Tin-base babbitt alloy is an essential material for industrial production as a kind of excellent sliding bearing liner material. In recent years, with the continuous emergence of large-scale, high-performance, heavy-duty machinery and equipment for industrial use, the traditional tin based babbitt alloy materials and preparation processes gradually show some drawbacks, which have been unable to meet new needs. The preparation process and composition optimization and modification of tin based babbitt alloy materials have become a research hotspot. The preparation process of tin based babbitt alloy mainly includes centrifugal casting, arc surfacing, brazing, laser cladding, in-situ microwave assisted casting, etc. The effects of different preparation processes on the composition of tin based babbitt alloy and the bonding strength of coating are analyzed; By adding strengthening elements, compounds, carbon materials, nanomaterials and other methods to optimize the composition of babbitt alloy, further improve the bonding strength of the coating. A series of research achievements have played a significant role in extending the life of bearings. In this paper, the research progress of preparation technology and composition modification of tin-base babbitt alloy is reviewed, and it is proposed that the design and development of new tin-base babbitt coating structure to improve the bonding strength and service life of tin-base babbitt alloy coating will continue to be the research focus and future development trend of scholars at home and abroad.

Due to its excellent characteristics of corrosion resistance, good flexibility and high construction efficiency, polyethylene gas pipelines have gradually replaced steel pipelines in urban gas transportation and become the main transportation method.Installation of polyethylene pipes mainly adopts electrofusion and hot fusion welding.The main welding defects are over welding, cold welding, unremoved oxide scale, dislocation of resistance wire, holes, and so on.Effect of different welding defects on strength of polyethylene welded joints and influence mechanism was studied.Welded joints with artificial defects were analyzed and tested by tensile, flattening, SEM and EDS test methods.The results of tensile and flattening mechanical properties showed that cold welding, over welding, unremoved oxide scale, and dislocation of resistance wire were main defects affecting strength of welded joints of polyethylene pipes, and unremoved oxide scale had the greatest impact on strength of welded joints, which was 0.4% of fracture stress of normal welded joints.SEM and EDS test results showed that molecular chain of pipe containing oxide skin was prone to crosslinking, chain scission, and poor fluidity.Diffusion of macromolecular chains of polyethylene base metal was hindered by hard plaques by thermal and oxygen aging and mechanical scratches.Under action of ultraviolet rays, light, heat, and so on, oxygen reacts with the molecular chain on surface of polyethylene to form a large number of hydroxyl radical groups, which made polyethylene molecular chain undergo thermo-oxidative aging and degradation, and greatly reduced strength of polyethylene joint.

Based on the analysis of the chemical composition and mechanical properties,the tensile strength,low temperature impact toughness,hardness,CTOD and HIC of welded joints had been studied on X100 pipeline steel with the selected welding process,welding filler metal and technical parameter. The results show that the welding performance of X100 pipeline steel is good,and the welding process put forward in this paper can meet the working and application requirements.

Formation of welding solidification crack was a high-temperature complex process affected by many factors. Although remarkable research was achieved, there were still some theoretical and technical problems. Research progress in the field of welding solidification crack was reviewed in the paper from aspects of mechanism of solidification crack, criterion for solidification crack and susceptibility test for solidification crack. In the research of formation mechanism of welding solidification crack, strength theory, liquid film theory, intergranular bridging theory and solidification compensation shrinkage theory were formed. Research on susceptibility criterion of solidification crack was mainly based on three aspects of stress, strain and strain rate to form a variety of criterion models. Testing methods of solidification cracking susceptibility were compared and analyzed. Advantages of transverse moving weldability (TMW) as a new testing method were pointed out. Research direction of solidification crack was also prospected, which could provide theoretical basis and technical support for the in-depth study of metallurgical mechanism of solidification crack and engineering application of controlling solidification crack.

