temperature to approximately 850 ℃, thus ensuring the resulting performance of the bonded joint.
SnAgCu solder alloys, such as Sn3.0Ag0.5Cu, Sn3.8Ag0.7Cu and Sn3.9Ag0.6Cu, are widely used for consumer electronics due to their good wettability, high mechanical properties and excellent thermal fatigue reliability. However, the high Ag content in SnAgCu solder can bring about a relatively high cost and poor drop impact reliability because of the formations of thicker brittle Ag3Sn compound during soldering. Therefore, the development of low Ag content SnAgCu solders to satisfy the requirements of electronic production has become a hot topic in this field. In this work, the effects of Sn0.3Ag0.7Cu, Sn1.0Ag0.5Cu and Sn3.0Ag0.5Cu solder on the melting character, wettability, mechanical properties and microstructures, phase composition were investigated by DSC, micro-joint strength tester, SEM, EDS and XRD. Under -55~125 ℃ cyclic conditions, the interfacial layer change of Sn-Ag-Cu solder joints was measured by TL-1000 high and low temperature test chamber. The results showed that, with the Ag content increased, the melting point was not changed, the wetting angle significantly decreased. And the wettability of three solders was improved under N2 atmosphere. Moreover, the mechanical properties of three solder joints were enhanced with the increase of Ag content. The matrix structure of Sn0.3Ag0.7Cu and Sn1.0Ag0.5Cu solder joint have a small amount of Ag3Sn and large Cu6Sn5 particles, and the distribution of particles were disordered. However, the matrix structure of Sn3.0Ag0.5Cu solder joint was obviously uniform. This is the reason that the mechanical properties of Sn0.3Ag0.7Cu and Sn1.0Ag0.5Cu solder joints were lower than that of Sn3.0Ag0.5Cu. In addition, the solder joints were subjected to a thermal cycling reliability test, it was found that the thickness of intermetallic compounds (IMCs) increased, and the morphology of IMCs was gradually changed from scallop-like to planar-like.
The improvement of steam parameters in fossil power plants requires the development of new kinds of 9% Cr martensitic heat-resistant steels, among which FB2 steel is a 100×10-6 (mass fraction) boron-containing steel and mainly used for manufacturing components with thick walls operating at high temperatures above 600 ℃. In the alloy system of martensitic heat-resistant steels, boron plays an important role in suppressing type IV crack of weld joints by the formation of heat affected zone (HAZ) with no fine grains in the normalized and intercritical zones, where there exhibit fine grains in conventional 9%Cr heat-resistant steels with no boron such as P91 steel. In this work, the formation process of HAZ in FB2 steel was investigated. The microstructures before and after thermal simulation were compared using OM and SEM. It was concluded that the austenization of FB2 steel at rapid heating rates (≥100 ℃/s) took place by shear mechanism, demonstrating austenite memory effect; while at slow heating rates (≤5 ℃/s), the austenization was by atom short range diffusion mechanism, without austenite memory effect. The special phase transformation of austenization is the main cause for the formation of HAZ with no coarsened grain in the overheated zone. Based on the previous results reported by other researchers, a preliminary model was proposed to describe how boron atoms change the austenite transformation type of FB2 steel during heating process, which developed the previous ideas about the phenomenon.
As the main corrosion form of coal- or heavy oil-fired boilers, dew point corrosion occurs when corrosive gases (SO3, HCl, NO2, et al) are cooled and converted to condensed acids. The condensed acids (H2SO4, HCl and HNO3) are much corrosive to steel, causing corrosion damage to plant materials. The service temperature is designed lower and lower to improve energy efficiency recently, which makes dew point corrosion more and more serious. Q315NS steel produced by appropriate alloy design is much suitable for those parts vulnerable to dew point corrosion in power and petrochemical industry due to its excellent corrosion resistance in H2SO4 solution. As an efficient and low-cost process, welding is an essential process in the utilization of Q315NS. The corrosion mechanism of the heat affected zone is much complex due to the presence of microstructure gradients, which is largely determined by the welding thermal cycle. However, there is little research elucidating the effect of welding thermal cycle on corrosion behavior of Q315NS steel in H2SO4 solution. In this work, the microstructure evolution and corrosion behaviour in the 50%H2SO4 (mass fraction) solution of welding heat affected zones of Q315NS was investigated by comparison with base metal using welding thermal simulation technique, scanning electron microscope and electrochemical measurements. The results show that the microstructures of ferrite and pearlite are observed in base metal, fine-grained region and incomplete recrystallization region, while coarse-grained region consists of granular bainite. All the equivalent circuits of Q315NS with or without welding thermal cycle contain a resistor of corrosion product and a capacitor of electric double layer, and all specimens have passivation behavior. The base metal and the incomplete recrystallization region have the lowest corrosion current density and the largest charge-transfer resistance, which means the best corrosion resistance, while the coarse-grained region has the highest corrosion current density and the least charge-transfer resistance. Rod-like shaped corrosion product was formed by deposition on the surface of the coarse-grained region specimen while a porous-structured corrosion product was formed on the surface of other specimens.
