Hot cracking during solidification can be a serious problem in aluminium casting alloys under certain conditions.This feature is well known,but still insuffi ciently investigated in shape casting.This study gives a br...Hot cracking during solidification can be a serious problem in aluminium casting alloys under certain conditions.This feature is well known,but still insuffi ciently investigated in shape casting.This study gives a brief overview of the factors inf luencing hot cracking during shape casting.Five different AlSi7MgCu alloys with varying Mg and Cu contents were examined.Theoretical models,including the cracking susceptibility coeffi cient(CSC)from Clyne and Davies,were considered.Thermodynamic calculations(terminal freezing range,TFR)of the behavior of the solid fraction during solidif ication were compared to an experimentbased hot cracking indexing(HCI)method.Scanning electron microscopy(SEM)was used to compare the existing microstructure and precipitated thermodynamic phases using the software ThermoCalc Classic(TCC).Furthermore,SEM was used to investigate crack surfaces initiated by a dog-bone shaped mold during casting.A good correlation between theoretical models and the experimental hot cracking index method was observed.展开更多
Baosteel’s first BTW1 austenitic high-manganese wear-resistant steel exhibits strong deformation-induced hardening characteristics.Compared with common low-alloy martensitic wear-resistant steels in the market, it ha...Baosteel’s first BTW1 austenitic high-manganese wear-resistant steel exhibits strong deformation-induced hardening characteristics.Compared with common low-alloy martensitic wear-resistant steels in the market, it has improved impact wear resistance, hard abrasive wear, erosion wear performance, and impact toughness.The metallurgical properties of such austenitic wear-resistant steel lead to the risk of failure because of hot cracking defects in the welded structure.In wear-resistant applications, evaluating hot cracking susceptibility is necessary to avoid the effect of welding defects.In this study, the Varestraint test is used to quantitatively analyze and evaluate the hot cracking susceptibility of BTW1 austenitic high-manganese wear-resistant steel.The test results show that by controlling the content of impurity elements and grain refinement, BTW1 austenitic high-manganese wear-resistant steel effectively reduces hot cracking tendency and has a low incidence of hot cracking under small strain conditions.The developed matching welding process can effectively avoid the influence of hot cracking susceptibility.展开更多
The hot cracking tendency of 7075 semi-solid alloy under different conditions was studied by critical diameter method.The experiment and simulation results show that the dendrite arms of the rod grow from the edge to ...The hot cracking tendency of 7075 semi-solid alloy under different conditions was studied by critical diameter method.The experiment and simulation results show that the dendrite arms of the rod grow from the edge to the center.The smaller the diameter of the rod is,the more obvious the directional growth of dendrite is,and the greater the tendency of hot cracking is.Compared with ordinary melt,for semi-solid slurry,increasing mould temperature or decreasing pouring temperature can significantly decrease hot cracking tendency of 7075 alloy,decreasing hot cracking grade from 256 to 100 mm^2.Furthermore,based on the RDG criterion,the effects of solidification conditions on the hot cracking tendency were discussed combined with simulation.At the same time,the application and development of RDG criterion were also researched.展开更多
The developing of large size superalloy vacuum induction melting(VIM)ingots is limited owing to hot cracking,The hot cracking behavior of the large size GH4742 superalloy VIM ingot was investigated via experiment and ...The developing of large size superalloy vacuum induction melting(VIM)ingots is limited owing to hot cracking,The hot cracking behavior of the large size GH4742 superalloy VIM ingot was investigated via experiment and simulation.The microstructure was examined by optical microscopy,and element segregation was investigated by electron probe microanalysis.The solidification temperature range and yield strength at high temperature(YSHT)were calculated by JMatPro software.The results show that the variations of microstructure and element segregation in different locations are caused by different cooling rates.Moreover,the larger secondary dendrite arm spacing and serious element segregation of Nb accelerate hot cracking of the VIM ingot.In addition,the solidification temperature range is wider,and YSHT is lower in center than at edge of the ingot.Therefore,the hot cracking susceptibility is the highest in the center of the GH4742 superalloy VIM ingot.The critical criterion of element segregation for hot cracking is that the partition coeffcient of Nb should be larger than 0.5.展开更多
The effect of rare earths （RE） ranging from 0.1% to 1.2%（mass fraction） on hot cracking resistant property of Mg-Al alloys was investigated. The results show that hot cracking resistant property of Mg-Al alloys re...The effect of rare earths （RE） ranging from 0.1% to 1.2%（mass fraction） on hot cracking resistant property of Mg-Al alloys was investigated. The results show that hot cracking resistant property of Mg-Al alloys remarkably declines with an increase of RE addition. The causes of the decline are as follows： First, grain coarsening of Mg-Al alloys caused by RE addition reduces the fracture strain required for hot crack initiation. Second, RE reduces the eutectic microstructure of Mg- Al alloys, and as a result, shortens the time that the feeding channel remains open, making it difficult to feed the alloy. Furthermore, RE elevates the eutectic reaction temperature, which leads to the decrease in the strength of the interdendritic liquid film at the late stage of solidification. Third, when a-Mg dendrites form continuous skeletons, the interdendritic Al11 RE3 phase tends to block the feeding channels and increases the difficulty of feeding. Last, the shrinkage ratio discrepancy between Al11RE3 phases and α-Mg matrix is prone to cause shrinkage stress and promote hot crack initiation.展开更多