Inertia friction welding technology was mainly used to connect medium carbon quenched and tempered steel 42CrMo with nickel base superalloy GH4169. Microstructure and mechanical properties of welded joints under different inertia were analyzed by means of optical electron microscope, scanning electron microscope, energy spectrum analyzer, microhardness tester and electronic tensile tester. The research results showed that when inertia was 168 kg·m², flash of 42CrMo and GH4169 inertial friction welded joints was well formed, and there was no obvious defect after flash was removed. Dynamic recrystallization occurred on both sides of weld interface, and uneven intermetallic compounds (Ni-Fe-Cr, Ni₃Al) and carbide layers (NbC, TiC) with thickness of 5 μm were observed, which also resulted in microhardness peak of 691 HV1.0 at the weld interface. According to tensile property test of welded joints, fracture location of samples under 168 kg·m² at room temperature and high temperature tensile (540 ℃) occurred on the 42CrMo base metal side. Fracture morphology observation showed typical ductile fracture, tensile strength at room temperature reached 1062.5 MPa, and tensile strength at high temperature reached 687.5 MPa, which were similar to 42CrMo base metal. This indicated that intermetallic compounds and carbide layers formed at the interface promoted metallurgical bonding of interface materials, and effectively improved mechanical properties of welded joints.

In the field of engineering, fatigue fracture of welded components is very easy to occur, which has attracted extensive attention in various industries.In order to analyze the microstructure and fatigue properties of typical welded components, T-joint were studied in this paper.Welding tests of T-joint of S355 steel were completed by MIG welding, and microstructure and hardness of welded joints were analyzed.The weld mainly contained pro-eutectoid, ferrite pearlite, granular bainite and a small amount of acicular ferrite.Composition and microstructure of fusion zone were not uniform, grain size was not consistent, and grains of overheated zone were coarser and uneven.Microstructure of normalized zone was mainly ferrite and pearlite, and grains were fine and uniform.Hardness of welded joints was higher than that of base metal, and hardness of heat-affected zone was the highest.Axial high-cycle fatigue tests on welded specimens of T-Joints were conducted, test data was processed and fitted by the least square method, and S-N curve was obtained.

The third-generation pipeline construction technology represented by automatic welding has been fully applied in the Sino-Russian East Route natural gas pipeline project and other national key oil and gas long-distance pipeline projects. Through combing quality management experience of automatic welding in pipeline engineering in recent years, focusing on five aspects of “Man, Machine, Material, Method and Environment”, quality control measures of girth weld were systematically summarized from aspects of welding procedure qualification, qualified welder control, welding material management, welding breaking-in control, joint welding, welding data acquisition and crater analysis. Combined with “Made in China 2025”, future development of oil and gas pipeline construction was prospected, which provided reference for the development and application of new welding technology and quality control.

This paper takes the shape of single weld as the starting point to study the influence of welding process parameters on the weld bead morphology and establish a regression equation to predict the weld bead size. The research shows that the welding current has the most significant effect on the weld bead size. When the welding current increases by 50 A, the weld width increases by 14% on average. There is a certain coupling relationship between different process parameters. At the same time, it reveals the law that the increase of welding current and welding speed causes the change of arc dynamic pressure, which leads to the transformation of weld shape from finger to bowl. The reason for the change from finger-like morphology to bowl-like morphology. Compared with various regression models, the response surface method based on the elimination of low correlation factors proposed by considering the coupling effect of process parameters has a good overall regression effect, with R² above 0.97 and the average error within 0.5%. The regression model established in this paper will provide accurate weld bead size information for the planning of arc surfacing layers and passes, and ensure that the robot welding pose meets the process requirements.

In girth welding of X60/625 bimetal clad pipe,because of the great differences of the chemical composition and physical properties of the two kinds of materials,it was prone to cause oxidation and dilution of nickel-based alloy layer,thus affecting the corrosion resistance of cladding layer and pipeline integrity.For the problem,the weldability and welding process of the X60/625 bimetal clad pipe were analyzed and studied in this article,so as to develop a girth welding process for X60/625 bimetal clad pipe.After nondestructive test,physical and chemical properties test,corrosion resistance test and dilution rate test of the weld,the results met the relevant standards and specifications.The welding process were successfully applied to first part clad pipeline of Saudi Arabia Halad Project,612 welded joints were finished and the weld pass rate was 99.7%.

Aiming at the weld-cracking problem in the process of using Tangsteel S380 to make welding-pipes,which happened in the tests of Flatten and Hole-expansion carried out on these pipes,microstructure-inspecting and property test were taken by the usage of SEM and microhardness tester. The results shown that Widmanstatten microstructures in the fusion-line and molten metal in the center of S380 weld are the main reasons causing the weld-cracking in the tests of Flatten. And then,some improving-measures were proposed for users and the rate of cracking had been greatly reduced.

