Strength and fracture behaviour of diffusion-bonded joints in Al-Li (8090) alloy

The shear strength () of overlap shear test pieces made by solid state diffusion bonding or by machining thin (2.5 or 4 mm thick) Al-Li 8090 alloy sheet has been determined for various overlap lengths (/). When / < 3 mm, was independent of / and equal to 188 to 202 MPa for the bonded joint and 199 to 209 MPa for the base metal sheet. The lower mean shear strength of the bonded joint was caused by the lower resistance of intergranular fracture in the planar grain boundary at the bond interface. The bond strengths were, however, greater than those previously reported for joints in 8090 alloy made by solid-state or liquid-phase diffusion bonding and about a factor of 7 greater than those for adhesive bonded joints.     

Strength and fracture behaviour of diffusion-bonded joints in Al-Li (8090) alloy

Al-Li 8090 alloy overlap shear test pieces machined from 3 mm thick diffusion-bonded sheets showed two fracture zones at the bond interface. Zone 1 at the ends of the overlap showed predominantly intergranular fracture and zone 2 at the centre of the overlap showed peel-type fracture. The load appeared to be carried entirely by zone 1. Only zone 1 fracture was obtained in the base metal test piece. The fracture zones were caused by the non-planar stress distribution and by the bending moments associated with this type of test piece. The planar bond interface may accentuate the tendency in these alloys towards low ductility and toughness in the short transverse direction.     

Microstructure of diffusion-bonded joints in Al-Li 8090 alloy

Joints were produced between Al-Li 8090 alloy sheet by solid state (SSDB) and transient liquid-phase (TLPDB) diffusion-bonding techniques. The bond interface was a planar, thermally stable, large-angle grain boundary in the SSDB joint. Non-planar grain boundaries in a band of coarse grains were present in the TLPDB joint. The origin of these microstructures and the measured shear strengths of the joints relative to that of the parent sheet are discussed.     

Strength and fracture behaviour of diffusion bonded joints in Al-Li (8090) alloy

Peel strengths at room temperature and under superplastic forming conditions at 530 °C were measured for diffusion-bonded joints in Al-Li 8090 alloy sheet. The bonds were made in the solid state, or via a transient liquid phase using interlayers. The effect of strain rate, sheet thickness and heat treatment were investigated. The significance of these results for the testing of DB joints and for their use in DB/SPF structures is discussed.     

Fatigue Behaviour of the Al-Li alloy 8090 compared to 2024

The high cycle fatigue and the fatigue crack propagation behaviour of the new Al-Li alloy 8090 were evaluated on 25 mm plate material and compared to the conventional high strength Al alloy 2024. The investigation covered changes of test direction, R-ratio and environment. The results revealed that for most conditions the Al-Li alloy proved to be equivalent or better than the conventional Al alloy.     

Nature of the interface between AA7072 alloy explosively clad to AA8090 aluminium alloy

Aluminium-lithium based alloy plates were explosively clad with Al-1 wt% Zn alloy sheets. Clad plates were evaluated for bond continuity, interface shape, microstructure, variation of elemental concentrations across the bond interface, and bond strength. Comparisons of selected characteristics were made with roll clad sheets developed earlier.Ultrasonic tests revealed the bond to be continuous at all locations except over 50 mm wide edges of the plates. Both straight and wavy shaped interfaces were observed, often alternating arbitrarily. Microstructures on each side of the interface were distinct and characteristic of the individual alloys bonded. No localized melting was observed in the interface regions. Elemental concentration varied sharply across the bond line in the as-clad condition, later changing to a smooth profile after heat treatment. The diffusion widths, when expressed as a percentage of the cladding thickness, were much smaller than the corresponding values of previously studied roll clad sheets.'Tensile shear strength' of the clad samples exceeded the shear strength of monolithic Al-1%Zn alloy, thus indicating good bonding. The bond strength values were marginally lower than those of roll clad sheets. These differences could, perhaps, be due to the differences in the extent of elemental diffusion across the bond interface between the two techniques.     