A novel Ni-based superalloy GH4151,with a γ′ volume fraction of about 55%and a service temperature capability up to 8oo oC,was investigated.Due to the different cooling conditions of various regions during the solid...A novel Ni-based superalloy GH4151,with a γ′ volume fraction of about 55%and a service temperature capability up to 8oo oC,was investigated.Due to the different cooling conditions of various regions during the solidification of ingots,significant cooling rate variations may lead to the occurrence of hot cracking.Conventional scanning laser microscope was utilised to investigate the solidification process and phase precipitation behaviour of the GH4151 under wide range cooling rates.The characteristics of L→γ transformation were analysed,and the growth rates of at each stage were calculated.The segregation behaviour was predicted using the Scheil equation,and the predicted results match well with the experimental results.The sensitivity coefficient for hot cracking was modified,and cracking sensitivity coefficient values for the alloy under different cooling rates were computed,revealing that the alloy is most susceptible to hot cracking at 10℃/min cooling rate.Therefore,controlling the cooling rate can reduce the possibility of hot cracking in ingot.展开更多
The prevention of hot cracking formation is of utmost importance in the production of the new Ni-Co based superalloys through the utilization of the electron beam smelting layered solidification technique(EBSL),as it ...The prevention of hot cracking formation is of utmost importance in the production of the new Ni-Co based superalloys through the utilization of the electron beam smelting layered solidification technique(EBSL),as it ensures exceptional homogeneity and dependable consistency of the specimens.In contrast to previous studies that focused on minimizing the liquid film and solidification range,our methodology adopts a distinct approach.In this research,a novel methodology was employed to mitigate internal stresses through the implementation of equiaxed grain layers via an alternately reduced cooling method.This ultimately resulted in the elimination of hot cracking.To be more specific,the transition from a columnar to an equiaxed structure was observed during the layer-by-layer construction process in the fabrication of the new Ni-Co based superalloy in EBSL.The EBSL-Ni-Co superalloy,when subjected to the alternating reduction cooling method,exhibited an internal stress of 49 MPa.This value represents a significant reduction of 83.8%compared to the internal stress observed when employing the linear reduction cooling method.Additionally,the solvus temperature of theγ-γ’eutectic phases in EBSL-Ni-Co superalloys produced by the alternating reduction cooling method is significantly higher.Intriguingly,the Nth layer of the EBSL-Ni-Co based superalloys produced by EBSL simultaneously heats treated with the preceding layers.And the low melting point phase gradually dissolved back into the matrix.The implementation of an alternating reduced cooling method successfully mitigated the formation of the liquid film in theγ-γ’eutectic phase and the buildup of internal stresses in the EBSL-Ni-Co superalloy during its manufacturing process.These discoveries open up a novel preparation procedure pathway for the manufacture of crack-free superalloys with superior mechanical characteristics using EBSL.展开更多
This study investigates the susceptibility of hot cracking and weldment heat treatment of Haynes 230 superalloy. The Varestriant test was conducted to evaluate this susceptibility. Welding was performed by gas tungste...This study investigates the susceptibility of hot cracking and weldment heat treatment of Haynes 230 superalloy. The Varestriant test was conducted to evaluate this susceptibility. Welding was performed by gas tungsten arc welding （GTAW） and plasma arc welding （PAW） with stress relief heat treatment and solid solution heat treatment. A tensile test is then performed to measure the changes in the mechanical properties of the heattreated material. The results indicate that the number of thermal cycles does not affect the susceptibility of Haynes 230 superalloy to hot cracking. However, it does increase the strain. In weldment of heat treatment, stress relief annealing increases the yield strength and tensile strength of the welded parts. The section of the tensile specimens shows fibrous fractures on the welded parts, regardless of whether they are heat-treated.展开更多
Laser powder bed fusion(LPBF)technology is a high-precision metal additive manufacturing(AM)technology.Due to the high specific strength of high strength aluminum alloys,high strength aluminum alloys fabricated by LPB...Laser powder bed fusion(LPBF)technology is a high-precision metal additive manufacturing(AM)technology.Due to the high specific strength of high strength aluminum alloys,high strength aluminum alloys fabricated by LPBF have broad application prospects in the field of light weighting.However,high strength aluminum alloys have high hot cracking susceptibility.In this study,an analysis of the hot cracking susceptibility as a function of processing parameters is presented for single tracks of LPBF processed(LPBFed)high strength Al–Cu–Mg–Mn alloy.The hot cracking in single tracks of LPBFed Al–Cu–Mg–Mn alloy is solidification cracking based on the experimental observations of microstructure.Combining Rosenthal simulations and spreading behavior of a single droplet,the critical scanning speed of single track with balling phenomenon was obtained.It was found that when the laser power was 200 W,the scanning speed exceeded 440.1 mm/s,the droplet will not be able to spread completely,which is consistent with the experimental result of 500 mm/s.Through the calculation and analysis of the microstructure and the existence time of the molten pool,it was pointed out that the reduction in the liquid phase caused by the high scanning speed,the shortening of the solidification time and the high stress caused by the high-temperature gradient promoted the generation of hot cracking.In summary,this work contains a practical guide to optimize processing parameters of LPBFed Al–Cu–Mg–Mn alloys,which provides a basis for fabricating thin walls and cubic samples without hot cracking.展开更多