The ring chain is widely used in mining machinery,marine ships and military vessels because of its large carrying capacity,reliable work,convenient disassembly and excellent transmission performance. In recent years,higher requirements for high strength and high toughness of the ring chain had been put forward with the development of marine equipment and coal mine machinery,as well as its welding technology have been gradually improved. Flash butt welding had been used in chain production as an efficient and applicable solid phase welding method. In view of the structural characteristics of high strength ring chain,the process characteristics of ring chain pulsed flash butt welding were summarized. The development of ring chain steel welding technology,the high strength ring chain flash butt welding technology and the research status of ring chain welding defects and quality control were analyzed. The development direction of high strength ring chain welding was predicted.

In view of problems such as excessive labor intensity, serious environmental pollution and low conductivity of manual welding in traditional electrolytic aluminum prebaked anode conductive device, the first 5 000 kN aluminum guide rod friction welding equipment and 4 000 kN cast steel claw friction welding equipment were innovatively developed in domestic and put into actual production. Involving automatic control design category, challenges faced by two sets of equipment in the development process were emphatically analyzed, such as characteristics of large inertia and large cross-section of welded parts, as well as requirements for small phase control errors, high-speed output accuracy of friction welding process, and fast data acquisition rates. Through scientific design, friction welding equipment could quickly, accurately collect input signals. In the meantime, it relied on construction of efficient control strategy to achieve real-time and precision control results, which met requirements of system stability and human-machine interaction quality, as well as speed, position, pressure, safety and other control parameters during the processing of large cross-section of same or different metals, and then met high requirements of friction welding process on quality and repeatability of processed welds. Because it could provide accurate data for welding process research, and then provide scientific path for pursuing the best joint quality. At present, due to limited basic theories and practical applications for large cross-section friction welding joints of same or dissimilar metals under conditions of thermal aging, thermal cycling and current load, research achievements of related equipment had a positive effect on integration of welding and control. Development trends of such equipment and automatic control systems were looked forward.

In order to investigate the effect of alloying elements Cr and C on the microstructure of high boron iron-based surfacing alloys, gas shielded surfacing technology was used to prepare different types of high-carbon ferrochromium on the surface of Q235 steel plate by adjusting the addition amount of high-carbon ferrochromium in the metal powder cored wire. The Fe-Cr-B-C surfacing alloy with high carbon and ferrochromium content was analyzed by metallographic microscope, scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), and other analysis and testing methods to analyze the effects of different alloying elements Cr and C content on the microstructure and properties of the surfacing alloy. The results show that the microstructure of the surfacing alloy is martensite + network (Fe, Cr)₃(B, C), and a small amount of M₇C₃. The volume fraction of (Fe, Cr)₃(B, C) increases, the martensite in the surfacing alloy has higher hardness, the alloy element Cr can make the matrix structure solid solution strengthening, and the network carbide (Fe, Cr)₃(B, C) As a wear-resistant skeleton, it hinders the extrusion and cutting of abrasives in the process of wear, so that the wear resistance of the surfacing layer is 3-4 times higher than that of 65Mn steel, and the wear mechanism is micro-furrowing.

The welded joints of dissimilar steels are widely used in ultra-supercritical power station boiler structure.The microstructure,elements and phase composition of T91/TP347H dissimilar welded joints in original state and service for 1×10⁵ h were analyzed by OM,SEM,EDS and XRD,and the microhardness of these two welded joints was tested by microhardness tester.The results showed that,compared with the original state,grains of TP347H base metal after 1×10⁵ h service were larger,the strengthening phases of T91 and TP347H in heat affected zone got coarsened,and M₂₃C₆ and Nb (C,N) precipitated in the weld.The maximum hardness of both welded joints appeared in the heat affected zone of T91,and the minimum hardness of both welded joints appeared in the base metal of TP347H.Compared with the original state,the hardness of welded joints after 1×10⁵ h service increased significantly.

