Be／Al激光焊接缺陷的研究

通过实验手段并改变激光焊接参数对Be／Al激光焊接缺陷的起因进行研究。实验分别采用热传导焊和深熔焊两种焊接模式制备样品。结果表明：激光焊接裂纹主要产生于热传导焊焊件的熔合区根部，其产生是热应力、焊件根部形状和杂质偏析其同作用的结果；而气孔主要出现在深熔焊焊接的焊件中：缺陷的种类和焊接参数有很大的关系。     

Measuring laser weldability of aluminium alloys using controlled restraint weldability test

Abstract

Al–Mg–Si alloys are known to be highly susceptible to solidification cracking. Weldability results of laminated AA6061-T6 plates are presented in this paper when welded in full penetration keyhole mode using a 1030 nm, 10 kW Yb:YAG disk laser welding source with different welding conditions. Making use of the controlled restraint weldability (CRW) cracking test, a boundary has been established between crack and no crack conditions for different preloads. The originality of the CRW test is the cross-shaped test coupon that partitions the pre-stress unequally along the welding path. The CRW is proven capable of ranking the solidification cracking behaviour of weld metals deposited under different welding conditions.     

Effect of solidification velocity on weld solidification process of alloy tool steel

Abstract

In order to clarify the effect of solidification velocity on the weld solidification process of alloy tool steel during the welding, the information about microstructure evolution was obtained by the concurrent experiments of liquid tin quenching and time resolved X-ray diffraction technique using intense synchrotron radiation. It was found from the experiments that the solidification mode was transferred from an FA to an A mode at the high solidification velocity. The effect of solidification velocity on the phase selection during solidification between the primary δ-ferrite and γ-austenite was theoretically proved by the Kurz, Giovanola and Trivedi (KGT) model. It is thus explained that the solidification cracking susceptibility of the weld metal of alloy tool steel was enhanced due to the δ to γ transition of the primary phase.     

Weldability of 1.6 mm thick aluminium alloy 5182 sheet by single and dual beam Nd: YAG laser welding

Abstract

The weldability of 1.6 mm thick 5182 Al–Mg alloy sheet by the single- and dual-beam Nd:YAG laser welding processes has been examined. Bead-on-plate welds were made using total laser powers from 2.5 to 6 kW, dual-beam lead/lag laser beam power ratios ranging from 3:2 to 2:3 and travel speeds from 4 to 15 m min^-1. The effects of focal position and shielding gas conditions on weld quality were also investigated. Whereas full penetration laser welds could be made using the 3 kW single-beam laser welder at speeds up to 15 m min^-1, the underbead surface was always very rough with undercutting and numerous projections or spikes of solidified ejected metal. This 'spikey' underbead surface geometry was attributed to the effects of the high vapour pressure Mg in the alloy on the keyhole dynamics. The undesirable 'spikey' underbead geometry was unaffected by changes in focal position, shielding gas parameters or other single-beam welding process parameters. Most full penetration dual-beam laser welds exhibited either blow-through porosity at low welding speeds (4–6 m min^-1) or unacceptable 'spikey' underbead surface quality at increased welding speeds up to 13.5 m min^-1. Radiography revealed significant occluded porosity within borderline or partial penetration welds. This was thought to be caused by significant keyhole instability that exists under these welding conditions. A limited range of dual-beam laser process conditions was found that produced sound, pore-free laser welds with good top and underbead surface quality. Acceptable welds were produced at welding speeds of 6 to 7.5 m min^-1 using total laser powers of 4.5–5 kW, but only when the lead laser beam power was greater than or equal to the lagging beam power. The improved underbead quality was attributed to the effect of the second lagging laser beam on keyhole stability, venting of the high vapour pressure Mg from the keyhole and solidification of the underbead weld metal during full penetration dual-beam laser welding.     

Failure mode transition and mechanical properties of similar and dissimilar resistance spot welds of DP600 and low carbon steels

Abstract

The present work addresses the microstructure and mechanical properties of similar and dissimilar resistance spot welds of low carbon steel (LCS) and dual phase steel (DP600). Correlations between the critical fusion zone size required to ensure pullout failure mode, the weld microstructure and the weld hardness characteristics were developed. Dissimilar DP600/LCS spot welds exhibit the lowest tendency to fail in interfacial failure mode. Effects of weld physical attributes and weld microstructure on the peak load and energy absorption of similar and dissimilar DP600/LCS resistance spot welds are analysed.     

