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

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

Causes and control of welding cracks in electron-beam-welded superalloy GH4169 joints

Welding joint of GH4169 alloy with a good formation was obtained. No macroscopic defects occurred in the joint. The weld had mainly a dendritic structure; the base metal was a solid solution of Ni, Cr, and Fe, and the strengthening-phase particles such as Ni3Nb were dispersively distributed along the grain boundary. The average tensile strength of the joint reached 743.7 MPa, and the Vickers hardness of the weld exceeded HV 300. Because of the segregation of the low-melting compound Ni3Nb at the grain boundary of the fusion zone, liquid cracks tended to occur as a result of welding stress. The formation of liquid cracks was inhibited by adding an alloying element, Mn, to the welding bath, because Mn diffused to the fusion zone and the high-melting phase Mn2Nb formed, and thus the overall properties of the joint were improved.

Controlling of weld hot cracks of aluminum alloy sheets by transverse pre-stressing

A new ideological and theoretical model—a technology to control weld hot cracks by transverse compressive pre-stress in the welding of aluminum alloy was put forward,which was further proved by the subsequent self-designed test setup.Experiments are conducted on the fishbone shaped specimen under conventional welding and welding with various pre-stress values.The experimental results turn out that,the initiation rate of the weld hot cracks decreases with increasing values of the compressive pre-stress.When the pre-stress reaches 0.3-0.4 of the yield stress,the cracks even disappear.In mechanical viewpoint,the researches here develop a new way to control weld cracks.

Cold Cracking of Flux Cored Arc Welded Armour Grade High Strength Steel Weldments

In this investigation, an attempt has been made to study the influence of welding consumables on the factors that influence cold cracking of armour grade quenched and tempered （Q＆.T） steel welds. Flux cored arc welding （FCAW） process were used making welds using austenitic stainless steel （ASS） and low hydrogen ferritic steel （LHF） consumables. The diffusible hydrogen levels in the weld metal of the ASS and LHF consumables were determined by mercury method. Residual stresses were evaluated using X-ray stress analyzer and implant test was carried out to study the cold cracking of the welds. Results indicate that ASS welds offer a greater resistance to cold cracking of armour grade Q＆T steel welds.

Development of the model for simulating weld metal solidification cracking in stainless steel

According to the characteristics of welding process, this paper divided the welding joint of a weldment into three zones: the liquid zone in the molten pool, the solid liquid co existing zone and the solid zone. In order to develop the stress/strain numerical model, the mechanical behaviors of the three zones were analyzed in detail. Moreover, Based on the solid fractions during solidification process and loading unloading deforming curves of stainless steel SUS310, this paper also studied the effects of deformation of molten pool, the rheologic properties and solidification shrinkage on stress/strain evaluating processes. Finally, the influence of the deformation in the molten pool was eliminated by element rebirth method. Furthermore, the algorithm of the thermal stress/strain for the solid metal formulated on the basis of the incremental thermo elastoplastic constitutive theory. As a result, a numerical simulation model of stress/strain distributions for welding solidification crack was developed.

NUMERICAL SIMULATION OF STRESS-STRAIN DISTRIBUTIONS FOR WELD METAL SOLIDIFICATION CRACKING IN STAINLESS STEEL

This paper analyzed the characteristics of welding solidification crack of stainless steels,and clearly re- vealed the the of the deformation in the molten - the pool and the solidification shrinkage on the stress - strain fields in the trail of molten - weld pool.Moreover, rheologic properties of the alloys in solid - liquid zone were also obtained by measuring the hading and unloading deform curves of the steels.As a result, a numerical model for simulation of stress - strain distributions of welding solidifi- cation crack was developed. On the basis of the model,the thesis simulated the driving force of solidifi- cation crack of stainless steels, that is, stress - strain fields in the trail of molten-weld pool with fi- nite element method.

Stress/strain distributions for weld metal solidification crack instain less steels

This paper has simulated the driving force of solidification crack of stainless steels, that is, stress/strain field in the trail of molten pool. Firstly, the effect of the deformation in the molten pool was eliminated after the element rebirth method was adopted. Secondly, the influence of solidification shrinkage was taken into account by increasing thermal expansion coefficients of the steels at elevated temperatures. Finally, the stress/strain distributions of different conditions have been computed and analyzed. Furthermore, the driving force curves of the solidification crack of the steels have been obtained by converting strain time curves into strain temperature curves, which founds a basis for predicting welding solidification crack.