Aiming at the defects formed in the manufacturing process of additive, it will cause irreversible influence on the workpiece. The current signal and voltage signal of CMT additive manufacturing process were analyzed, and an online monitoring method of CMT additive manufacturing defects based on time series algorithm was proposed. Different welding conditions were set, the original current and voltage signals of good group and defective group were collected, and SAX (Symbolic aggregate approximation) algorithm was used to preprocess the data. Random forest model was used to reclassify numerical data to achieve real-time monitoring effect. At the same time, in order to highlight the superiority of SAX algorithm, a comparative test group was set up, and the original current data was directly put into the random forest model for classification. The experimental results showed that the accuracy of the test set of the original current group was 80%, and that of the SAX algorithm data preprocessing group was 96%.

In view of the quality control problem of surface forming in magnesium alloy arc additive manufacturing,process parameters of AZ91 magnesium alloy TIG arc additive manufacturing such as current,wire feeding speed and material increasing speed,and the width of cladding layer were modeled by Design-Expert software.The influence of various process parameters on the width of the additive layer was explored.The additive current was optimized by the mathematical model of the main process parameters and dimensions of the additive.Then the layer width of the straight wall component was controlled according to the current optimization value.The results showed that the greatest influence on the layer width was the additive current,followed by the additive speed,and the smallest influence was the wire feed speed.The layer width fluctuations of the single-pass multi-layer component prepared by the optimized process additively were small from top to bottom,and the layer width deviation value was reduced from 4.54 mm to 0.94 mm,which improved the quality of AZ91 magnesium alloy additive forming.

Based on the thermal elastic-plastic analysis method of ABAQUS finite element software, the residual stress and residual deformation of 32 mm thick Q345qD steel butt joint under gas metal arc welding (GMAW) are studied in this paper. Combined with experimental test and numerical simulation, the effects of four different welding sequences on the residual stress distribution and residual deformation of butt joint are studied. The element birth and death is used to simulate the filling of deposited metal, and the volume heat source with double ellipsoid distribution is used to realize the movement of welding arc. The results show that the numerical simulation results are in good agreement with the experimental results. The change of welding sequence has little effect on the temperature field; compared with that of welding the other side after welding, the overall stress level of transverse residual stress by alternating welding on both sides is smaller, and the maximum value is reduced from 356 MPa to 278 MPa, which is reduced by 21.91%; while the overall stress level of longitudinal residual stress by alternating welding on both sides is larger, and the maximum value of longitudinal residual stress is increased from 303 MPa to 368 MPa, which is increased by 21.45%. The deformation in the thickness direction of alternating welding on both sides is smaller, and the maximum value is reduced from 1.67 mm to 0.04 mm, which is reduced by 97.60%.

The shear strength and fatigue strength are important mechanical properties of the laser lap welded joints. However,no standard has regulated the dimensions of the tensile and fatigue specimens. In this paper,the reasonable widths of the laser lap welded specimens were determined by taking account of the residual stress. The shear strengths of specimens with residual stress and without residual stress were studied. It was found that the residual stress reduced the shear strength of the laser lap welded joints by change the stress state and micro-deformation uniformity.

Gleeble-3500 thermal simulator was used to simulate the welding heat of Q690D low alloy high strength steel,and the microstructure of the heat-affected zone under different peak temperatures and cooling times during the first and second welding thermal cycles was obtained.The microstructure observation,hardness test,impact property test and fracture morphology analysis were carried out.The result showed that in the first welding thermal cycle,with the increase of the peak temperature of the welding thermal cycle,microstructure of samples became gradually coarsened,the granular bainite transformed to upper bainite and lath martensite,the hardness increased and the impact performance deteriorated.When the thermal cycle peak temperature was 900℃,the maximum impact energy absorption was 78.95 J.When the peak temperature was 1 350℃,the minimum impact energy absorption was only 17 J.The impact fracture changed from ductile fracture to dissociative fracture.At the same peak temperature,as the cooling time t_8/5 increased,the hardness of the sample decreased and the impact performance deteriorated.In the second welding thermal cycle,microstructure of samples was coarse grain,mainly lath martensite,the hardness was higher and the impact performance continued to deteriorate.The lowest value of impact energy absorption was only 24.99 J,and the impact fracture was mainly cleavage fracture and quasi cleavage fracture,which indicated that the performance of samples deteriorated due to the second welding thermal cycle.