Welding: Solidification and microstructure

Parameters that control the solidification of castings also control the solidification and microstructure of welds. However, various physical processes that occur due to the interaction of the heat source with the metal during welding add a new dimension to the understanding of the weld pool solidification. Conventional theories of solidification over a broad range of conditions can be extended to understand weld pool solidification. In certain cases, because of rapid cooling rate effects, it is not unusual to observe nonequilibrium microstructures. Recent developments in the application of computational thermodynamics and kinetic models, studies on single-crystal welds, and advanced in-situ characterization techniques have led to a better understanding of weld solidification and microstructures. Editor’s Note: A hypertext-enhanced version of this article is available on-line atwww.tms.org/pubs/journals/JOM/0306/David-0306.html For more information, contact S.A. David, Oak Ridge National Laboratory, Metals & Ceramics Division, Building 4508, MS 6095, Oak Ridge, Tennessee 37831-6095; (865) 574-4804; fax (865) 574-4928; e-mail Davidsa1@ornl.gov.     

Ultrasonic hardness measurements

Abstract

When austenitic high-alloy steel weld metal sustains single-phase solidification generally described as A-mode solidification, this is well-known to result in heightened solidification cracking susceptibility.^1–3 To reduce the solidification cracking susceptibility of austenitic stainless steel, it is known to be effective to undertake component modification such as to obtain a solidification mode called the AF mode or FA mode involving the ä phase being crystallized or retained.^{1, 2} To obtain a complete γ solidification mode in the case of high Nibase alloys, such as Fe–36%Ni alloy, however, it is necessary to arrange for high Cr addition in order to achieve component modification facilitating AF or FA mode solidification such as affects austenitic stainless steel. The result of such addition, however, is that impairment of base metal properties also self-evidently cause heavy loss of hot workability in a way that makes this approach difficult to describe as effective.     

High quality welding of stainless steel with 10 kW high power fibre laser

Abstract

The objectives of this research are to investigate penetration characteristics, to clarify welding phenomena and to develop high quality welding procedures in bead on plate welding of type 304 austenitic stainless steel plates with a 10 kW fibre laser beam. The penetration depth reached 18 mm at the maximum at 5 mm s^?1. At 50 mm s^?1 or lower welding speeds, however, porosity was generated at any fibre laser spot diameter. On the other hand, at 100 mm s^?1 or higher welding speeds, underfilling and humping weld beads were formed under the conventionally and tightly focused conditions respectively. The generation of spatters was influenced mainly by a strong shear force of a laser induced plume and was greatly reduced by controlling direction of the plume blowing out of a keyhole inlet. The humping formation was dependent upon several dynamic or static factors, such as melt volume above the surface, strong melt flow to the rear molten pool on the top surface, solidification rate and narrow molten pool width and corresponding high surface tension. Its suppression was effective by producing a wider weld bead width under the defocused laser beam conditions or reduction of melt volume out of keyhole inlet under the full penetration welding conditions. Concerning porosity, X-ray transmission in situ observation images demonstrated that pores were formed not only from the tip of the keyhole but also at the middle part because of high power density. The keyhole behaviour was stabilised using a nitrogen shielding gas, resulting in porosity prevention. Consequently, to produce high quality welds in 10 kW high power fibre laser welding, the reduction procedures of welding defects were required on the basis of understanding their formation mechanism, and 10 kW fibre laser power could produce sound deeply penetrated welds of 18 mm depth in a nitrogen shielding gas.     

Gas tungsten arc welding studies on similar and dissimilar combinations of Al-Zn-Mg alloy RDE 40 and Al-Li alloy 1441

Abstract

The present study is concerned with gas tungsten arc welding of two high strength aluminium alloys, namely, an Al-Zn-Mg alloy (RDE 40) and an Al-Li based alloy of Russian grade 1441. One of the critical requirements of these two alloys is that they should be weldable. In the present work, weldability aspects of these alloys were studied in terms of solidification cracking tendency, microstructure, tensile properties, and microhardness across the welds. These studies were extended to dissimilar welds between RDE 40 and 1441 produced via conventional gas tungsten arc (GTA) welding as well as pulsed current GTA welding. It was found that RDE 40 welds were less sensitive to solidification cracking and weld metal porosity compared with 1441 alloy. The superior weldability of RDE 40 was related to the equiaxed nature of the fusion zone and a lower sensitivity to moisture pickup. It was possible to produce RDE 40-1441 welds without defects. Pulsed current welding of RDE 40 to 1441 showed improved mechanical properties compared with conventional GTA welding, and these were related to the refinement of the fusion zone microstructure.     