2A14 aluminum alloy is the important raw materials of aerospace, which belongs to the heat treatment aluminum alloy. Friction stir welding (FSW) can weld aluminum alloy with high quality, and can avoid the pores and cracks of fusion welding effectively. In order to obtain better mechanical properties of FSW joints, the surface nanocrystallization method is introduced into FSW technology. By means of the hybrid surface nanocrystallization (HSNC) method of both supersonic fine particles bombarding (SFPB) and surface mechanical rolling treatment (SMRT), a smooth gradient nanostructured (GNS) layer was formed on the surface of 2A14 aluminum alloy before FSW. The FSW joints microstructure and fracture morphology of the original and HSNC specimens were researched by OM, SEM and TEM. The results showed that nanostructure layer zone (NLZ) was formed when GNS with shape similar to the "S" line was distributed in the thermal-mechanical affected zone (TMAZ) and the nugget zone (NZ) of the HSNC specimen. The lowest micro-hardness and fracture position of the original specimen occurred on the TMAZ of advancing side (AS). The lowest micro-hardness and fracture position of the HSNC specimen occurred on the NZ. The tensile strength of HSNC specimen was 6.4% higher than the original sample. The elongation of HSNC specimen was 14.1% more than the original specimen. The fracture mode of both specimens was toughness fracture. The fracture morphology of the HSNC was isometric dimple when the fracture morphology of original specimen were non-isometric dimple and avulsion dimple. Analysis showed that the NLZ of the FSW joints was beneficial to improving the strength and the plastic deformation capability simultaneously.
It has been recognized that low temperature martensitic transformation can reduce harmful tensile stress and generate beneficial compressive stress in weld zone of single pass welded joints. The influence of martensitic transformation is even greater in 9%Cr steel because of its high hardenability and low transformation temperature (about 100~400 ℃). However, compressive stress was confined in certain parts of weld zone in multi-pass butt-welded 9%Cr steel pipes. In this work, stress evolution in a multi-pass butt-welded 9%Cr steel pipe was predicted using Abaqus software, and the effect of martensitic transformation was further investigated. The simulated results show that the overall pattern for the axial and hoop stresses appears to be similar, despite the lower magnitudes for axial stress. The maximum compressive stress was found in the final weld pass, and the maximum tensile stress was formed in the weld pass adjacent to the final weld pass. Stress in weld passes adjacent to weld root is relatively low. Tensile stress due to thermal contraction in the final weld pass was relieved by martensitic transformation and clear compressive stress was formed. However, little effect of martensitic transformation was found on the significant tensile residual stress in weld passes adjacent to the final weld pass. The final weld pass has the primary effect on the formation of residual stress. Compressive stress was indeed generated by martensitic transformation in former weld pass, however it was relieved by weld thermal cycle of latter weld pass. As a result, the effect of martensitic transformation appears to be confined to the final weld pass. The influence of martensitic transformation is greater around outer surface than that around inner surface.
Compared with the low heat input welding steel structures, the high strength low alloy (HSLA) steel structures after high heat input welding keep high temperature with longer time, and the cooling speed is slower, then the austenite crystal grains of coarse-grained heat affected zones (CGHAZ) grow up sharply, and coarse upper bainite (UB) and ferrite side plate (FSP) are generated easily in original austenite crystal, thus toughness of CGHAZ deteriorates seriously. At present, the approach of improving toughness of CGHAZ is to produce massive interleaved acicular ferrite (AF) in the original austenite crystal. However, with the improvement of welding capability for thick plate, welding heat input will be greater, and the hold time of high temperature will be more prolonged. In this case, AF coarsens much seriously, thus the improvement of CGHAZ toughness is limited severely. In this work, a new method for improving the toughness of CGHAZ in high heat input welding steels by studying the distribution map of HAZ impact value was proposed. This new method changes the grain boundary ferrite (GBF) and AF of the CGHAZ to polygonal ferrite (PF) of the fine-grained heat affected zones (FGHAZ) at same peak temperature, which improves the toughness of CGHAZ significantly. Comparing the microstructures and toughness of CGHAZ in Ti-V-N and Al-Ti-V-N micro alloy welding steels, the transformation condition and nucleation mechanism of PF in the CGHAZ of Al-Ti-V-N steel were analyzed. It is found that micron oxide inclusions is a key factor to inducing the nucleation of massive PF in CGHAZ, and nanoscale carbonitride is a key factor to draging and pinning the grain boundaries of austenite and ferrite. Therefore, the effective combination of above two factors guarantees the generation of a large number of PF, which improves the impact toughness greatly at low temperature.
P92 steel is a typical 9%~12%Cr ferrite heat-resistant steel with good high temperature creep resistance, relatively low linear expansion coefficient and excellent corrosion resistance, so it is one of important structural materials used in supercritical thermal power plants. Fusion welding technology has been widely used to assemble the parts in thermal power plant. When the supercritical unit is in service, its parts are constantly subjected to combination of tensile, bending, twisting and impact loads under high temperature and high pressure, and many problems such as creep, fatigue and brittle fracture often occur. It has been recognized that welding residual stress has a significant impact on creep, fatigue and brittle fracture, so it is necessary to study the residual stress of P92 steel welded joints. The evolution and formation mechanism of welding residual stress in P92 steel joints under multiple thermal cycles were investigated in this work. Based on SYSWELD software, a computational approach considering the couplings among thermal, microstructure and mechanics was developed to simulate welding residual stress in P92 steel joints. Using the developed computational tool, the evolution of residual stress in Satoh test specimens was studied, and welding residual stress distribution in double-pass welded joints was calculated. In the numerical models, the influences of volume change, yield strength variation and plasticity induced by phase transformation on welding residual stress were taken into account in details. Meanwhile, the hole-drilling method and XRD method were employed to measure the residual stress distribution in the double-pass welded joints. The simulated results match the experimental measurements well, and the comparison between measurements and predictions suggests that the computational approach developed by the current study can more accurately predict welding residual stress in multi-pass P92 steel joints. The simulated results show that the longitudinal residual stress distribution around the fusion zone has a clear tension-compression pattern. Compressive longitudinal residual stresses generated in the fusion zone and heat affected-zone (HAZ) in each pass, while tensile stresses produced near the HAZs. In addition, the numerical simulation also suggests that the transverse constraint has a large influence on the transverse residual stress, while it has an insignificant effect on the longitudinal residual stress.