Fracture toughness and fatigue crack propagation behavior of 8090 Al-Li alloy

We present results of fatigue tests of high-strength 8090 Al-Li alloy and data on its fatigue crack growth resistance. High strength combined with fairly high crack growth resistance and endurance limit results in much better service characteristics compared to other high-strength aluminum alloys. We discuss results of tensile and impact tests of Charpy specimens and the critical values of theJ-integral andK _1c for 10-mm-thick specimens in the T-L and L-T orientations subjected to complete and partial aging. The experimental results are compared with published data for 8090 and other high-strength aluminum alloys. We suggest a numerical method for the evaluation of fatigue strength according toda/dN-K diagrams.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 45–58, January – February, 1995.     

Effect of layered microstructure and its evolution on superplastic behaviour of AA 8090 Al–Li alloy

Abstract

Superplastic forming grade sheets of AA 8090 Al–Li alloy were observed to contain layers of different microstructure and microtexture across the thickness cross-section. Superplastic behaviour and its relationship to the concurrent microstructural and microtextural evolution of this sheet were studied at 803 K by tensile testing of specimens taken from the full thickness and the near surface and midthickness layers. Initially, the surface layers contained nearly equiaxed and relatively coarse grains with a strong S {123}〈634〉 type texture, whereas the midthickness section had elongated fine grains and a dominant Bs {011}〈211〉 texture. The stress–strain rate (σ–ε) curves exhibited minimum flow stress for the full thickness material. Varying levels of grain growth and texture weakening occurred in the above two layers, the extent of which depended on whether the layers were in separated form or as coexistents in the full thickness material. The maximum values of strain rate sensitivity index for the full thickness, surface, and centre materials were 0.82, 0.64, and 0.56, respectively. The corresponding ductility values were 475, 420, and 286% at ε=1×10^-3 s^-1.     

Critical strength criteria for DB/SPF processing of Al-Li 8090 alloy

Peel strengths are reported for solid state or transient liquid-phase diffusion-bonded (DB) joints between aluminium-lithium 8090 alloy sheets. The joints were tested under superplastic forming (SPF) conditions at 530 °C and with a progressively increasing peel angle,, in the range 0°–60°. The sheet deformed superplastically with or without peel fracture of the joints. A deformation model is proposed which predicts a critical combination of peel strength and superplastic flow stress for DB/SPF processing of the 8090 alloy and indicates peel fracture will occur when sheet thicknesses exceed 2 and 0.8 mm in solid state and transient liquid-phase diffusion-bonded joints, respectively.     

Diffusion bonding of an aluminium-lithium alloy (AA8090) using aluminium-copper alloy interlayers

Diffusion bonds have been produced between sheets of an Al-Li-Cu-Mg-Zr alloy using aluminium-4% copper vapour deposited metallic interlayers. Microstructural changes occurred both in the parent alloy and in the bond interface after diffusion bonding cycles and post-bonding heat treatments were analysed. Different metallographic techniques (light microscopy, scanning and transmission electron microscopy) have been used. Diffusion bonding trials were carried out using the same alloy (AA8090), both in non-superplastic (T6) and superplastic conditions. Differences in their behaviours in relation to diffusion bonding were observed.     

Behavior of short fatigue cracks initiated from a notch in 8090 Al-Li alloy

The rates of growth of short fatigue cracks initiated from a notch are much greater than the rates of growth of long fatigue cracks for the same values of K. A decrease in the strength of materials caused by aging affects the behavior of long cracks. The geometric form of the notch strongly affects the behavior of short cracks. The growth rate of a short crack initiated from a sharp notch decreases and attains a minimum value at a length of 0.45 mm, which is far beyond the region of its influence. However, short cracks initiated from blunt notches exhibit slower growth in the region of stress concentration than outside this region. Strain fields induced by deformation of the tip of the notch are not the only factor inhibiting the propagation of short cracks from notches. To explain the behavior of a short crack initiated at a notch, one must take into account some other factors, in particular, crack closure.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 39–44, January – February, 1995.     