Weld pool geometry during keyhole laser welding of thin steel sheets

Abstract

In industrial applications of laser welding it is often essential to obtain full penetration welds at high processing rates using minimal heat input. Keyhole welding meets these requirements when process parameters are kept close to the boundary where complete penetration switches to partial penetration welding. In the present work weld pool behaviour at the edge of the full penetration regime has been studied. Four types of keyhole penetration mode were observed. The first type is a completely developed keyhole through the material thickness and open in the root region, whereas the second type is closed at the root. The third mode is unstable and results in intermittent penetration involving periods of open and closed keyhole conditions interspersed with periods of lack of fusion. The fourth mode is a partial penetration mode. A possible explanation of the weld pool transient behaviour is presented based on three-dimensional reconstructions of the weld pools.     

Stray grain formation,thermomechanical stress and solidification cracking in single crystal nickel base superalloy welds

Abstract

In the present study, the effects of stray grain formation and thermomechanical stresses on solidification cracking in welds of single crystal Ni-base superalloys have been investigated. Welds were made in an asymmetric crystallographic orientation under three different processing conditions. As welding speed and power increased, stray grain formation became extensive, but only on one side of the weld. Solidification cracking also became more extensive and occurred mostly along the stray grain boundaries. The three welding processes have been simulated using the finite element method (FEM). The calculation results showed that thermomechanical stresses increase with welding speed and power, leading to increased susceptibility to cracking. These results agree well with experimental observations.     

Solidification and microstructure modelling of welds in aluminium alloys 5754 and 6111

Abstract

A solidification and microstructure modelling approach has been developed to predict weld metal and heat affected zone (HAZ) characteristics. The freezing range and phase evolution in the weld metal region were predicted using thermodynamic and diffusion controlled growth calculations. The calculated freezing range was correlated with the weld solidification cracking tendency. A simplified analytical model was suggested to describe thermal cycles that are experienced by the HAZ. This analytical model was coupled with a published microstructure model for age hardenable alloys to predict the hardness variations across the HAZ. The above integrated approach was evaluated using experimental welds made on non­age hardenable 5754 (Al–Mg) and age hardenable 6111 (Al–Mg–Si) alloys using gas tungsten arc, electron beam, and gas metal arc welding processes.     

Microstructural development in single crystal nickel base superalloy welds

Abstract

The welding behaviour of alloy PWA 1480, a single crystal nickel base super alloy, has been investigated. The ability to successfully weld single crystal nickel base superalloys is desirable for fabrication procedures as well as for potential repair applications in both aircraft and land based turbine systems. The microstructural development in welds of this alloy was characterised and analysed using a geometrical model developed earlier in the study of Fe–Cr–Ni single crystal welds. The microstructural features in the nickel base alloy welds and, in particular, the dendritic growth patterns, were accurately described by this model. However, several potential difficulties with the welding of the nickel base superalloys were identified. First, there is frequent formation of stray crystals which result in the loss of the single crystal nature of the weld. Second, dendritic zones are formed in the weld and these may result in a degradation of the weld properties, even if the single crystal character of the weldment remains intact. In addition, extensive cracking was found in these welds and this subject is dealt with in a companion paper.     

End cracking during one-sided submerged-arc welding

Abstract

The mechanical behaviour of end cracking during one-sided SAW was computer-simulated by thermal elastic analysis with the introduction of a dummy element. Experiments on small sized specimens and on-site large specimens have been carried out. The rotational deformation, the mechanism of formation of the end cracking and factors affecting it were investigated. It was concluded that for preventing the formation of end cracks, the end opening displacement and the rate of end opening should be reduced as far as possible. Theoretical analysis and on-site large specimen experiments showed that end cracking can be effectively prevented only if the arrangement of tack welds over the weld end is appropriate, and the stiffness and restraint of slit tab plate are moderate. By the aid of the above mentioned study, end cracking was successfully avoided in over 130 specimens of 12, 14, 16, 18 and 20mm thickness with weld length more than 1000m.     

Correlation between weld formation and processing parameters in CO2 laser welding of steels

Abstract

Laser welding, which has undergone rapid development in the past few decades, is one of the most important applications in laser materials processing. Although some general data are available, precise welding parameters are equipment specific. In the present study, a series of autogenous laser welds on mild and stainless steels has been investigated, using a Trumpf 3·0 kW CO² laser system, to establish welding parameter windows. The correlation between laser power, welding speed, and weld bead profile for bead on plate welding has been obtained. For a constant laser power, penetration depth reaches a stable value as welding speed exceeds 11 000–13 000 mm min^-1. This value is defined as the penetration threshold. Lower welding speed produces deeper penetration. However, under such conditions, the unstable keyhole and weld pool could result in undercut and porosity. The maximum penetration achievable for sound welds on both mild steel and stainless steel was investigated. The correlation between penetration threshold and power level was also established. The parameter windows established for autogenous welds can be adopted effectively on butt jointsif welding speed is reduced by 25%.     