Pipeline steels for sour oil and gas containing H2S generally suffer from either hydrogen-induced cracking (HIC) or sulfide stress corrosion cracking (SSC). Oil and gas containing high concentration H2S are noxious to pipeline steels because of the hydrogen-induced corrosion. In this study, HIC susceptibility of welded MS X70 pipeline steels was evaluated in NACE “A” solution at room temperature. Meanwhile, microstructure and regions near a HIC crack in the MS X70 base steel and its welded joint were analyzed through OM, SEM and EBSD. The hydrogen trapping efficiency was also investigated by measuring the permeability (J∞) and the effective hydrogen diffusivity (Deff). The results showed that both base metal and welded joint were highly susceptible to HIC and the later steel sample was more vulnerable than the former. This higher susceptibility could be primarily attributed to the following effects: the higher hydrogen trapping efficiency of bainitic lath microstructure in the welded joint; the more low angle grain boundary in the welded joint also made it easier to crack by improving the hydrogen trapping efficiency of high angle grain boundary; the less amount of coincidence site lattice grain boundary and Σ13b、Σ29b lead to higher HIC susceptibility by decreasing the resistance to crack of high angle grain boundary.
Aluminium alloys were widely applied in rail transit, ships and aerospace owing to their unique properties, such as low density, high strength and stiffness, outstanding corrosion resistance and low temperature performance. As a type of structure material, aluminium alloy joining was inevitable. However, these alloys were often considered very difficult to weld using traditional fusion welding technique since the welding seams were often accompanied with metallurgical defects, large deformation and stress. Friction stir welding (FSW), an innovative solid-state welding technology invented at the welding institute (TWI), was seen by designers as an effective joining methods in welding aluminium alloys due to low heat input, small stress-strain and environment friendly. In this work, 0.8 mm thick plate of 6061-T6 aluminium alloy was successfully welded by use of high rotational speed fiction stir welding technology. The microstructure and mechanical property of the butt joints prepared by high rotational speed friction stir welding were analysed in detail. The results show that the well surface topography and excellent bonding interface existed in the nugget zone (NZ) were observed. Both of the microhardness of the weld seam was lower than that of the substrate. The lowest microhardness of the butt joints located between the thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). Compared with the conventional rotational speed, the number of β-Mg2Si, Al2CuMg and Al8Fe2Si precipitated phases existed in the NZ was more, which made the microhardness in the NZ improved significantly. The rod-shaped precipitates (Mg2Si) have the greatest influence on the microhardness. The excellent mechanical properties were obtained at the rotational speed of 8000 r/min and welding speed of 1500 mm/min. The maximum tensile strength was 301.8 MPa, which was 85.8% of the as-received 6061-T6 (351.7 MPa). And the toughness-brittleness fracture mode appeared.
High-strength steel has the advantages of high strength, low cost and good hot and cold workability, etc., which is widely used in various fields of national economy as engineering steel, such as bridge, vehicle, ship, pressure vessel and so on. As increasing strength, the plasticity and toughness of high strength steel have not meet the demand in some industrial areas, especially the low temperature impact toughness. Recently, a Fe-Cr-Ni-Mo steel with high-strength and high-toughness has been deve-loped and has been successfully used to prepare high pressure vessels. In this work, metal active gas (MAG) welding with multi-pass welding was used to join a Fe-Cr-Ni-Mo high-strength and high-toughness steel. The microstructure and fracture morphologies of welded joint are investigated by SEM, EPMA and TEM and the micro-hardness, tensile strength and Charpy impact energy are tested as well. The results show that the morphologies of welded metal (WM) consist of columnar crystal (CC) and equiaxed crystal (EC), where the upper WM is predominantly CC and the proportion of EC increases in the lower WM. The microstructure of upper WM is tempered martensite for the faster cooling rate. Because the higher content of alloying elements in lower WM improves the hardening tendencies, the lower WM is granular bainite. The heat affected zone near WM is coarsen martensite and has the highest hardness (621 HV), which is significantly higher than that of the base metal (BM) (410 HV). The hardness of the upper WM is 365 HV, which is lower than that of BM and the lower WM has higher hardness (450 HV). Therefore, the upper tensile sample of welded joint was broken in the WM and the fracture strength is 1109 MPa and lower than that of BM (1190 MPa). While the fracture position of lower tensile sample is in the BM and the strength is about 1183 MPa. The welded joint of experimental Fe-Cr-Ni-Mo steel has higher strength and the welding factor is not lower than 0.93. Moreover, the impact energy of WM is 53 J.