Herein,a hot cracking initiation criterion based on the characteristics of solidification liquid film and the microstructure was proposed,which integrated both the mechanical and non-mechanical factors during solidifi...Herein,a hot cracking initiation criterion based on the characteristics of solidification liquid film and the microstructure was proposed,which integrated both the mechanical and non-mechanical factors during solidification.The criterion also took the effect of the shrinkage volume of the solid-liquid two-phase in the mushy zone,the flow behavior of the liquid film and the microstructure on the feeding behavior into account.Meanwhile,the effect factors of hot cracking initiation such as alloy composition,microstructure,mold design and process condition were included in this criterion,and it could quantitatively calculate whether hot cracks occurred under a certain state or not during solidification.The criterion was utilized to predict whether hot cracks occurred in Al-4.0 wt%Cu alloy in different initial solidification states or not,which was consistent with the experimental results and verified its reliability.According to the criterion expression,Vfeeding*was related with five effect factors includingη,ΔP*,l*,r*and n,in which r*and n were in positive correlation with Vfeeding*whileη,ΔP*and l*were in negative correlation with that,which provided a good instructive significance for mold design,process optimization and composition and microstructure regulation of alloys and simultaneously further enriched the mechanism and influencing factors of hot cracking initiation.Furthermore,a multiscale simulation method for calculating the characteristic parameters of hot tearing behavior during solidification was also provided in this study.展开更多
In this study,Al–4Cu alloy specimens with spherical grains and liquid flms were obtained by isothermal reheating treatment.The hot cracking of the solidifcation process was determined using a modifed constrained rod ...In this study,Al–4Cu alloy specimens with spherical grains and liquid flms were obtained by isothermal reheating treatment.The hot cracking of the solidifcation process was determined using a modifed constrained rod casting experimental apparatus,and the efect of liquid flm characteristics at the end of solidifcation on hot cracking initiation of Al–4Cu alloys was systematically investigated by combining molecular dynamics simulations and other methods.With the extension of soaking time,the liquid fraction(liquid flm fraction at the end of solidifcation)and grain shape factor increased with higher isothermal reheating temperatures.Additionally,the widened flling channel decreased the hot cracking initiation temperature and the critical hot cracking shrinkage stress was found to increase,thus reducing the hot cracking severity in Al–4Cu alloys.Molecular dynamics simulations revealed that with the extension of soaking time,the composition of the liquid flm changed at diferent isothermal reheating temperatures,but the short-range structure and atomic ordering of the liquid flm remained the same.The activity of the liquid flm increased in equilibrium,leading to a decrease in viscosity and an increase in fuidity,which contributed to the flling behaviour.After isothermal reheating at 640℃for 60 min,the liquid fraction reached the maximum,and the viscosity of the liquid flm was the minimum.In addition,almost no hot cracks were found.展开更多
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 crack...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.展开更多
The hydrogen-induced delayed cracking(HIDC)behaviors of two types of 1500 MPa grade hot stamping steels(HSSs)have been investigated by the method of slow strain rate tensile test and hydrogen permeation,where one is m...The hydrogen-induced delayed cracking(HIDC)behaviors of two types of 1500 MPa grade hot stamping steels(HSSs)have been investigated by the method of slow strain rate tensile test and hydrogen permeation,where one is manufactured by compact strip production(CSP)process which is a revolution to the traditional HSS and the other by the traditional cold rolling process.The results show that the performance of HSS produced by CSP is superior to that of the traditional HSS,due to lower hydrogen embrittlement index,lower hydrogen diffusion coefficient and lower hydrogen content.It has been found that HIDC behavior is closely associated with inclusions.The inclusions of HSS produced by CSP are mainly spherical Al-Ca-O and CaS,while the inclusions in the traditional HSS are TiN+AI2O3+MnS with sharp edges and corners.Based on these results,the influence of composition,shape and distribution of inclusions in HSS on HIDC and the mechanism of HIDC from the perspective of inclusions were analyzed and discussed.展开更多
Hot tearing is one of the most serious defects during the casting solidification process.In this study,a new type of multichannel"cross"hot tearing device was designed.The hot cracks initiation and propagati...Hot tearing is one of the most serious defects during the casting solidification process.In this study,a new type of multichannel"cross"hot tearing device was designed.The hot cracks initiation and propagation were predicted by the relationship between temperature,shrinkage force and solidification time during the casting solidification process.The reliability and practicability of the multichannel"cross"hot tearing device were verified by casting experiments and numerical simulations.The theoretical calculation based on Clyne-Davies model and numerical simulation results show that the hot tearing tendency decreases in the order:2024 Al alloy>Al-Cu alloy>Al-Si alloy at a pouring temperature of 670°C and a mold temperature of 25°C.Feeding of liquid films at the end of solidification plays an important role in the propagation process of hot tearing.The decrease of hot tearing tendency is attributed to the feeding of liquid film and intergranular bridging.展开更多
The impact of rare earth element La on the microstructure and hot crack resistance of ADC12 alloy was analyzed. The additive amount of La was 0%, 0.3 wt%, 0.6 wt% and 0.9 wt%, respectively. The results showed that, wi...The impact of rare earth element La on the microstructure and hot crack resistance of ADC12 alloy was analyzed. The additive amount of La was 0%, 0.3 wt%, 0.6 wt% and 0.9 wt%, respectively. The results showed that, with the increase of the additive amount from 0% to 0.6 wt%, the grain shape of α-Al gradually varied from developed dendritic crystal into fine dendritic crystal, equiaxed crystal and spheroidal crystal; eutectic silicon varied from needle-like or tabular shape into fine rod like shape; the hot crack force of the alloy also gradually decreased. However, when the additive amount of La reached 0.9 wt%, the excessive amount of rare earth elements was segregated within grain boundary area, forming intermetallic compounds. Therefore, the grain size of α-Al, eutectic silicon and the hot crack force of the alloy all increased. In the case that the additive amount of La reached 0.6 wt%, the best metamorphism effect and most excellent hot cracking resistance capacity of alloy were presented. The poisoning effect of rare earth element on eutectic silicon and the constitutional supercooling caused by rare earth element were the major causes for alloy modification, alloy refinement, and the main reasons for the increased hot cracking resistance.展开更多