Roles of dendrite tip undercooling and solid state diffusion in microsegregation of Fe–Nb welds

Abstract

The contributions of dendrite tip undercooling and solid state diffusion to the final degree of microsegregation of gas tungsten arc welds in Fe–Nb alloys were experimentally determined. With a partitioning coefficient of ～0·28, niobium is expected to highly segregate to the liquid during solidification. However, weld microsegregation modelling using afinite difference technique indicated that almost complete homogenisation occurs as a result of solid state diffusion during solidification and cooling. The final degree of microsegregation could be accounted for considering only solid state diffusion. Examination of specimens that had been liquid tin quenched during welding showed that a significant amount of tip undercooling occurred during solidification. The degree of undercooling and amount of solid state diffusion measured were compared with predicted values using solidification models. It was found that although tip undercooling resulted in a higher niobium concentration during initial solidification, its impact on the final degree of microsegregation was small because of the overwhelming effect of solid state diffusion.     

Characterisation of solidification cracking in pulsed Nd:YAG laser welding of 2024 aluminium alloy

Abstract

Aluminium alloy AA2024 is one of the most susceptible alloys to solidification cracking. The influence of Nd:YAG laser welding parameters on cracking severity of AA 2024 alloy was studied. Welding was performed in two modes: single spot welding and overlap spot welding. In single spot welding mode, the formation of columnar zone increases cracking severity in the fusion zone. In overlap spot welding mode, the solidification cracks were characterised as: cracks propagated from previous spot, cracks initiated from the fusion line with the previous spot, and cracks initiated from the fusion line with the base metal. It was established that in comparison there is very limited tendency for initiation of new cracks from the fusion line with the weld metal of a previous spot.     

Experimental and numerical analysis of solidification cracking behaviour in fibre laser welding of 6013 aluminium alloy

Abstract

Experimental observation and numerical modelling were employed to investigate the solidification cracking behaviour during fibre laser welding of 6013 aluminium alloy. The solidification cracking initiation location and propagation path were studied using a high speed camera system and via metallurgical analysis. A three-dimensional thermomechanical finite element model of fibre laser welding of aluminium alloys was developed, which considered cylindrical volumetric heat source, temperature dependent material properties, solidification shrinkage and stress relaxation in the weld molten pool. The transient evolution and distribution of mechanical strain in the brittle temperature range (BTR) were analysed in detail to find the factors which drove the crack initiation and propagation. The results showed that the solidification cracking initiated near the fusion line and then propagated along the centreline of the weld, which was the result of the strain distribution characteristic in BTR.     

Model for inclusion formation in low alloy steel welds

Abstract

Oxide inclusion formation in low alloy steel welds is described with a model as a function of weld metal composition and process parameters. The model was developed by coupling thermodynamic and kinetic considerations. Thermodynamic calculations considered the effect of the alloying elements and kinetic equations allowed, describing oxide formation during weld cooling. The model showed that weld cooling rate influences the inclusion characteristics and therefore must be taken into consideration. This inclusion model is capable of predicting the composition, size, number density, and oxidation sequence of inclusions and was verified with published data. The inclusion model was coupled with numerical heat transfer and fluid flow models to extend the model to a wide range of welding process conditions.     

Influence of scandium on weldability of 7010 aluminium alloy

Abstract

The commercial 7000 series aluminium alloys are based on medium strength Al–Zn–Mg and high strength Al–Zn–Mg–Cu systems. The medium strength alloys are weldable, whereas the high strength alloys are non-weldable. This is because the amount of copper present in these alloys gives rise to hot cracking during solidification of welds. As a result, the high strength Al–Zn–Mg– Cu base alloys are not used for applications where joining of components by welding is an essential step. In the present study, using a combination of qualitative Houldcroft test and quantitative Varestraint test, it is shown that a small addition of scandium to the commercial 7010 alloy reduces the hot cracking susceptibility during solidification of welds produced by the gas tungsten arc welding process. The improvement in weldability is found to be the result of the considerable grain refinement in the weld structure following the scandium addition. The results of microhardness and tensile tests are further described within the context of the present work to demonstrate that the 7010+Sc welds also exhibit a combination of improved strength and ductility.