The heat generation, heat transfer and plasticized material flow in friction stir welding determine directly the microstructure evolution and mechanical properties of weld joints. Numerical simulation of these thermo-physical phenomena is of great significance for getting a deep insight into the underlying mechanisms and optimizing the process parameters of friction stir welding. This article reviews the progress status in numerical simulation of heat generation, heat transfer and plasticized material flow behaviors in friction stir welding, and outlines the unsolved problems. The research work targeting these issues, which has been conducted by the authors' group, is introduced. According to the stress characteristics at the tool-workpiece interface, the expressions of sticking rate and friction coefficient are developed, and this measurement-calculation method lays foundation for improving the accuracy of numerical analysis. Through synthetically considering the characteristics of complex-shaped tools, a three dimensional model of friction stir welding process is established. Three types of tools are taken into consideration, i.e., normal CT (conical-pin tool), ST (conical-pin with 4 flats tool) and TT (conical-pin with 3 flats tool). For the cases in application of these tools, the heat generation, temperature profile, and material flow velocity are analyzed quantitatively. A mathematical model for the whole friction stir welding process including plunge stage, dwell stage, welding stage, and cooling stage is established for numerical analysis of transient development in heat generation rate, temperature and material flow fields in each stages. Based on the status review, the trend in numerical simulation of frictions stir welding is outlooked, and the research focus for next step is proposed.
Marine engineering steel is the key material for the construction of major marine infrastructure projects. Due to the harsh environment in the deep sea, the mechanical properties such as strength, low temperature toughness and so on of the marine steel are required to be higher. In this work, the weldability of a Fe-Cr-Ni-Mo high-strength steel was studied, and the microstructure and impact toughness of the steel after welding thermal cycling at different peak temperatures were analyzed. The results show that the average impact toughness of characteristic heat affected zone under different temperatures increases first and then decreases with the increase of peak temperature (Tp). The microstructures of coarse grain heat-affected zone (CGHAZ, Tp=1320 ℃) and fine grain heat-affected zone (FGHAZ, Tp=1020 ℃) are quenched martensite. Because of the coarse grain size, the impact toughness of CGHAZ is poor, which is lower than that of FGHAZ. The microstructure of inter-critical heat-affected zone (ICHAZ, Tp=830 ℃ and Tp=760 ℃) is composed of quenched martensite and tempered martensite. Due to the randomness of the proportion of the interfaces between the mixed microstructures near the V-notch, the impact energy values of ICHAZ fluctuates greatly. The homogeneous fine grain structure in ICHAZ (Tp=830 ℃) has a crack arrest effect during the impact deformation, which makes the characteristic zone have the best impact toughness. Although the grain size in ICHAZ (Tp=760 ℃) is also fine, the existence of the ultra-fine grain zones (the grain size in which is only 1~2 μm) benefits the formation of secondary voids under the impact load. The undissolved M2C and MC precipitations in matrix promote the connecting of secondary voids and then form the secondary cracks. As a result, the impact toughness of the characteristic zone is poor, and becomes the weak region of HAZ.
X70 pipeline steel with thick specifications (40.5 mm) for 3500 m deep sea reached the international advanced level in the wall thickness and service depth. Due to the high heat input during the welding process, the corrosion resistance of inside welding and outside welding would vary depending on the microstructure differences. The corrosion resistance of the welded joints of X70 pipeline for deep sea was studied by the immersion test, the weight loss test, the electrochemical test in this work. The components of the passive film were analyzed by XRD and the microstructure was observed by SEM. The results show that the corrosion resistance of the weld metal is the best. The corrosion resistance of the heat affected zone follows. The corrosion resistance of the base metal is the worst. And for the same area, the corrosion resistance of the inside welding is better than that of the outside welding. The formation of dense Fe3O4 passivation film can effectively slow down the progress of the reaction, and the corrosion products of Fe2O3, FeOOH and Fe(OH)3 which are loose in the outer layer, have no protective effect on the matrix. The microstructure of the weld metal with the best corrosion resistance is mostly the intragranular nucleation ferrite and martensite-austenite (M-A) constituent is fine and uniform. The microstructure gradient of the heat affected zone is the largest, the M-A constituent is coarse and the corrosion resistance is inferior to the weld metal. The base metal consists of ferrite and bainite, the bainite is island-like distribution and the corrosion resistance is the worst. Microstructure of the inside welding is more refined, owing to the influence of outside welding thermal cycle, and the volume fraction of M-A constituent in inside welding is higher than that of the outside welding, so the corrosion resistance is better than that of the outside welding.