Microstructural characterization of CO2 laser welds in the Al-Li based alloy 8090

The microstructural development of the Al-Li-Cu-Mg-Zr alloy 8090 has been studied after autogenous CO₂ laser welding. Sheets ranging in thickness from 1–4 mm were welded at speeds of between 20–120 mm s^–1 and powers from 1.5–3.8 kW. Optical microscopy, scanning and transmission electron microscopy were used to study the as-received base metal, the heat-affected zone and the solidified fusion zone. The base metal was supplied in a superplastically formable condition and thus had an unrecrystallized grain structure containing 1–2 m sized sub-grains with sub-micrometre and precipitates in the matrix. In the fusion zone, the as-solidified grain structure was columnar at the interface with the base metal but became equiaxed in the central region of the weld pool. The weld depth and top bead width both increased with decreasing welding speed and increasing beam power within the limits investigated. The fusion zone microstructure was cellular-dendritic. Intermetallic precipitates, which are rich in copper, magnesium, silicon (and presumably lithium), formed in the cell/dendrite boundaries. Very fine-scale precipitates were present in the as-solidified -Al matrix but there was no evidence for the , S and T₁ phases. The heat-affected zone was only 100 m wide and was characterized by regions of partial melting. Radiographs of welds reveal that porosity occurred predominantly along the weld centre-line. In partial penetration welds, two types of pores were observed: near spherical and irregular. However, in fully penetrating welds, only the spherical type of porosity was present. Overall volume fractions of porosity were measured from metallographic sections and were found to vary with welding speed and weld type, i.e. partial or full penetration.     

Development of mechanical properties in AA 8090 alloy produced by extrusion processing

Abstract

The effects of extrusion processing parameters on the mechanical properties of an AA 8090 alloy were monitored using a combination of hardness, tensile, andfracture toughness tests, and using light, transmission electron microscopy, and scanning electron microscopy. It was found that variations in the processing parameters affect the tensile properties to a greater extent in the as extruded condition than in the heat treated condition. In the former, the property changes occur as a result of both variation of grain structure and the solutionising effect during the process. In the latter, the tensile properties are controlled by the precipitation processes that occur, and the toughness remains essentially unaffected by changes in the processing conditions. Improved combinations of strength, ductility, and toughness are achieved when the material is subjected to suitable preaging treatments, which modify the precipitate morphology within the microstructure; the fracture surface characteristics of both tensile and fracture toughness test specimens reflect the microstructural changes.

MST/1115     

Relating microtexture and dynamic micro hardness in an extruded AA8090 alloy and AA8090-8 vol% SiCp composite

The present study involves combined measurements of microtexture and dynamic ultra micro hardness (DUH) in hot extruded AA8090 aluminum alloy and its composite reinforced with 8 vol% SiC_p. Both the materials show strong crystallographic fiber textures—111 and 001. The dynamic micro hardness shows a clear pattern of difference between these two fiber textures, 111 oriented grains being harder and stiffer. The difference in θ/d between the fibers, where θ and d are the average cell misorientation and cell size, respectively, was marginal in the alloy and thus cannot explain the observed hardness difference. The hardness difference can be explained from the difference in Taylor factors between the respective fibers. Elastic stiffness values estimated from microtexture and DUH follow a similar trend qualitatively.     

8090Al-Li合金的凝固机制

研究了8090Al-Li 合金在缓慢冷却时的凝固过程,结果表明:L→α-Al+L′反应是基本凝固过程,贯彻始终。由于溶质元素 Cu 的严重偏析,使得合金终凝固温度显著降低,凝固温度区间扩大,终凝温度在525℃左右,但熔体的90%以上在635—590℃之间凝固,在最后凝固区溶质大量富集,从而形成了 T_2相等低熔点共晶化合物。杂质元素 Fe,Si 具有强烈偏析倾向。在缓冷过程中,形成粗大的 Al_3Fe,AlLiSi,Al_7Cu_2Fe 等金属间化合物。