According to the functional requirements,the design principle of automatic all-direction tube-sheet argon-arc welding machine was put forward in this paper. Then the key problems of this machine were described,such as reliable wire feeding,accurate positioning,flexible water and gas supplying. Finally,a tube-plate welding head with high performance was designed. This welding head with the conductive ring and static positioning,dual water and gas supply,has the advantages of compact structure,reliable operation,strong applicability etc. The production practice shows that the welding head can effectively ensure the quality of the welded joint,and significantly improves the production efficiency.

In order to study distribution of welding temperature field of T-joint of girder with corrugated steel web, based on thermo-elastic-plastic theory and finite element theory, temperature field of T-shaped joint by CO₂ shielded arc welding was simulated and verified by experiments at 20℃ with Simufact Welding software, double ellipsoid heat source model and configuring the relevant parameters.Influence of different ambient temperature on cooling rate of molten pool was analyzed.The results showed that type of heat source and the related parameters used in the simulation were reasonable, welding heat input was sufficient, welding penetration was moderate, and bottom plate and web were connected well.Distribution of heat-affected zone of weldment obtained by simulation was an irregular closed ellipse and showing a certain temperature gradient, which was in good agreement with the test results.During the welding simulation process, temperature of the weld reached melting point of material 1 500℃.The isotherms presented dense in front of the heat source and sparse in the rear with a certain gradient, which was similar to heat transfer in the actual welding process.In the post-weld cooling process, temperature of the weld and its near area dropped promptly, temperature gradient fell apace, and radiation range of temperature field increased constantly.Cooling rate of metal molten pool was inversely proportional to ambient temperature.The research results obtained in this paper could provide reference for the analysis of welding temperature field of similar bridges.

Gas Tungsten Arc Weld (GTAW),which was characterized as flexible operation,high welding quality,is widely used in critical welding production,especially in the backing welding of pressure pipelines,boilers and pressure Vessels. In the process of GTAW,the use of different welding methods has large influence on the welding quality. In this paper,we take the GTAW of small-diameter pipeline as an example to analyze the effect of three welding methods on the appearance forming of weld pass and the welding quality,and to summarize an appropriate method during the site-welding of pipelines. Therefore,the pass rate of site-welding of pressure pipelines was increased.

Basic characteristics of superplasticity of coarse-grained aluminum alloys at home and abroad were described. The superplastic deformation mechanism of coarse-grained aluminum alloy was classified and summarized, which mainly included diffusional creep mechanism, grain boundary slip mechanism with diffusion creep, dislocation slip/creep mechanism, liquid phase regulation mechanism, dynamic recrystallization mechanism, cooperative grain boundary sliding mechanism and cavities growth mechanism. In view of urgency of developing superplasticity of aluminum alloy at low cost, application of coarse-grained materials was forecasted. It was pointed out that in addition to large grains, superplasticity under high strain rate and low temperature condition was also the current research direction of aluminum alloy deformation.

The cracks in 304 stainless steel welds and base metal were analyzed during the overhaul,it was found that welding slag in the weld and lack of penetration were the reasons of cracking of welded joint. Metallographic analysis of base metal showed that carbides precipitate in grain boundary of 304 stainless steel,which likely results in the intergranular corrosion that could cause the cracking of base metal. During the welding process,the multi-pass welding easily caused the intergranular corrosion of heat affected zone,which can lead to cracking. During the cooling process after manufacturing of 304 stainless steel forging flange,it should be avoided remaining at sensitization temperature for too long time,and fast cooling after solution should be taken to avoid intergranular corrosion. The welded joint of 304 stainless steel was qualified by welding inspection after repairing by strictly controlling interpass temperature and low heat input during and fast welding.

The solid wire of high strength and toughness was successfully developed by vacuum melting technique. The microstructure and mechanical properties of deposited metal were analyzed in detail by means of optical microscopy (OM),scanning electron microscopy (SEM), transmission electron microscopy (TEM),instrumented impact tests. Results showed that the highest tensile strength and toughness of deposited metal can reached to 920 MPa and 66.7 J (-60℃),respectively,for the alloying systems of Mn-Ni-Cr-Mo-Ti. With the content of alloying elements being increased,the microstructure transformed from bainite to the mixture of granular bainite and low carbon martensite. However, the higher content of alloying elements in deposited metal can promote the formation of martensite,and its appearance is adverse to toughness. The existence of the film-like residual austenite which can improve the impact toughness was found between the lath-like bainite ferrites.