The 7075 aluminum alloys have major applications in commercial, transportation industry and military air carriers, owing to their associated light weight, high strength, good machinability, high fracture toughness and low fatigue crack growth. Several welding techniques, such as metal inert gas (MIG) welding, tungsten inert gas (TIG) welding, laser welding and friction stir welding (FSW), have been applied to weld the 7075 aluminum alloys. However, their applications are limited because of the lower weld strength, slower welding speed and other significant limitations of them. Among the different welding techniques, plasma-MIG hybrid welding is a new fabrication technique with many advantages such as stable welding process, no weld spatter, the decreased pores, small grain size and high joint quality. Up to now, the study mainly focuses on coaxial plasma-MIG hybrid welding, and it is rare in dealing with the hot cracking susceptibility of 7000 series aluminum alloys welded by paraxial plasma-MIG hybrid welding. In this work, the paraxial plasma-MIG hybrid welding system was used to weld 7075-T6 aluminum alloy plates. The quantitative relationship between plasma-MIG hybrid welding parameters of 7075 aluminum alloy and weld penetration was established by linear regression orthogonal test. Hot ductility tests were studied by using the thermal simulated test to determine the brittleness temperature range of the alloy. Welding hot cracking susceptibility tests were conducted by using the fish bone method, and the type and cause of the hot cracking were analyzed by SEM, EDS and OM. The results indicated that the brittleness temperature range of 7075 aluminum alloy was 470~620 ℃. When the heat inputs of plasma-MIG hybrid welding were 2.52, 2.95 and 3.42 kJ/cm respectively, the welding hot cracking susceptibility decreased and then increased with the heat input increasing. The type of cracking in partially melted zone of base metal was liquation cracking, and that of weld zone was solidification cracking. When the heat input was 2.95 kJ/cm, the welding hot cracking sensitivity was the least, and the welding cracking was solidification cracking. Compared to MIG welding joints, the hot cracking susceptibility of plasma-MIG hybrid welding joints decreased by 47% under the same conditions.
S31042 steels with 25%Cr (mass fraction) and 20%Ni have been served as super-heaters and re-heaters in ultra-super critical (USC) plants, owing to their outstanding corrosion resistance and creep rupture strength. And the reliability of joints at high temperature has attracted much attention since the S31042 steels have been joined successfully by linear friction welding. In this work, the microstructures and mechanical properties of linear friction welded S31042 steel joint subjected to ageing treatment were investigated by using OM, SEM, TEM and mechanical test at 700 ℃. The recrystallized grains and nanoscale NbCrN particles have been stable during the high-temperature ageing, and the joint exhibited excellent performance due to the grain refinement strengthening and precipitation strengthening. The average size of M23C6 phase in weld zone, thermo-mechanically affected zone and heat affected zone increased with the ageing time. After ageing treatment at 700 ℃ for 500 h, σ phase precipitated at boundary junctions in thermo-mechanically affected zone. The average size of σ phase increased with the ageing time, as well as the volume fraction of the σ-phase. With the formation of σ phase, the fracture site of joints shifted from the parent material to the areas adjacent to the weld zone, and the high-temperature mechanical properties of joints were sharply decreased.
Titanium and its alloys with fine corrosion resistance and specific tenacity are widely used in the fields of astronautics, chemical industry and so on. While the pipeline steels with low price and good mechanical properties are always used in petroleum industry. For now, composite panels are widely used in petrochemical industry, aerospace engineering and other fields, which can combine the respective features of the dissimilar materials together so as to meet the special requirements and save a lot of rare and precious metals. Previous studies have showed that the joining of titanium and steel suffered from two major challenges: one was the emergence of continuous distributed intermetallics of TiFe and TiFe2 in the weld, which could cause brittle fracture with low strength; the other was the occurrence of residual stresses that were caused by the great differences in thermal properties between titanium and steel. This work is aimed to join the explosion-bonded TA1/Cu/X65 trimetallic sheets (titanium flyer plate with thickness 2 mm, copper intermediate plate 1 mm, and X65 base plate 12 mm) with Cu-based flux-cored wires by the tungsten inert gas (TIG) welding. The microstructure and mechanical properties of welded joint was characterized by using SEM, EDS, TEM, XRD and tensile and microhardness tests. The results indicated that the filler metals for each weld layer have obvious zoning by using solid solution phases and intermetallic compounds. There was about 150 μm width Ti-Cu reaction zone between the Ti weld and transition layer weld. The microstructures of Cu-Ag-Mo-Nb/ER50-6 transition interface were composed of Fe-based and Cu-based solid solution. The intermediate copper played an important role in reducing the high temperature residence time of welded joints so as to reduce the interdiffusion of Ti, Fe element. Consequently, the hard-brittle Ti-Fe intermetallic compounds were partly replaced by Cu-based solid solution and Ti-Cu, Ti-Ag intermetallic compounds with relatively good ductility and toughness. The average tensile strength of the butt joints is 507 MPa at room temperature, mainly of that of X65 was obtained. ERTi-1 weld metal exhibited higher hardness than Cu-Ag-Mo-Nb weld metal, and their microhardness values were 507 and 447 HV100, respectively. In addition, the microhardness in reaction zone presented a slightly drop. The lowest values occurred in ER50-6 weld metal.
With the rapid development of Chinese high-speed railway system, the urgent demand for lighter weight structures is increasing, and aluminum alloys are widely applied into manufacturing the railway train and critical safety components. As a medium strength aluminum alloy, the 7020 aluminum alloy shows a great potential. Hybrid laser welding has currently become one of the most important welding techniques for medium and high strength aluminum alloys. Nevertheless, intrinsic defects such as pores and shrinkages physically determine the fatigue resistance of the welded joint. Based on in situ synchrotron radiation X-ray computed microtomography (SR-μCT), the population, location and size of gas pores within AA7020 hybrid welded joints are firstly identified and counted. The critical size of gas pores, affecting the fatigue properties of welded joints, is acquired by combining the statistical results of the pores and the average grain size of the hybrid weld. Meanwhile, the qualitative relationship between pore size, effective stress and fatigue life is discussed through in situ fatigue life data via SR-μCT and fracture morphology. By using the finite element analysis, detailed works have been performed on the stress state near the pores of different positions inside the joint. Through the simulation analysis, the stress concentration coefficient around the pores firstly increases, then decreases, and finally tends to a stable trend as the location of the pore-like defect is transferred from the surface to the inside. Besides, the influence of porosity on fatigue crack initiation, fatigue crack growth and sudden breaking process is also analyzed using fatigue crack growth experiment. In conclusion, the results show that the critical pore size of hybrid laser welded joint can be qualitatively identified as 30 μm; the SR-μCT and fracture analysis show that larger surface and sub-surface pores are more likely to initiate fatigue cracks, and the fatigue crack propagation experiment further shows that the porosity has very little effect on the long crack growth but significant influence on the crack front.
