Asymmetrical solidification structures and fluid flow in GTA welds of 7004 aluminum alloy sheets

Weld solidification structures in aluminum alloys and other materials have been studied by several investigators. In results reported to date, the welds were symmetrical about their center lines. While studying solidification structures of Gas-Tungsten Arc (GTA) welds in 7004 aluminum alloy sheet, however, some asymmetrical welds were produced. Formerly graduate student at the University of Waterloo.     

Asymmetrical solidification structures and fluid flow in GTA welds of 7004 aluminum alloy sheets

Metallurgical and Materials Transactions B - Weld solidification structures in aluminum alloys and other materials have been studied by several investigators. In results reported to date, the welds...     

Grain refinement in magnetically stirred GTA welds of aluminum alloys

The mechanisms of grain refinement have been examined for magnetically stirred gas tungsten arc (GTA) welds completely penetrating thin sheets of several aluminum alloys. Grain refinement in unstirred welds may be brought about by adding sufficient titanium to produce heterogeneous nucleation by Ti-rich particles. In some alloys magnetic stirring is shown to extend the range of welding conditions which produce a partially equiaxed structure, and to widen the equiaxed fraction of partially equiaxed welds. This is attributed to magnetic stirring lowering the temperature gradient, allowing nucleation and growth of Al-rich grains further ahead of the columnar interface growing in from the fusion boundaries. In alloys with low Ti levels, magnetic stirring may cause refinement by sweeping grains from the partially molten zone ahead of the advancing solidification interface. This mechanism requires that the partially molten zone be sufficiently wide, and that the grain size in this zone remain small.     

Grain structure and solidification cracking in oscillated arc welds of 5052 aluminum alloy

The effect of arc oscillation on grain structure and solidification cracking in GTA welds of 5052 aluminum alloy was investigated using a four-pole magnetic arc oscillator and a modified fish-bone crack test. Two different mechanisms of crack reduction were identified: one in the low frequency range of arc oscillation and the other in the high frequency range. The former was the alteration of the orientation of columnar grains, while the latter was grain refining. Neither mechanism was operative in the intermediate frequency range and solidification cracking was severe, especially when the amplitude of arc oscillation was small. Alteration of grain orientation was obtained in welds made with transverse and circular arc oscillations, but not longitudinal arc oscillation. Grain refining, on the other hand, was achieved in welds made with all three types of arc oscillation patterns. The differences between the response of alloy 5052 to arc oscillation and that of alloy 2014 observed previously were discussed.     

Dispersoid-free zones in the heat-affected zone of aluminum alloy welds

The local microstructure in the heat-affected zone 1 (HAZ1) of a laser beam-welded Al-Mg-Si-Cu aluminum alloy is investigated closely. Dispersoid-free zones (DFZs), where the dispersoids of the base material (BM) are dissolved, are found in the vicinity of the fusion line (FL). They are not uniformly surrounding a grain, but oriented toward the FL. Their width can be as large as 10 μm. Detailed analysis has revealed a decreased silicon concentration as well as a decreased hardness of the oriented dispersoid-free zones (ODFZs). Two mechanisms for the formation of this welding metallurgical feature are discussed.     

Grain structures in gas tungsten-arc welds of austenitic stainless steels with ferrite primary phase

The grain structures were investigated in full penetration gas tungsten-arc (GTA) welds in sheets of 304 and 321 austenitic stainless steels for a range of welding conditions. In type 321 steel welds, fine equiaxed ferrite dendrites were observed in the ferrite phase. The equiaxed structure was ascribed to heterogeneous nucleation of ferrite on Ti-rich cuboidal inclusions present in this steel, since these inclusions were observed at the origin of equiaxed dendrites. In type 304 welds, the ferrite grains were columnar, except in less complete penetration specimens, where a few coarse equiaxed dendrites appeared to originate from the weld surface. The secondary austenitic grain structure was columnar in both steels. In type 304 steel, the columnar austenitic grain structure did not necessarily correspond to the primary ferrite grains. In type 321 steel, the secondary austenite was columnar despite the equiaxed structure of the primary ferrite. Factors which affect the columnar-to-equiaxed transition (CET) are discussed. The failure to form equiaxed austenitic grains in type 321 steel is ascribed to austenite growing across the space between ferrite grains instead of renucleating on the primary equiaxed ferrite.     

Effect of arc oscillations on the temperature distribution and microstructure in GTA tantalum welds

A two-dimensional mathematical model was developed to calculate the thermal history in thin tantalum sheets, GTA welded with arc oscillations. The model, based on the finite difference solution of the unsteady heat flow equation, was employed to calculate the temperature distribution for several arc oscillation conditions. The obtained results were used to explain experimentally observed improved microstructures in the fusion zone achieved by welding with arc oscillations. Indications are given for using the mathematical model in choosing the optimum arc oscillation conditions required for improving the microstructure of the weld. He is presently on Sabbatical leave at Lewis Research Center Cleveland, OH 44135.     