Bobbin-tool friction stir welding (BT-FSW) was used to realize welding of 10 mm thick 6082-T6 aluminum alloy sheet. Characteristics of thread processing on the surface of pin was conducive to inducing single shear deformation of material around the pin, increasing complexity of deformation of material around the pin while improving strain rate, and thus helping to weaken microstructure of material in the area of BT-FSW joint. The results showed that when rotational speed was 300 r/min, welding speed was 400 mm/min and press-in depth was 0.5 mm, BT-FSW joints with good surface and internal macroscopic shape were obtained, and macroscopic shape of welded joints was typical “dumbbell”. Under the influence of high degree of plastic deformation, dynamic recrystallization of grains in BT-FSW joints occurred, resulting in fine equiaxed grains. Due to different distribution of friction heat input in different areas of welded joints, hardness distribution of welded joints exhibited typical W shaped distribution. Tensile strength of welded joints was about 77% of base material and elongation was about 69% of base material.

For the common problem of side-wall incomplete fusion in narrow groove welding of oil and gas pipelines, welding process and properties of X65 pipeline steel by narrow groove rotating arc welding were studied. Rotating arc experimental system of X65 pipeline steel plate was built, high-speed camera synchronous acquisition system was used to study behavior of rotating arc droplet, and influence of rotating arc frequencies on droplet transfer was analyzed. Through macroscopic metallographic observation of weld section morphology, influence of rotation frequency of narrow groove rotating arc GMAW on formation of weld was investigated. The results showed that high-frequency rotating arc accelerated transfer frequency of droplet, reduced size of droplet, and promoted transfer process of droplet. When rotation frequency was in the range of 5~30 Hz, good formation of weld could be obtained. Under the action of high-frequency stirring of rotating arc, surface texture of the obtained weld was very dense, and welded joints had good tensile and bending properties.

Welding materials production in China has reached more than half of world output,and become the world's welding material production and consumption power. With the development of national economy,industry of welding materials is more and more playing an important role. Status of the welding materials industry in China during the period of "12th five-year plan" was analyzed,and the existing problems were points out. The development trend during the period of "13th five-year pan" were put forward.

A high-power ultrasonic cavitation facility was used to investigate the cavitation erosion behavior of 316 L welded joints before and after PWHT in liquid LBE at 550℃. After different time of cavitation erosion test,SEM and AFM were used to analyze the surface topography, roughness and hardness. It is found that the roughness and the maximum erosion depth increase with the increase of erosion time,and work hardening is found on all the surface of welded joints with and without solution treatment. Plastic deformation appears on the surface of welded joint without solution treatment,and after solution treatment,the hardness of welded joint increases greatly but the plasticity is decreased. The results and analyses showed that the welded joint with solution treatment exhibits better cavitation erosion resistance.

In order to research the effect of laser cleaning pretreatment on the welding quality of 30Cr3 ultra-high strength steel, surface morphology, element composition and matrix microstructure were observed by scanning electron microscope before and after cleaning. The results showed that the proper laser cleaning process could effectively remove the oxide layer on the surface of 30Cr3 ultra-high strength steel, which made the surface smoother without damaging the base material. The test samples after laser cleaning were welded by electron beam. After welding, it was observed that the weld surface was well formed without undercut, depression and other defects, and the internal quality and mechanical properties of the weld fulfilled the requirements. Then welded joints were made into metallographic specimen to observe its microstructure and morphology. After laser cleaning pretreatment, welded joints were fully formed without crack, undercut and other defects under low power mirror. Through high power observation, grains in the center of the weld were fine and uniform. Therefore, the laser cleaning pretreatment of 30Cr3 ultra-high strength steel could effectively remove the contaminated layer on the surface, and the welding quality met the requirements.