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-de...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.展开更多
Based on the conventional uniaxial pre-tensile stress method during welding, this study presents a new method of welding with biaxial pre-stress. With the help of numerical simulation, experiments were carried out on ...Based on the conventional uniaxial pre-tensile stress method during welding, this study presents a new method of welding with biaxial pre-stress. With the help of numerical simulation, experiments were carried out on the self-designed device. Except for the control on residual stress and distortion us-welded, the experimental results also show its effect on the prevention of hot cracks, thus this method can make up for the disadvantage of the conventional pre-stress method. Hot cracks disappear when the value of pre-stress surpasses 0. 2 σs（yield limit）. Welded thin plates with low-level residual stress, little distortion and no hot cracks are obtained with longitudinal pre-tensile stress level between 0. 6σsand 0. 7σs and precompressive stress between 0. 2 σs and 0. 3 σs in transverse direction.展开更多
The hot deformation behaviors of steel D2 in the range of 900 ℃ to 1 160 ℃ and strain rate range of 0.01 s -1 to 10 s -1 have been studied by using Processing Map developed on the basis of dynamic materia...The hot deformation behaviors of steel D2 in the range of 900 ℃ to 1 160 ℃ and strain rate range of 0.01 s -1 to 10 s -1 have been studied by using Processing Map developed on the basis of dynamic materials model. The efficiency of energy dissipation η is taken as a function of temperature and strain rate to obtain a Processing Map. In the Processing Map of steel D2, there are two zones of cracking susceptivity with high dissipation efficiency η of 46 % and 63 % respectively. One zone is in the range of 900 ℃ to 980 ℃ and the strain rate range of 0.01 s -1 to 0.06 s -1 , and the other from 1 140 ℃ to 1 160 ℃ and 8 s -1 to 10 s -1 . The experiment proves that there are microstructural brittle transgranular fractures and macroscopic thermal cracks in the two zones respectively. The map also revealed that deformation in these two zones is of instable flowing , so these two zones should be avoided when choosing hot deformation conditions.展开更多
The low frequency electromagnetic casting(LFEC)was used to prevent hot cracking during the solidification process of GH4742 superalloy ingot.The effects of LFEC on the solidification macrostructure of the ingot were i...The low frequency electromagnetic casting(LFEC)was used to prevent hot cracking during the solidification process of GH4742 superalloy ingot.The effects of LFEC on the solidification macrostructure of the ingot were investigated through experiments and simulation.The results show that the average grain size decreases after application of LFEC.At the same time,the fraction of equiaxed grains increases compared with the ingots that without LFEC.In addition,the average grain size decreases and the fraction of equiaxed grains increases with increasing the current frequency.When the current frequency increases from 5 Hz to 20 Hz,the average grain size decreases from 5.39 mm to 4.74 mm,and the fraction of equiaxed grains increases from 41.21%to 55.24%.The distribution of Lorentz force,melt flow field and temperature field in the melt was simulated using COMSOL Multiphysics software.It is found that the Lorentz force increases and the forced convection is enhanced with increasing the current frequency,thus the melt flow velocity and heat transfer in the melt are promoted.It can facilitate the heterogenous nucleation in the melt,resulting in grain refinement,and further preventing hot cracking of large size ingots.展开更多
Cryogenic steels,i.e.,steels with maximum toughness at particularly low temperature,are increasingly becoming the focus of research.Cryogenic steels are usually alloyed with 5%–9%nickel.Ni can also be substituted by ...Cryogenic steels,i.e.,steels with maximum toughness at particularly low temperature,are increasingly becoming the focus of research.Cryogenic steels are usually alloyed with 5%–9%nickel.Ni can also be substituted by manganese as an austenite former.These high-manganese cryogenic grades are a cost-effective alternative to nickel-alloyed steels for use in liquefied natural gas storage tanks.The Mn content can then be more than 20 wt.%and lead to problems in production,particularly in the continuous casting process.In continuous casting of high-Mn-grades,quality issues and even breakout may result from the initial solidification behavior of the steel grades at high temperatures.Hot cracks form when a critical load is exceeded during solidification,close to the solidus temperature of the steel.A selected high-Mn-steel grade was characterized with respect to liquidus and solidus temperatures by means of thermal analysis and computational thermodynamics.In addition,so-called submerged split chill tensile tests were carried out to further understand the crack sensitivity of the solidifying shell for high-manganese cryogenic steels.The results reveal the presence of coarse hot tears,and also,a high frequency of hot cracks was observed at the location with the maximum accumulated strain,which is in line with the applied cracking criterion of Pierer and Bernhard for this investigation.In summary,the initial solidification phase of continuous casting poses a high risk of cracking for high-manganese cryogenic steel.展开更多
基金financially supported by the Austrian Research Promotion Agency FFG
文摘Hot cracking during solidification can be a serious problem in aluminium casting alloys under certain conditions.This feature is well known,but still insuffi ciently investigated in shape casting.This study gives a brief overview of the factors inf luencing hot cracking during shape casting.Five different AlSi7MgCu alloys with varying Mg and Cu contents were examined.Theoretical models,including the cracking susceptibility coeffi cient(CSC)from Clyne and Davies,were considered.Thermodynamic calculations(terminal freezing range,TFR)of the behavior of the solid fraction during solidif ication were compared to an experimentbased hot cracking indexing(HCI)method.Scanning electron microscopy(SEM)was used to compare the existing microstructure and precipitated thermodynamic phases using the software ThermoCalc Classic(TCC).Furthermore,SEM was used to investigate crack surfaces initiated by a dog-bone shaped mold during casting.A good correlation between theoretical models and the experimental hot cracking index method was observed.