Three Dimensional Numerical Simulation for the Driving Force of Weld Solidification Cracking

The double ellipsoidal model of heat source is used to analyze the thermal distributions with a three dimensional finite element method (FEM). In the mechanical model, solidification effects are treated by a dynamic element rebirth scheme. The driving force is obtained in the cracking susceptible temperature range. Moreover, this paper presents the effect of solidification shrinkage, external restraint, weld start locations and material properties on the driving force. The comparison between the simulated driving force and the experimental measurements of the material resistance predicts the susceptibility of weld metal solidification cracking.

New advances for cast iron welding in China in last decade

In this paper. the research activities in the field cast iron welding in China in China in last decade are introduced. Investigations related to hot cracking and cold cracking of cast iron welding are reviewed Development of new consumables for welding ductile iron.compacted graphite iron and austempered ductile iron are also described.

A new method--the inverse strain method(ISM)for preventing welding hot cracking of high strength aluminium alloy LY12CZ

In this paper a new method for preventing welding hot cracking—the inverse strain method(ISM)is developed on the principle of welding mechan- ics.Effectiveness and feasiblity of method in preventing welding hot cracking of high strength aluminum alloy LY12CZ by synchronous rolling during welding (SRDW)along both sides of the weld at a suitable distance behind the welding arc are examined.Experimental resulte indicate that welding hot cracking of LY12CY can be effectively prevented and the mechanical properties of welded joint can also be improved by the method.It is an important new solution for preventing hot cracking in welding of sheet metal.

Study on an Explosion Treated Steel Weld Metal with Prepared Crack

A softening zone with recrystallized grain around the prepared crack tip in the shock waves treated C-Mn steel weld metal was observed. It is suggested that a dynamic recovery occurred around the prepared crack tip even at a low explosion pressure (5 GPa) because of the stress and strain concentration at the crack tip when shock waves pass through. This result supports the previous work on the improved mechanical properties of a shock treated welded joint with residual crack from the viewpoint of microstructure.

A Computer Aided System for Simulating Weld Metal Solidification Crack

A computer-aided system for simulating weld solidification crack has been developed by which a welding engineer can carry out the welding solidification crack simulation on the basis of a commercial finite element analysis software package. its main functions include calculating the heat generations of the moving arc. mesh generation, calculating stress-strain distributions with element rebirth technique.

Investigation on Cracking in the Surfacing Welding Layer of Ni_3Al Based Alloy

Investigation has been made into the causes of cracking in the Surfacing welding layer of Ni3Al based alloy by analysing both the liqu id-to-solid transformation in the molten pool and the distribution of thermal stress within the surfacing welding layer. The results show that cracking in the surfacing welding layer is directly related to the producing of eutectic phase β' (NiAl) in the interdendritic region and high thermal stress within the surfacing welding layer. When the process of electric arc surfacing welding is changed from along straight line to along' Z' pattern, cracking in the surfacing welding layer of Ni3Al based alloy is prevented due to being reduced of both the cooling rate of liquid in the molten pool and the moving speed of the heat source. Reducing the melting volume of the substrate material by lowering the output power of electric arc welding would make the content of iron atoms in the molten pool decrease. and this also can reduce the trend of the eutectic reaction in the interdendfitic region and is helpful to Suppress cracking in the surfacing welding layer.

The effect of solidification behaviour of austenitic stainless steel on the solidification cracking susceptibility

On the basis of quantitative evaluation of susceptibility to solidification cracking with Trans-Varestraint-Test, the microstructures of two stainless steels of 316L and alloy 800H with different Creq/Nieq ratios during solidification process were analyzed with several methods. It is concluded that the susceptibility to solidification cracking of 316L-stainless steel is much lower than that of alloy 800H due to different solidification behaviors of the weld metal of the two materials. The weld metal of alloy 800H solidifies in the form of primary austenite whose boundaries are straight and smooth and easily wetted by low melting-point liquid phases, which increases the susceptibility to solidification cracking; while the 316L weld metal solidifies into primary austenite/ferrite. Owing to a series of dynamic microstructure changes during solidification such as peritectic reactions, migration of austenitic boundaries and nailing of δ-ferrite to the boundaries, the grains become finer, the orientations of columnar grains get disordered and the boundaries are curved and complex. Also high temperature δ-ferrite exists, segregation of impurities at boundaries decreases and the boundaries are hard to be wetted by liquid films, which reduces the cracking susceptibility.