In this paper, advantages of ultrasonic spot welding were introduced, in comparison with that of resistance spot welding and laser welding. And its welding equipment, welding mechanism and investigation status in the lithium-ion battery manufacturing were demonstrated. Meanwhile, the key effects on the macroscopic appearance, microstructure and mechanical properties were systematically illustrated. To obtain the sound robustness and good quality of ultrasonic spot welded joint, a real-time controller and a new tool geometry design for the sonotrode were effective methods, but numerous unsolved issues existed. In addition, some perceptions on how to further enhance the process robustness of ultrasonic spot welded joint were proposed in this paper.

Mo-Re alloys are one of promising materials for the future application. But Mo-Re alloys are sensitive to carbon,nitrogen and oxygen elements,which are easily oxidized. Pores and brittle compounds formed in joints cause serious drops in properties. Therefore,many scholars researched on the weldability of Mo-Re alloys. The research status of electron beam welding,laser welding,resistance spot welding,vacuum brazing and friction stir welding of Mo-Re alloys at home and abroad were summarized. The common problems in welding were analyzed and summarized,and reduction in brittle compounds,pores and cracks is the key to obtain good joints.

Aiming at the current situation that nuclear-grade 690 nickel-alloy welding wire depends on the imported products,its localization was studied. WHS690M welding wire meets the requirement of AP1000 and CAP1400 nuclear power plant,which has good welding performance. At different welding parameters,mechanical properties of the weld metal at room temperature and high temperature are relatively stable, and the tensile strength reaches 485 MPa at 350℃. Strain-to-fracture (STF) test based on Gleeble-3500 thermo-mechanical simulator was performed to evaluate the DDC susceptibility of Inconel 52M and WHS690M. Combined with OM,SEM and EBSD,the characteristics of grain orientation and precipitation were analyzed. According to the test results,the critical cracking strain is 2.1% for Inconel 52M,while 3.2% a-bout for WHS690M at 1 050℃。

To address corrosion failure of titanium alloys in deep submerged marine service environment, femtosecond laser etching was used to prepare corrosion-resistant super-hydrophobic micro- and nano-structure on surface of titanium alloy. Mechanism of laser etching parameters on the surface etched structure and interaction effect law of surface microstructure, hydrophobic property and salt spray corrosion behavior were investigated. The results showed that surface microstructure etched by femtosecond laser could trap air between microstructures and form an air protective film between liquid and substrate material. The support of air film structure to liquid improved hydrophobic ability of titanium alloy surface, reduced contact time and contact area between corrosive liquid and substrate metal, and further improved corrosion resistance of titanium alloy.

A kind of link long journey cover plate centering mechanism was designed to solve the difficulty,which the machine vision are susceptible to cover plate cutting edge,oxide skin and dust in workshop,in existing hydraulic support link robot welding workstation. It is mainly made up of the main body frame,driving mechanism,centering module,supporting module,control system,shield and peripheral accessories. It adopts a special mechanism which can achieve two directions centering by one direction power,with servo control system to achieve precise centering of link cover plate. The actual welding production test shows that the centering accuracy of centering mechanism is no more than 0.5 mm for link cover plates with different shapes and sizes,and the maximum unilateral clearance of link cover plates after assembly is measured to be 2 mm,meeting the welding requirements. With the increase of the length-width ratio of the cover plate,the rigidity of the cover plate gradually weakens and the phenomenon of vertical bending occurs,which leads to the increase of the maximum gap difference between the two sides of the link cover plate. At the assembly gap of 2 mm,the robot welding process is stable and the cover plate weld seam is good. The link cover plate centering mechanism has high centering accuracy,strong adaptability and high practical value.

Weldability and joint softening mechanism of 2 mm thick 6061 aluminum alloy sheet were deeply analyzed and discussed by means of test and numerical simulation. Tensile test results showed that with the increase of heat input, macro forming of welded joints presented transition from incomplete penetration to excellent forming and burn through, and tensile strength first increased and then decreased. Under the optimal process of 2.6 kW laser power and 1.5 m/min welding speed, strength of welded joints reached 98% that of base metal, almost as strong as base metal, and softening degree of welded joints was the lowest. Microstructure characterization equipment (SEM, OM) was used to observe and measure position and size of pores, height of weld collapse, maximum distance of undercut, maximum distance of bite edge and length of columnar crystal trunk in the epitaxial solidification zone with collapse displacement, and causes of joint softening were analyzed and summarized in all aspects. A macro-micro coupled finite element model with cellular automata model (FEM-CAM) for cross scale microstructure evolution was established. Accuracy of FEM was verified by comparing simulation results under different welding parameters and molten pool morphology under experimental measurement. Taking simulation results of macro temperature field as heat input, driving force of dendrite growth was calculated based on solid-liquid interface solute balance method. CAM simulation results of primary dendrite length variation of columnar crystals along the thickness direction in epitaxial solidification zone were consistent with measurement results of OM characterization. Therefore, coupling model established in this paper could reveal dynamic evolution mechanism and growth law of dendrite during solidification process of molten pool.