文摘Baosteel’s first BTW1 austenitic high-manganese wear-resistant steel exhibits strong deformation-induced hardening characteristics.Compared with common low-alloy martensitic wear-resistant steels in the market, it has improved impact wear resistance, hard abrasive wear, erosion wear performance, and impact toughness.The metallurgical properties of such austenitic wear-resistant steel lead to the risk of failure because of hot cracking defects in the welded structure.In wear-resistant applications, evaluating hot cracking susceptibility is necessary to avoid the effect of welding defects.In this study, the Varestraint test is used to quantitatively analyze and evaluate the hot cracking susceptibility of BTW1 austenitic high-manganese wear-resistant steel.The test results show that by controlling the content of impurity elements and grain refinement, BTW1 austenitic high-manganese wear-resistant steel effectively reduces hot cracking tendency and has a low incidence of hot cracking under small strain conditions.The developed matching welding process can effectively avoid the influence of hot cracking susceptibility.
基金Project(17YF1407100)supported by the Shanghai Sailing Program of ChinaProject(17PJ1408600)supported by Shanghai Pujiang Program of China
文摘The hot cracking tendency of 7075 semi-solid alloy under different conditions was studied by critical diameter method.The experiment and simulation results show that the dendrite arms of the rod grow from the edge to the center.The smaller the diameter of the rod is,the more obvious the directional growth of dendrite is,and the greater the tendency of hot cracking is.Compared with ordinary melt,for semi-solid slurry,increasing mould temperature or decreasing pouring temperature can significantly decrease hot cracking tendency of 7075 alloy,decreasing hot cracking grade from 256 to 100 mm^2.Furthermore,based on the RDG criterion,the effects of solidification conditions on the hot cracking tendency were discussed combined with simulation.At the same time,the application and development of RDG criterion were also researched.
基金supported by the National Natural Science Foundation of China(Nos.Ui708253 and 51571052)。
文摘The developing of large size superalloy vacuum induction melting(VIM)ingots is limited owing to hot cracking,The hot cracking behavior of the large size GH4742 superalloy VIM ingot was investigated via experiment and simulation.The microstructure was examined by optical microscopy,and element segregation was investigated by electron probe microanalysis.The solidification temperature range and yield strength at high temperature(YSHT)were calculated by JMatPro software.The results show that the variations of microstructure and element segregation in different locations are caused by different cooling rates.Moreover,the larger secondary dendrite arm spacing and serious element segregation of Nb accelerate hot cracking of the VIM ingot.In addition,the solidification temperature range is wider,and YSHT is lower in center than at edge of the ingot.Therefore,the hot cracking susceptibility is the highest in the center of the GH4742 superalloy VIM ingot.The critical criterion of element segregation for hot cracking is that the partition coeffcient of Nb should be larger than 0.5.
文摘The effect of rare earths （RE） ranging from 0.1% to 1.2%（mass fraction） on hot cracking resistant property of Mg-Al alloys was investigated. The results show that hot cracking resistant property of Mg-Al alloys remarkably declines with an increase of RE addition. The causes of the decline are as follows： First, grain coarsening of Mg-Al alloys caused by RE addition reduces the fracture strain required for hot crack initiation. Second, RE reduces the eutectic microstructure of Mg- Al alloys, and as a result, shortens the time that the feeding channel remains open, making it difficult to feed the alloy. Furthermore, RE elevates the eutectic reaction temperature, which leads to the decrease in the strength of the interdendritic liquid film at the late stage of solidification. Third, when a-Mg dendrites form continuous skeletons, the interdendritic Al11 RE3 phase tends to block the feeding channels and increases the difficulty of feeding. Last, the shrinkage ratio discrepancy between Al11RE3 phases and α-Mg matrix is prone to cause shrinkage stress and promote hot crack initiation.
基金supported by the National Science and Technology Major Project(J2019-VI-0006-0120)National Natural Science Foundation of China(52074092).
