An experimental study on the dynamic fracture of extruded AA6xxx and AA7xxx aluminium alloys

The dynamic fracture behaviour of extruded AA6xxx and AA7xxx aluminium alloys is investigated using an instrumented Charpy test machine and V-notch specimens. The specimens are made from extruded flat profiles with a rectangular cross-section of 10 mm thickness and 83 mm width. The material is in T6 temper, i.e. the peak hardness condition. The alloys have either recrystallized or fibrous grain structure. For each alloy six different Charpy impact tests are carried out in two series. In Series 1, the notch is parallel to the thickness direction of the profile (i.e. through the thickness), while the notch is perpendicular to the thickness direction in Series 2 (i.e. lying in the plane of the flat profile). In each series, the longitudinal direction of the specimen is parallel, 45° or 90° to the extrusion direction. Comprehensive fractographic investigations are carried out for the different tests and alloys. It is found that the dissipated energy is practically invariant to specimen orientation and notch direction for the recrystallized alloy. For the fibrous alloys the dissipated energy is lower when the longitudinal direction of the specimen is 90° to the extrusion direction, i.e. when the notch is parallel with the fibrous grain structure. Further, the energy dissipation is higher for Series 2 than for Series 1 due to substantial delamination and secondary cracking in Series 2. The precipitate-free zones (PFZs) formed adjacent to the grain boundary are weak areas, preferable for crack initiation and growth. This is seen in the fracture surface as facets with high density of small dimples and is more pronounced for specimens with the notch parallel to the fibre direction.     

A thermomechanical process for grain refinement in precipitation hardening AA6xxx aluminum alloys

A new thermomechanical processing method, consisting of conventional rolling and a continuous non-isothermal annealing process, has been designed to achieve substantial grain refinement through cost-efficient routes in heat treatable aluminum alloys. The method has been implemented on an alloy of interest for automotive applications and a highly-stable fine-grained microstructure with a very weak texture has been achieved.     

Influence of rolling direction on strength and ductility of aluminium and aluminium alloys produced by accumulative roll bonding

Sheets from commercial purity aluminium AA1050 and aluminium alloy AA6016 were processed by accumulative roll bonding to obtain an ultrafine-grained microstructure. The accumulative roll bonded samples showed a significant increase in specific strength paired with high ductility. Despite a strongly elongated grain structure, tensile testing of samples oriented 45° to the rolling direction revealed considerable improvement in elongation to failure compared to the samples oriented parallel to the rolling direction. From hydraulic bulge tests, it was observed that the accumulative roll bonded samples reached higher burst pressures and slightly lower equivalent strains in comparison to the as-received conventionally grain-sized samples. This behaviour reflects the extraordinary mechanical properties of the ultrafine-grained materials and indicates promising metal sheet formability.     

Substructure morphology in aluminium alloys AA 3003 and AA 3004

Abstract

The industrial alloys AA 3003 and AA 3004 have been experimentally rolled using a wide range of temperatures and reductions. The substructures obtained were extensively examined, and it is shown that when the rolling reductions are low the relationship between processing parameters and subgrain size (d) follows the well established form d^?m=a+bln Z, where Z is the Zener–Hollomon parameter. As the reduction is increased the orientations of the subgrains to the rolling direction decreases, and the subgrain size also decreases. It is concluded that the existing relationships require modification if they are to be useful in the industrial context.

MST/676     

Control of earing quality in AA 5052 and AA 5454 aluminium alloys

Abstract

The effect of discrete operations during thermomechanical processing on the earing behaviour of two aluminium alloys (AA 5052 and AA 5454) has been investigated. In the homogenized condition AA 5454 contained a fine dispersoid distribution while AA 5052 was dispersoidfree. It was found that for the dispersoid-containing material the hot-rolling process was less dominant. Nevertheless, for both alloys the earing quality could be adjusted by suitable modifications to hot-rolling, cold-reduction, and annealing cycles. Pole-figure analysis indicated differences in hot-rolled texture caused by variations in processing, which became more significant as the total hot reduction was increased.