According to research status of 7xxx high strength aluminum alloy welding, it was mainly introduced from four kinds of fusion welding process including TIG, MIG, laser welding and laser-arc hybrid welding. The problems existed in fusion welding process of 7xxx high strength aluminum alloy were described in detail from aspects of welding defects, formation of weld, softening of welded joints, fatigue property and corrosion resistance. For TIG, MIG, laser welding and laser-arc hybrid welding of 7xxx high strength aluminum alloy, welding defects and low formation quality of weld and strengthened phase dissolved in welded joints could lead to softening, poor corrosion resistance and low fatigue life of welded joints. Optimizing welding parameters, reducing welding defects and improving formation quality of weld were beneficial to improve softening, corrosion resistance and fatigue life of welded joints. Selecting appropriate welding wire (containing Ti, Zr elements and Er, Sc rare earth elements) could make weld produce precipitation strengthening, fine grain strengthening and solid solution strengthening, which could improve softening problem of welded joints. Selecting appropriate post-welding heat treatment could improve size, shape and distribution of strengthening phase, and also improve softening problem of welded joints, while it could also improve corrosion resistance of welded joints.

TC4 titanium alloy welded joints were remelted by oscillated laser welding, and effect of oscillated frequency on porosity, microstructure, and mechanical properties of TC4 titanium alloy welded joints was studied. The results showed that remelting oscillated laser welding could effectively eliminate porosity defects of TC4 titanium alloy welded joints. With the increase of oscillated frequency, effect of eliminating porosity was enhanced, and elimination effect of large porosity became more pronounced. In remelting oscillated laser welded joint, microstructure of weld was acicular α' martensite and part of α phase, and microstructure of heat-affected zone was composed of primary α phase, acicular α' martensite and β phase. With the increase of oscillated frequency, stirring effect of laser on molten pool was enhanced, grains in weld gradually broke and became finer, while grains growth in heat-affected zone became more favorable and grains gradually became coarser. The increase of oscillated frequency changed microstructure of welded joints and lead to performance variation of welded joints. Increasing oscillated frequency changed microstructure of welded joints changed, and then changed mechanical properties of welded joints. With the increase of oscillated frequency, hardness in weld of TC4 remelting welded joints increased, while hardness of heat-affected zone gradually decreased, and tensile properties of welded joints showed a downward trend. When oscillated frequency increased to 120 Hz and 150 Hz, fracture area changed from base metal to heat-affected zone.

Nickel base alloy covered electrode ENiCrMo-3 type Ni625 for all position welding was developed by reasonably adjusting ratio of carbonate,fluoride and oxide in the electrode and controlling alloying element content of deposited metal. The results showed that all performances of the developed electrode can meet the requirement and it exhibits good technological performances in all position welding,excellent intergranular corrosion resistance and hot cracking susceptibility.

The microstructure and mechanical properties of 0.1 mm silver-plated braided copper wire and 0.07 mm copper micro-resistance spot welded joint were studied. The influences of different parameters on the mechanical properties of the joint were analyzed. The microscopic analysis of the typical joint was performed by scanning electron microscopy. The microstructure characteristics were analyzed. The results show that the joint is mainly formed by brazing-welding and fusion welding,and the four processes of silver element wetting and spreading,silver element diffusion,copper element melting and metal solidification are experienced in the formation process. There is a copper-silver two-phase and a single-rich copper phase formed in joint interface morphology,and the latter soldered joint microstructures have the characteristics of fusion welding. Within the parameters used in the tests,the maximum shear force of the soldered joint can reach 45 N. The welding time and the welding current have a great influence on the soldered joint performance within a certain range. When the resistance heat reaches the saturation interval,the shear resistance of the joint change little.