WELD ROOT MAGNIFICATION FACTORS FOR SEMI-ELLIPTICAL CRACKS IN T-BUTT JOINTS

Many researchers have focused their efforts on fatigue failures occurring on weld toes. In recent years, more and more fatigue failures occur on weld roots. Therefore, it is important to explore the behaviour of weld root fatigues. This paper investigates numerically the Magnification factors （Mk） for types of semi-elliptical cracks on the weld root of a T-butt joint. The geometry of the joint is determined by four important parameters： crack depth ratio, crack shape ratio, weld leg ratio and weld angle. A singular element approach is used to generate the corresponding finite element meshes. For each set of given four parameters of the semi-elliptical root crack, the corresponding T-butt joint is numerically simulated and its Mk at the deepest point of the weld root crack is obtained for the respective tension and shear loads. The variation range of the four parameters covers 750 cases for each load, totaling 1500 simulations are completed. The numerical results obtained are then represented by the curve to explore the effects of four parameters on the Mk. To obtain an approximate equation representing Mk as a function of the four parameters for each load, a multiple regression method is adopted and the related regression analysis is performed. The error distributions of the two approximate equations are compared with the finite element data. It is confirmed that the obtained approximate functions fit very well to the database from which they are derived. Therefore, these two equations present a valuable reference for engineering applications in T-butt joint designs.

INVESTIGATION ON BRITTLE FRACTUREPROPAGATION－ARRESTING CHARACTERISTICS OF STEEL PIPE

In order to study crack propagation-arresting characteristics of steel pipe construction, a new test installation was designed. The experiments of 20 steel pipes, pipes with longitudinally weld, pipes with circumferential weld and steel pipe with sleeve were conducted. The testing results indi- cated that circumferential weld in pipe with overrnatching weld had beneficial effect on crack prop- agation arrcsting propcrtics and the mechanical split sleeve can be used as crack arrester to prevent crack propagation.

Weldability of Low Carbon Transformation Induced Plasticity Steel

Transformation induced plasticity （TRIP） steel exhibited high or rather high carbon equivalent （CE） because of its chemical composition, which was a particularly detrimental factor affecting weldability of steels. Thus the weldability of a TRIP steel （grade 600） containing （in mass percent, %） 0.11C-1. 19Si-1.67Mn was extensively studied. The mechanical properties and impact toughness of butt joint, the welding crack susceptibility of weld and heat affected zone （HAZ） for tee joint, control thermal severity （CTS） of the welded joint, and Y shape 60° butt joint were measured after the gas metal arc welding （GMAW） test. The tensile strength of the weld was higher than 700 MPa. Both in the fusion zone （FZ） and HAZ for butt joint, the impact toughness was much higher than 27 J, either at room temperature or at -20 ℃, indicating good low temperature impact ductility of the weld of TRIP 600 steel. In addition, welding crack susceptibility tests revealed that weldments were free of surface crack and other imperfection. All experimental results of this steel showed fairly good weldability. For application, the crossmember in automobile made of this steel exhibited excellent weldability, and fatigue and durability tests were also accomplished for crossmember assembly.

Local melting mechanism and its effects on mechanical properties of friction spot welded joint for Al-Zn-Mg-Cu alloy

Local melting and the eutectic film and liquation crack formation mechanisms during friction spot weld- ing （FSpW） of Al-Zn-Mg-Cu alloy were studied by both experiment and finite element simulation. Their effects on mechanical properties of the joint were examined. When the welding heat input was high, the peak temperature in the stir zone was higher than the incipient melting temperature of the Al-Zn-Mg-Cu alloy. This resulted in local melting along the grain boundaries in this zone. In the retreating stage of the welding process, the formed liquid phase was driven by the flowing plastic material and redistributed as a ＂U-shaped＂ line in the stir zone. In the following cooling stage, this liquid phase transformed into eutectic films and liquation cracks. As a result, a new characteristic of＂U＂ line that consisted of eutectic films and liquation cracks is formed in the FSpWjoin. This ＂U＂ line was located in the high stress region when the FSpW joint was loaded, thus it was adverse to the mechanical properties of the FSpW joint. During tensile shear tests, the ＂U＂ line became a preferred crack propagation path, resulting in the occurrence of brittle fracture.

A simpliied fatigue assessment method for transverse illet welded joints

Under the as-welded condition the fatigue crack initiation period was considered nonexistent and Linear Elastic Fracture Mechanics（LEFM） was used to calculate fatigue strength for a range of weld geometries. Fracture mechanics assessment of welded joints requires accurate solutions for stress intensity factor（SIF）. However, the solutions for the SIF of complex welded joints are dificult to determine due to the complicated correction factors. Three methods for SIF prediction are discussed on illet welded specimens containing continuous or semi-elliptical surface cracks, including the traditional correction method Mk, the approximate correction method Kt, and the suggested additional crack size method（ac＋ae）.The new additional crack parameter ae is used to replace the stress concentration effect of weld proile Mk, which simpliies the calculation process. Experimental results are collected to support fatigue strength assessment of the additional crack size method.