In recent years, the increasing application demand for Mg alloys in automobile, rail transport, aviation and aerospace industries brings about the growing prominence of seeking reliable techniques to join Mg alloys. As a solid state welding method, friction stir welding (FSW) exhibits unique advantages in joining Mg alloys, and thus arouses widespread research interest. This paper emphatically reviewed the research status of conventional friction stir butt-welding of Mg alloys, and highlighted the welding process, microstructure evolution, texture characteristics, mechanical behavior and their interaction mechanisms. It was indicated that the texture plays a vital role in FSW joint performance of wrought Mg alloys, which is quite different from that in the FSW Al alloy joints. The specific strong texture formed in the weld is the main factor that gives rise to the impediment to achieving equal-strength joints to base materials. At the same time, some focuses like the weldability and the factors that influence joint performance in other types of FSW like lap welding, spot welding and double-sided welding; the weldability, interface bonding mechanism, joint performance and its affecting factors and optimization methods in dissimilar FSW between Mg alloys and other materials like Mg alloys of other grades, Al alloys and steels, were summarized and discussed. Finally, the future research and development directions in FSW of Mg alloys were prospected.
9%Cr heat-resistant steels have been abundantly used in boilers of modern thermal plants. The 9%Cr steel components in thermal plant boilers are usually assembled by fusion welding. Many of the degradation mechanisms of welded joints can be aggravated by welding residual stress. Tensile residual stress in particular can exacerbate cold cracking tendency, fatigue crack development and the onset of creep damage in heat-resistant steels. It has been recognized that welding residual stress can be mitigated by low temperature martensitic transformation in 9%Cr heat-resistant steel. Nevertheless, the stress mitigation effect seems to be confined around the final weld pass in multi-layer and multi-pass 9%Cr steel welded pipes. The purpose of this work is to investigate the method to break through this confine. Influence of martensitic transformation on welding stress evolution in multi-layer and multi-pass butt-welded 9%Cr heat-resistant steel pipes for different inter-pass temperatures (IPT) was investigated through finite element method, and the influential mechanism of IPT on welding residual stress was revealed. The results showed that tensile residual stress in weld metal (WM) and heat affected zone (HAZ), especially the noteworthy tensile stress in WM at pipe central, was effectively mitigated with the increasing of IPT. The reasons lie in two aspects, firstly, there is more residual austenite in the case of higher IPT, as a result, lower tensile stress is accumulated during cooling due to the lower yield strength of austenite; secondly, the higher IPT suppresses the martensitic transformation during cooling of each weld pass, thus the tensile stress mitigation due to martensitic transformation was avoided to be eliminated by welding thermal cycles of subsequent weld passes and reaccumulating tensile residual stress. The influence of IPT on welding residual stress relies on the combined contribution of thermal contraction and martensitic transformation. When the IPT is lower than martensite transformation finishing temperature (Mf), thermal contraction plays the dominant role in the formation of welding residual stress, and tensile stress was formed in the majority of weld zone except the final weld pass. While, compressive stress was formed in almost whole weld zone due to martensitic transformation when the IPT is higher than martensite transformation starting temperature (Ms).
Al matrix composites (AMCs) have been used in the aerospace and automotive industries due to the desirable properties including high specific strength, superior wear resistance and low thermal expansion. However, the traditional fusion welding process of AMCs usually brings defects such as pores, particles segregation and detrimental phases, which limits the application of AMCs. So more and more attentions are paied on friction stir welding (FSW), a solid state welding method possessing great potential in the welding of AMCs. In this work, to acquire high quality and excellent fatigue property of friction stir welded SiCp/6092Al composite joint, 3 mm-thick rolled SiCp/6092Al composite plates with T6 state were conducted by FSW at a constant rotational rate of 1000 r/min, and at a low welding speed of 50 mm/min and a high welding speed of 800 mm/min, respectively. Microstructure evolution, mechanical properties and high cycle fatigue behavior of the FSW joints were evaluated. The results showed that high welding speed resulted in a much rougher surface of scale-like ripple and the morphology of the nugget zone was different from that of the joint at low welding speed. Significant enhancement of the hardness and tensile strength were achieved in the joints at the high welding speed, but the fatigue properties were not improved for the joints with unpolished surfaces. The fatigue limit of the joint at low welding speed was 150 MPa, however the fatigue limit reduced to 140 MPa at the high welding speed. For the joints with polished surfaces, obviously enhanced fatigue limit was achieved at the high welding speed of 800 mm/min compared to that of the joint at the low welding speed of 50 mm/min. Different fracture characteristics were observed in the specimens with unpolished surfaces at various cyclic stress loading. Under a low cyclic stress loading, crack initiated at the scale-like ripple on the surface of the specimen; under a high cyclic stress loading, crack also initiated at the scale-like ripple at the low welding speed, while the crack initiated at the swirl zone in the bottom of the nugget zone at the high welding speed. The results of three-dimension surface topography showed that a large surface roughness was achieved on the surface of the joint at the high welding speed, resulting in lower fatigue limit compared to that of the joint at the low welding speed. For the specimens with polished surfaces, the fatigue limit was improved by 40~65 MPa compared to that of the specimens with unpolished surfaces. In this case, a high fatigue limit of 205 MPa was obtained in the joint at the high welding speed of 800 mm/min, and all the specimens failed at the lowest hardness zone and nearby.