文摘A novel Ni-based superalloy GH4151,with a γ′ volume fraction of about 55%and a service temperature capability up to 8oo oC,was investigated.Due to the different cooling conditions of various regions during the solidification of ingots,significant cooling rate variations may lead to the occurrence of hot cracking.Conventional scanning laser microscope was utilised to investigate the solidification process and phase precipitation behaviour of the GH4151 under wide range cooling rates.The characteristics of L→γ transformation were analysed,and the growth rates of at each stage were calculated.The segregation behaviour was predicted using the Scheil equation,and the predicted results match well with the experimental results.The sensitivity coefficient for hot cracking was modified,and cracking sensitivity coefficient values for the alloy under different cooling rates were computed,revealing that the alloy is most susceptible to hot cracking at 10℃/min cooling rate.Therefore,controlling the cooling rate can reduce the possibility of hot cracking in ingot.
基金support from the National Key Research and Development Program of China(Grant No.2019YFA0705300)the National Natural Science Foundation of China(GrantNo.52004051)the Innovation Team Projectfor Key Fields of Dalian(Grant No.2019RT13).
文摘The prevention of hot cracking formation is of utmost importance in the production of the new Ni-Co based superalloys through the utilization of the electron beam smelting layered solidification technique(EBSL),as it ensures exceptional homogeneity and dependable consistency of the specimens.In contrast to previous studies that focused on minimizing the liquid film and solidification range,our methodology adopts a distinct approach.In this research,a novel methodology was employed to mitigate internal stresses through the implementation of equiaxed grain layers via an alternately reduced cooling method.This ultimately resulted in the elimination of hot cracking.To be more specific,the transition from a columnar to an equiaxed structure was observed during the layer-by-layer construction process in the fabrication of the new Ni-Co based superalloy in EBSL.The EBSL-Ni-Co superalloy,when subjected to the alternating reduction cooling method,exhibited an internal stress of 49 MPa.This value represents a significant reduction of 83.8%compared to the internal stress observed when employing the linear reduction cooling method.Additionally,the solvus temperature of theγ-γ’eutectic phases in EBSL-Ni-Co superalloys produced by the alternating reduction cooling method is significantly higher.Intriguingly,the Nth layer of the EBSL-Ni-Co based superalloys produced by EBSL simultaneously heats treated with the preceding layers.And the low melting point phase gradually dissolved back into the matrix.The implementation of an alternating reduced cooling method successfully mitigated the formation of the liquid film in theγ-γ’eutectic phase and the buildup of internal stresses in the EBSL-Ni-Co superalloy during its manufacturing process.These discoveries open up a novel preparation procedure pathway for the manufacture of crack-free superalloys with superior mechanical characteristics using EBSL.
文摘This study investigates the susceptibility of hot cracking and weldment heat treatment of Haynes 230 superalloy. The Varestriant test was conducted to evaluate this susceptibility. Welding was performed by gas tungsten arc welding （GTAW） and plasma arc welding （PAW） with stress relief heat treatment and solid solution heat treatment. A tensile test is then performed to measure the changes in the mechanical properties of the heattreated material. The results indicate that the number of thermal cycles does not affect the susceptibility of Haynes 230 superalloy to hot cracking. However, it does increase the strain. In weldment of heat treatment, stress relief annealing increases the yield strength and tensile strength of the welded parts. The section of the tensile specimens shows fibrous fractures on the welded parts, regardless of whether they are heat-treated.
基金This work was financially supported by the National Natural Science Foundation of China(Grant Nos.61575074,51805184 and 61475056)the Self-Research and Development Plan of Naval University of Engineering(Nos.2022505010 and 2022501140)the Plan for Strengthening Basic Disciplines of College of Ship and Ocean.The authors thank the Analytical and Testing Center of HUST for EBSD measurement.
文摘Laser powder bed fusion(LPBF)technology is a high-precision metal additive manufacturing(AM)technology.Due to the high specific strength of high strength aluminum alloys,high strength aluminum alloys fabricated by LPBF have broad application prospects in the field of light weighting.However,high strength aluminum alloys have high hot cracking susceptibility.In this study,an analysis of the hot cracking susceptibility as a function of processing parameters is presented for single tracks of LPBF processed(LPBFed)high strength Al–Cu–Mg–Mn alloy.The hot cracking in single tracks of LPBFed Al–Cu–Mg–Mn alloy is solidification cracking based on the experimental observations of microstructure.Combining Rosenthal simulations and spreading behavior of a single droplet,the critical scanning speed of single track with balling phenomenon was obtained.It was found that when the laser power was 200 W,the scanning speed exceeded 440.1 mm/s,the droplet will not be able to spread completely,which is consistent with the experimental result of 500 mm/s.Through the calculation and analysis of the microstructure and the existence time of the molten pool,it was pointed out that the reduction in the liquid phase caused by the high scanning speed,the shortening of the solidification time and the high stress caused by the high-temperature gradient promoted the generation of hot cracking.In summary,this work contains a practical guide to optimize processing parameters of LPBFed Al–Cu–Mg–Mn alloys,which provides a basis for fabricating thin walls and cubic samples without hot cracking.
基金the National Natural Science Foundation of China(No.51875365).