Grain refining potency of LaB6 on aluminum alloy

Al-LaB6 alloy was successfully prepared by aluminum melt reaction method.Microstructure analysis of this alloy was carried out by field emission scanning electron microscopy(FESEM),Raman spectroscopy and transmission electron microscopy(TEM).It was found that cubic LaB6 particles were highly dispersed in aluminum matrix with a uniform edge length of about 4.5 μm.Grain refining potency of LaB6 on commercial pure aluminum was also investigated.It was shown that LaB6 could act as an effective and stable nucleation substrate for α-Al during solidification process,due to their crystallographic similarity.The coarse grains of commercial pure aluminum were obviously refined to small equiaxed ones by addition of 0.5% Al-5LaB6 alloy at 720℃.     

Some observations on cyclic deformation structures in the high-strength commercial aluminum alloy AA 7150

Load-controlled fatigue testing of the aluminum alloy AA 7150 has been conducted using four-point bending with an R ratio of +0.1 over a range of maximum stress levels from 60 to 120 pct of the 0.2 pct proof stress. The alloy, in the form of 12.5-mm rolled plate, was investigated in underaged (UA), peak-aged (PA), and overaged (OA) conditions, corresponding to a change in average precipitate sizes from 5 nm in the UA condition to 21 nm in the OA condition. Three orientations of the plate were investigated. Orientation and aging condition influenced the degree of surface topographical development but not fatigue life. Detailed transmission electron microscopy (TEM) of the fatigued surface indicated that deformation in all aging conditions occurred by planar slip. Slip was generally restricted to a single slip system within each grain, and subgrain boundaries offered little resistance to dislocation movement facilitating long slip line lengths (measured up to 310 μm) between adjacent high-angle grain boundaries. Planar slip observed in the OA condition is attributed to shearing of large strengthening precipitates, which is promoted by long slip line lengths. No evidence of surface specific changes in slip character was observed.     

Heterogeneity of crystallographic texture in friction stir welds of aluminum

Over the past decade, friction stir welding (FSW) has rapidly become an important industrial joining process, particularly in the aluminum industry. Included among the advantages of FSW are such important attributes as improved weld strength and the elimination of cracking and porosity. During the friction stir process, the metal undergoes a tortuous deformation path that is not yet fully understood. The crystallographic texture that evolves during FSW contains sharp spatial gradients that undoubtedly influence the integrity of the weld and surrounding region in subsequent performance. The locally measured textures are discussed in the context of the material flow required to produce such textures, ultimately resulting in an estimate of the flow field present during FSW.     

Three-dimensional heat flow and solidification during the autogenous GTA welding of aluminum plates

A theoretical and experimental study of heat flow and solidification during the autogenous GTA welding of aluminum plates was carried out. The theoretical part of the study involves the development of a computer model which describes three-dimensional heat flow during welding. The model, though valid for any plate thickness, is particularly useful for moderately thick plates since both full- and partial-penetration welds can be considered. The experimental part of the study, on the other hand, involves the measurement of the thermal response of the workpiece during welding, and the examination of the configuration, grain structure, and subgrain structure of the fusion zone. The experimental results were compared with the calculated ones and the agreement was very good. With the help of the computer model, the effects of welding parameters on weld penetration in moderately thick plates were discussed. These parameters are the heat input per unit length of the weld, the thickness of the workpiece, the preheating of the workpiece, and the power-density distribution of the heat source. Formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA     

Serrated flow in aluminum 6061 alloy

An investigation of serrated flow in 6061 aluminum alloy at room temperature has been made using photoelastic coatings with emphasis on two basic types of serrations. This has shown that Type A serrations are associated with Lüders bands that travel (in an ideal case) the length of the gage section under an increasing load. Secondary serrations in this type of deformation are due to disturbances in the propagation of the Lüders front. The rising load during propagation of the Type A Lüders bands is associated with a change in the gage section strain gradient that occurs at the end of “homogeneous” deformation. Type B serrations involve bands that move only during the load drop. The effect of machine stiffness on the deformation accomplished by Type B bands is also considered. Formerly Graduate Student, University of Florida, Formerly Graduate Student, University of Florida, On leave from Technical University of Istanbul, Istanbul, Turkey     