Welded joints of hydrogen-containing coal gas transmission pipelines are prone to hydrogen enrichment due to their severe microstructure inhomogeneity and residual stress in them, and thus lead to the decrease of plasticity and toughness. In order to investigate the effect of local hydrogen enrichment on the safety of hydrogen-containing coal gas transport pipelines, a three dimensional numerical simulation model was established to investigate the hydrogen diffusion behaviour considering the combined effect of microstructure inhomogeneity and residual stress in X80 spiral welded pipeline by using ABAQUS software. Results showed that both microstructure inhomogeneity and residual stress could lead to hydrogen diffusion. The distribution of hydrogen concentration in the pipeline was similar to that of hydrostatic stress distribution. That is, the higher the hydrostatic stress value, the higher the corresponding hydrogen concentration, indicating that the influence of residual stress on the hydrogen diffusion behaviour is greater than that of microstructure inhomogeneity. The enriched hydrogen concentration at the center region of the welded joint with the highest residual stress was 2.7 times higher than that without considering residual stress. Equivalent charging hydrogen pressure was put forward to reflect the degree of hydrogen enrichment in weld metal. Slow strain rate tension (SSRT) tests were subsequently performed on weld metal specimen at equivalent charging hydrogen pressure to investigate the effect of hydrogen enrichment on hydrogen embrittlement (HE) susceptibility. The SSRT tests performed in nitrogen gas and simulated coal gas were used for comparison. The HE index increased from 18.56% in simulated coal gas to 32.53% in equivalent charging hydrogen pressure, increasing by 75.27%. Therefore, the residual stress is a non-ignorable factor, because it could lead to hydrogen enrichment and could significantly influence HE susceptibility in welded joint. The determination of hydrogen enrichment in welded joint by using numerical simulation method is the basis to evaluate the safety of coal gas transmission pipeline.
In recent years, the welding of dissimilar metals such as steels and aluminum alloys has attracted much more attentions due to weight reduction, especially in automobile and railway vehicle manufacturing industry. However, many challenges and problems need to be addressed in order to obtain high quality welding joints between steels and aluminum alloys resulting from their differences of thermal-physical properties. The formation of intermetallic compounds (IMCs) in the course of welding will lower the mechanical properties of the joints. Up to now, a few techniques have been tried to weld aluminum alloys and steels, including solid welding and fusion welding. In this work, dissimilar metals of 5182-O and HC260YD+Z were welded by cold metal transfer (CMT) arc-brazing using AlSi5 as filler metal. The macro and micro morphologies of the overlap joint were investigated using OM, XRD, SEM and EDS analyses. The hardness and shear strength of the joints were tested. Results show that welding line energy can affect the thickness of IMCs existing on the brazing interface and thus depress the combination properties because of the different fracture modes. When the welding speed and wire feed speed are 9 mm/s and 5 m/min respectively, the IMCs thickness is about 6 μm, and the shear strength of the jonts can reach to 160 MPa. Two typical fracture modes of fusion interface fracture and brazing interface fracture were observed. The fracture mode of the position near arc striking is "fusion interface". With the increasing of welding energy, the thickness of IMCs is increased and the fracture mode near arc extinguishing is changed from "fusion interface" to "brazing interface". When the output power of CMT equipment is 150~210 J/mm at welding beam length, the IMCs thickness is less than 9 μm, which benefits the shear strength performance of the joints, and the fracture mode of "fusion interface" can be easily obtained.
With the extensive exploitation of ocean resources, the steels used in ocean engineering have been developed towards the trend of high strength-toughness and thick plates, which consequently causes welding problem and high risk of stress corrosion cracking (SCC). The heat-affected zone (HAZ) of high-strength low-alloy steel undergoes phase transformation during welding thermal cycle and it's generally considered to be most vulnerable to SCC. E690 steel, as a newly-developed high strength steel, is currently the leading kind of steel used in ocean platform for its excellent performance. However, there is few research about its SCC behavior in marine atmosphere, especially in SO2-polluted atmosphere. Therefore, it's of great importance to investigate the SCC behavior and mechanism of simulated HAZ of E690 steel in this environment. However, the HAZ is a narrow zone including various microstructures; thus, the individual performance of different microstructures is inconvenient to study. In this work, various microstructures in HAZ, including coarse grained heat-affected zone (CGHAZ), fine grained heat-affected zone (FGHAZ) and intercritical heat-affected zone (ICHAZ), were simulated by heat treatment according to real HAZ microstructures of E690 steel. A comparative study of SCC behaviors of various HAZ microstructures in simulated SO2-containing marine atmosphere was conducted by using U-bend specimen corrosion test under dry/wet cyclic condition. The results indicated that various HAZ microstructures have high susceptibility to SCC in this environment. The SCC susceptibility of CGHAZ and ICHAZ is very high with a high crack growth rate while that of FGHAZ and parent metal is relatively modest. SCC cracks were initiated after 5 d of cyclic corrosion test for U-bend specimen of various microstructures. The microcracks were initiated from the corrosion pits, which were induced by the galvanic corrosion between martensite-austenite (M-A) constituents and ferritic matrix.