文摘Herein,a hot cracking initiation criterion based on the characteristics of solidification liquid film and the microstructure was proposed,which integrated both the mechanical and non-mechanical factors during solidification.The criterion also took the effect of the shrinkage volume of the solid-liquid two-phase in the mushy zone,the flow behavior of the liquid film and the microstructure on the feeding behavior into account.Meanwhile,the effect factors of hot cracking initiation such as alloy composition,microstructure,mold design and process condition were included in this criterion,and it could quantitatively calculate whether hot cracks occurred under a certain state or not during solidification.The criterion was utilized to predict whether hot cracks occurred in Al-4.0 wt%Cu alloy in different initial solidification states or not,which was consistent with the experimental results and verified its reliability.According to the criterion expression,Vfeeding*was related with five effect factors includingη,ΔP*,l*,r*and n,in which r*and n were in positive correlation with Vfeeding*whileη,ΔP*and l*were in negative correlation with that,which provided a good instructive significance for mold design,process optimization and composition and microstructure regulation of alloys and simultaneously further enriched the mechanism and influencing factors of hot cracking initiation.Furthermore,a multiscale simulation method for calculating the characteristic parameters of hot tearing behavior during solidification was also provided in this study.
基金supported by the National Natural Science Foundation of China(No.51875365).
文摘In this study,Al–4Cu alloy specimens with spherical grains and liquid flms were obtained by isothermal reheating treatment.The hot cracking of the solidifcation process was determined using a modifed constrained rod casting experimental apparatus,and the efect of liquid flm characteristics at the end of solidifcation on hot cracking initiation of Al–4Cu alloys was systematically investigated by combining molecular dynamics simulations and other methods.With the extension of soaking time,the liquid fraction(liquid flm fraction at the end of solidifcation)and grain shape factor increased with higher isothermal reheating temperatures.Additionally,the widened flling channel decreased the hot cracking initiation temperature and the critical hot cracking shrinkage stress was found to increase,thus reducing the hot cracking severity in Al–4Cu alloys.Molecular dynamics simulations revealed that with the extension of soaking time,the composition of the liquid flm changed at diferent isothermal reheating temperatures,but the short-range structure and atomic ordering of the liquid flm remained the same.The activity of the liquid flm increased in equilibrium,leading to a decrease in viscosity and an increase in fuidity,which contributed to the flling behaviour.After isothermal reheating at 640℃for 60 min,the liquid fraction reached the maximum,and the viscosity of the liquid flm was the minimum.In addition,almost no hot cracks were found.
文摘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.
基金The authors would like to express sincere gratitude to Dr.Na Luo for her insightful comments and helpful assistance to the revision of this manuscript.This work was supported by National Natural Science Foundation of China(No.51871172).
文摘The hydrogen-induced delayed cracking(HIDC)behaviors of two types of 1500 MPa grade hot stamping steels(HSSs)have been investigated by the method of slow strain rate tensile test and hydrogen permeation,where one is manufactured by compact strip production(CSP)process which is a revolution to the traditional HSS and the other by the traditional cold rolling process.The results show that the performance of HSS produced by CSP is superior to that of the traditional HSS,due to lower hydrogen embrittlement index,lower hydrogen diffusion coefficient and lower hydrogen content.It has been found that HIDC behavior is closely associated with inclusions.The inclusions of HSS produced by CSP are mainly spherical Al-Ca-O and CaS,while the inclusions in the traditional HSS are TiN+AI2O3+MnS with sharp edges and corners.Based on these results,the influence of composition,shape and distribution of inclusions in HSS on HIDC and the mechanism of HIDC from the perspective of inclusions were analyzed and discussed.
基金This work was financially supported by the National Natural Science Foundation of China(Grant No.51875365).
文摘Hot tearing is one of the most serious defects during the casting solidification process.In this study,a new type of multichannel"cross"hot tearing device was designed.The hot cracks initiation and propagation were predicted by the relationship between temperature,shrinkage force and solidification time during the casting solidification process.The reliability and practicability of the multichannel"cross"hot tearing device were verified by casting experiments and numerical simulations.The theoretical calculation based on Clyne-Davies model and numerical simulation results show that the hot tearing tendency decreases in the order:2024 Al alloy>Al-Cu alloy>Al-Si alloy at a pouring temperature of 670°C and a mold temperature of 25°C.Feeding of liquid films at the end of solidification plays an important role in the propagation process of hot tearing.The decrease of hot tearing tendency is attributed to the feeding of liquid film and intergranular bridging.
基金Supported by National Natural Science Foundation of China(Nos.51364035,51165032)Specialized Research Fund for the Doctoral Program of Higher Education(No.20133601110001)+3 种基金Loading Program of Science and Technology of College of Jiangxi Province(No.KJLD14003)Production and Teaching and Research Cooperation plan of Nanchang Non-party Experts and Doctor(No.2012-CYH-DW-XCL-002)Postgraduate Innovative Foundation of Jiangxi ProvinceOpen Project Program of Jiangxi Engineering Research Center of Process and Equipment for New Energy,East China Institute of Technology(No.JXNE2015-09)
文摘The impact of rare earth element La on the microstructure and hot crack resistance of ADC12 alloy was analyzed. The additive amount of La was 0%, 0.3 wt%, 0.6 wt% and 0.9 wt%, respectively. The results showed that, with the increase of the additive amount from 0% to 0.6 wt%, the grain shape of α-Al gradually varied from developed dendritic crystal into fine dendritic crystal, equiaxed crystal and spheroidal crystal; eutectic silicon varied from needle-like or tabular shape into fine rod like shape; the hot crack force of the alloy also gradually decreased. However, when the additive amount of La reached 0.9 wt%, the excessive amount of rare earth elements was segregated within grain boundary area, forming intermetallic compounds. Therefore, the grain size of α-Al, eutectic silicon and the hot crack force of the alloy all increased. In the case that the additive amount of La reached 0.6 wt%, the best metamorphism effect and most excellent hot cracking resistance capacity of alloy were presented. The poisoning effect of rare earth element on eutectic silicon and the constitutional supercooling caused by rare earth element were the major causes for alloy modification, alloy refinement, and the main reasons for the increased hot cracking resistance.