Microporosity prediction in aluminum alloy castings

A comprehensive methodology that takes into account solidification, shrinkage-driven interdendritic fluid flow, hydrogen precipitation, and porosity evolution has been developed for the prediction of the microporosity fraction and distribution in aluminum alloy castings. The approach may be used to determine the extent of gas and shrinkage porosity, i.e., the resultant microporosity which occurs due to gas precipitation and that which occurs when solidification shrinkage cannot be compensated for by the interdendritic fluid flow. A solution algorithm in which the local pressure and microporosity are coupled is presented, and details of the implementation methodology are provided. The models are implemented in a computational framework consistent with that of commonly used algorithms for fluid dynamics, allowing a straightforward incorporation into existing commercial software. The results show that the effect of microporosity on the interdendritic fluid flow cannot be neglected. The predictions of porosity profiles are validated by comparison with independent experimental measurements by other researchers on aluminum A356 alloy test castings designed to capture a variety of solidification conditions. The numerical results reproduce the characteristic microporosity profiles observed in the experimental results and also agree quantitatively with the experimentally measured porosity levels. The approach provides an enhanced capability for the design of structural castings.     

Grain refinement in 7075 aluminum by thermomechanical processing

A thermomechanical process for grain refinement in precipitation hardening aluminum alloys is reported. The process includes severe overaging, deformation, and recrystallization steps. Microstructural studies by optical and transmission electron microscopy of grain refinement in 7075 aluminum have revealed that precipitates formed during the overaging step create preferential nucleation sites for recrystallizing grains. The relationship between precipitate density following severe overaging and recrystallized grain density has been investigated; the results show that the localized deformation zones associated with particles larger than about 0.75 μ m can act at preferential nucleation sites for recrystallizing grains. The density of particles capable of producing nucleation sites for new grains is approximately ten times greater than the density of recrystallized grains. A close relationship between dislocation cell size after the deformation step and recrystallized grain density has also been established. Both quantities saturate for rolling reductions larger than approximately 85 pct. The grain size produced in 2.5 mm thick sheet by the optimum processing schedule is approximately 10 μm in longitudinal and long transverse directions and 6 μm in the short transverse direction.     

Effect of activity differences on hydrogen migration in dissimilar titanium alloy welds

The effect of alloy composition on hydrogen activity was measured for seven titanium alloys as a means to determine the tendency for hydrogen migration within dissimilar metal welds. The alloys were: Ti-CP, Ti-3A1-2.5V, Ti-3Al-2.5V-3Zr, Ti-3Al-2Nb-lTa, Ti-6A1, Ti-6A1-4V, and Ti-6Al-2Nb-lTa-0.8Mo. Hydrogen pressure-hydrogen concentration relationships were determined for temperatures from 600 ‡C to 800 ‡C and hydrogen concentrations up to approximately 3.5 at. pct (750 wppm). Fusion welds were made between Ti-CP and Ti-CP and between Ti-CP and Ti-6A1-4V to observe directly the hydrogen redistribution in similar and dissimilar metal couples. Hydrogen activity was found to be significantly affected by alloying elements, particularly Al in solid solution. At a constant Al content and temperature, an increase in the volume fraction ofΒ reduced the activity of hydrogen in α-@#@ Β alloys. Activity was also found to be strongly affected by temperature. The effect of temperature differences on hydrogen activity was much greater than the effects resulting from alloy composition differences at a given temperature. Thus, hydrogen redistribution should be expected within similar metal couples subjected to extreme temperature gradients, such as those peculiar to fusion welding. Significant hydrogen redistribution in dissimilar alloy weldments also can be expected for many of the compositions in this study. Hydride formation stemming from these driving forces was observed in the dissimilar couple fusion welds. In addition, a basis for estimating hydrogen migration in titanium welds, based on hydrogen activity data, is described.     

ICP-AES法测定纯铝及铝合金中9种元素

采用电感耦合等离子体发射光谱法对纯铝及各种铝合金中硅、锰、铬、铁、钛、铜、镁、镍、锌的分析方法进行了试验研究 ,包括试样的溶解方法 ,内标及共存元素的干扰试验 ,通过试验找到了一个分析纯铝和铝合金的准确可靠的方法 ,此方法灵敏度及准确度高 ,操作简便、快速     

Fracture toughness of alloy 690 and EN52 welds in air and water

The effect of low- and high-temperature water with high hydrogen on the fracture toughness of alloy 690 and its weld, EN52, was characterized using elastic-plastic J _IC methodology. While both materials display excellent fracture resistance in air and elevated-temperature (>93 °C) water, a dramatic degradation in toughness is observed in 54 °C water. The loss of toughness is associated with a hydrogen-induced intergranular cracking mechanism, where hydrogen is picked up from the water. Comparison of the cracking behavior in low-temperature water with that for hydrogen-precharged specimens tested in air indicates that the critical local hydrogen content required to cause low-temperature embrittlement is on the order of 120 to 160 ppm. Loading-rate studies show that cracking resistance is improved at rates above ∼ 1000 MPa √m/h, because there is insufficient time to produce grain-boundary embrittlement. Electron fractographic examinations were performed to correlate cracking behavior with microstructural features and operative fracture mechanisms.