Pure titanium is often used in the manufacture of pressure vessels due to its excellent corrosion resistance. Pressure vessels are generally operated in various corrosive media and subjected to varying degrees of corrosion. Stress corrosion cracking is one of the most dangerous forms of damage to pressure vessels used in various fields. Welded joints become the weak link of the pressure vessels because of the uneven microstructure and welding residual stress, which could cause stress corrosion and directly affect the overall performance and service life of pressure vessels. At present, as a method to improve the mechanical and corrosion properties of materials, shot peening has been widely studied. However, shot peening often leads to the increase of surface roughness and even causes defects such as cracks and surface damage, which will affect the effect of improving the corrosion resistance of materials. It remains to be further studied that the specific influence of surface roughness on the stress corrosion resistance of metal materials. In this work, the stress corrosion behavior of the original samples, ultrasonic shot peening (USSP) samples and USSP with surface polished samples of TA2 titanium welded joints in 10%HCl solution were studied by slow strain rate tension (SSRT) experiment. OM, TEM and SEM were used to observe the microstructure and corrosion fracture morphology of the welded joints. The surface roughness and residual stress of different processed samples were measured, and the corrosion mechanisms were analyzed. The results showed that both stress corrosion and hydrogen embrittlement occurred in pure titanium welded joints in this system, and weld metal (WM) was the weakest link in the welded joint. The stress corrosion cracking susceptibility index (ISCC) of the original sample in this system was 25.61%, indicating a tendency of stress corrosion. The ISCC of the USSP sample was reduced by 28.78%, and that of the USSP with surface polished (1500#) sample was reduced by 53.3%; both of them had no obvious tendency of stress corrosion in the system. The roughness of the USSP surface could cause stress concentration to form a crack source, which was similar to the pitting propagation. USSP with surface polished treatment reduced the surface roughness, achieving the homogenization of the stress distribution and increasing the elongation and the plasticity of the samples, which could further improve the stress corrosion cracking resistance.
As a solid-state welding technology, linear friction welding has unique advantages in machining dissimilar titanium alloy blade disc. However, there still lacks sufficient support in basic applied research, and the mechanism of interface formation is still under study. In this work, the microstructure of the welded joint between TC11 and TC17 titanium alloys was analyzed by OM, SEM and TEM, respectively. The results showed that common grains and common grain boundaries are formed at the weld interface. In the common grain, a phase boundary is formed in the weld interface. Elements diffusion is observed on both sides of the common grain boundary and the phase boundary in the common grain. Under the action of rejection, adsorption and towing of solute elements in the formation of common grains and common grain boundary, the observed diffusion distance of elements in the phase boundary of the common grain is longer than the one in the common grain boundary. The composition change at the phase boundary of the weld zone is greater than the one inside the phase. A large number of small needle-like α phases are formed at the weld interface that has a large number of deformed twins. The structure of the interface in common grains consists of two interfaces (recrystallization growth interfaces of both sides) and two growth regions (ordered and disordered). The dynamic recrystallization also has an ordered and disordered crystallization process similar to that of solidification crystallization.
Welding is widely used for pipeline connection. Composition, microstructures and properties of the welded joints are highly heterogeneous and the resultant corrosion such as galvanic corrosion between different parts is widely present and influence the long-time service and safety. In this sense, the fundamental research in the electrochemical behavior of such joint parts is required. Electrochemical corrosion behavior of simulated X80 steel welded joint, accurately modeled by wire beam electrode (WBE) technique, was investigated by classical electrochemical techniques and microelectrode array (MEA) technique. A new index, namely the galvanic corrosion intensity factor, was proposed and verified to succeed in characterizing the degree of galvanic corrosion. Results showed that microstructure of granular bainite mixed with ferrite showed the highest positive open circuit potential and lowest polarization resistance. Furthermore, the corrosion tendency of the isolated electrodes that constituted the X80 steel welded joint was found to increase in the following order: fine grain heat affected zone (FGHAZ) < intercritical heat affected zone (ICHAZ) < base metal (BM) < coarse grain heat affected zone (CGHAZ) < weld metal (WM). Due to the difference in potential and the polarization characteristics, the WM displayed the highest polarization resistance but the most positive current density. The CGHAZ possessed a lower polarization resistance and a higher positive current density. In comparison, the FGHAZ and ICHAZ performed a lower polarization resistance but higher negative current densities. The WM and CGHAZ acted as the main anode, while the FGHAZ and ICHAZ acted as the main cathode and the galvanic current polarity of some BM electrodes changed with time during the immersion test. The intensity of galvanic corrosion of simulated X80 steel welded joint plateaued with immersion time. The results revealed that WM and CGHAZ were the weak links in the simulated X80 pipeline steel welded joints during its long-term service.