文摘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.
文摘Based on the conventional uniaxial pre-tensile stress method during welding, this study presents a new method of welding with biaxial pre-stress. With the help of numerical simulation, experiments were carried out on the self-designed device. Except for the control on residual stress and distortion us-welded, the experimental results also show its effect on the prevention of hot cracks, thus this method can make up for the disadvantage of the conventional pre-stress method. Hot cracks disappear when the value of pre-stress surpasses 0. 2 σs（yield limit）. Welded thin plates with low-level residual stress, little distortion and no hot cracks are obtained with longitudinal pre-tensile stress level between 0. 6σsand 0. 7σs and precompressive stress between 0. 2 σs and 0. 3 σs in transverse direction.
文摘The hot deformation behaviors of steel D2 in the range of 900 ℃ to 1 160 ℃ and strain rate range of 0.01 s -1 to 10 s -1 have been studied by using Processing Map developed on the basis of dynamic materials model. The efficiency of energy dissipation η is taken as a function of temperature and strain rate to obtain a Processing Map. In the Processing Map of steel D2, there are two zones of cracking susceptivity with high dissipation efficiency η of 46 % and 63 % respectively. One zone is in the range of 900 ℃ to 980 ℃ and the strain rate range of 0.01 s -1 to 0.06 s -1 , and the other from 1 140 ℃ to 1 160 ℃ and 8 s -1 to 10 s -1 . The experiment proves that there are microstructural brittle transgranular fractures and macroscopic thermal cracks in the two zones respectively. The map also revealed that deformation in these two zones is of instable flowing , so these two zones should be avoided when choosing hot deformation conditions.
基金financially supported by the Major Projects in Aviation Engines and Gas Turbines (Grant No.2019-VI-0020-0136)the National Key Research and Development Program of China (Grant Nos.2022YFB3705101&2022YFB3705102)+1 种基金the National Natural Science Foundation of China (Grant No.U1708253)the Fundamental Research Funds for the Central Universities,China (Grant No.N2302005)。
文摘The low frequency electromagnetic casting(LFEC)was used to prevent hot cracking during the solidification process of GH4742 superalloy ingot.The effects of LFEC on the solidification macrostructure of the ingot were investigated through experiments and simulation.The results show that the average grain size decreases after application of LFEC.At the same time,the fraction of equiaxed grains increases compared with the ingots that without LFEC.In addition,the average grain size decreases and the fraction of equiaxed grains increases with increasing the current frequency.When the current frequency increases from 5 Hz to 20 Hz,the average grain size decreases from 5.39 mm to 4.74 mm,and the fraction of equiaxed grains increases from 41.21%to 55.24%.The distribution of Lorentz force,melt flow field and temperature field in the melt was simulated using COMSOL Multiphysics software.It is found that the Lorentz force increases and the forced convection is enhanced with increasing the current frequency,thus the melt flow velocity and heat transfer in the melt are promoted.It can facilitate the heterogenous nucleation in the melt,resulting in grain refinement,and further preventing hot cracking of large size ingots.
基金supported by National Natural Science Foundation of China(Grant Nos.52174324,51974213 and 52204351)the China Postdoctoral Science Foundation(2022M722487)+1 种基金Open fund project(Grant No.FMRUlab23-05)supported by Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Educationsupport under the scope of the COMET program within the K2 Center“Integrated Computational Material,Process and Product Engineering(IC-MPPE)”(Project No.886385).
文摘Cryogenic steels,i.e.,steels with maximum toughness at particularly low temperature,are increasingly becoming the focus of research.Cryogenic steels are usually alloyed with 5%–9%nickel.Ni can also be substituted by manganese as an austenite former.These high-manganese cryogenic grades are a cost-effective alternative to nickel-alloyed steels for use in liquefied natural gas storage tanks.The Mn content can then be more than 20 wt.%and lead to problems in production,particularly in the continuous casting process.In continuous casting of high-Mn-grades,quality issues and even breakout may result from the initial solidification behavior of the steel grades at high temperatures.Hot cracks form when a critical load is exceeded during solidification,close to the solidus temperature of the steel.A selected high-Mn-steel grade was characterized with respect to liquidus and solidus temperatures by means of thermal analysis and computational thermodynamics.In addition,so-called submerged split chill tensile tests were carried out to further understand the crack sensitivity of the solidifying shell for high-manganese cryogenic steels.The results reveal the presence of coarse hot tears,and also,a high frequency of hot cracks was observed at the location with the maximum accumulated strain,which is in line with the applied cracking criterion of Pierer and Bernhard for this investigation.In summary,the initial solidification phase of continuous casting poses a high risk of cracking for high-manganese cryogenic steel.