MST/41     

Simultaneously enhanced strength and ductility of 6xxx Al alloys via manipulating meso-scale and nano-scale structures guided with phase equilibrium

Excellent comprehensive mechanical properties including good formability, high strength and high ductility are prior demands for Al-Mg-Si-Cu alloys. This study utilizes calculation of phase diagram(CALPHAD) to simplify the alloy design and meet these demands. Specifically, CALPHAD was used to finely tune the Mg/Si atomic ratio in solid solution and accurately control the type and content of second phases, especially to avoid the formation of the harmful constituent phase β-Al Fe Si. Constituents and dispersoids of only-Al Fe Mn Si phase were found in the alloy prepared. An optimized microstructure with fine grains, micro scale constituents, densely distributed submicron scale dispersoids and extremely dense nano precipitates provides effective impediment to dislocation gliding and induces transgranular fracture. Therefore, the designed alloy has better comprehensive mechanical properties than other 6 xxx series aluminum alloys, including excellent formability, strength and ductility. The low T4P strength of 149 MPa as well as the high elongation of 26.1% implies the alloy's applicability to automobile body panel forming. The yield strength was rapidly improved from 149 MPa to 277 MPa during the paint bake ageing,because the number density of precipitates is twice as high as that of some other 6xxx alloys. Meanwhile,the elongation was kept at a high level of 20.0%.     

Bending in aluminium alloys AA 6111 and AA 5754 using the cantilever bend test

Abstract

Bendability is an important forming parameter in many applications, but particularly in automotive parts where the formed parts in structures can be quite complex, and where outer skins are joined to inner panels through the hemming process. In this paper the bend performance of two aluminium based automotive alloys, the heat treatable skin alloy, AA 6111 and the non-heat treatable structural alloy AA 5754, are assessed by the cantilever bend test. This test enables the loadndash;bend angle relationship to be monitored, and provides a bend surface that can be examined for different bend angles, since the bending pin does not contact the specimen surface in the local region of the bend. The results demonstrate that the cantilever bend test can differentiate between different bend performance, and the differences relate to the damage process involved in bending. In the heat treatable AA 6111 the bendability is dependent on the alloy temper, which controls the bend angle at which large surface cracks appear on the surface. This fracture process is a result of differences in the development of surface topography and surface damage with bending strain. The AA 5754 alloy has similar behaviour, but the performance is superior to the best of the AA 6111 tempers, and reflects a lower rate of surface topography development, and an absence of significant surface cracking over the bend angle range investigated.     

Comparison of hardenability and over-aging precipitation behaviour of three 7xxx aluminium alloys

The purpose of the paper is to compare hardenability and aging precipitation behaviour of three 7xxx aluminium alloys (7150, 7055 and 7056). The hardenability has been studied by Jominy test. The aging precipitation behaviour has been investigated according to their hardness and electrical conductivity after over-aging treatment. The micro-structural characterisation has been observed by scanning electron microscope and transmission electron microscope. These results show that AA7056 has the highest hardenability and sensitivity to over-aging treatment compared with AA7150 and AA7055. The highest w(Zn)/w(Mg) ratio and the lowest Cu content of AA7056 are beneficial for inhibiting quench precipitates and decreasing the stability of meta-stable η′ phase.     

Dissimilar friction stir lap welding of AA 5754-H22/AA 6082-T6 aluminium alloys: Influence of material properties and tool geometry on weld strength

Dissimilar friction stir welding (FSW) of heat (AA 6082-T6) and non-heat (AA 5754-H22) treatable aluminium alloys, in lap joint configuration, was performed in this work. The base material plates were 1 mm thick. Welds were performed combining different plates positioning, relative to the tool shoulder, in order to assess the influence of base materials properties on welds strength. Three different tools were tested, one cylindrical and two conical, with different taper angles. Welds strength was characterized by performing transverse and tensile–shear tests. Strain data acquisition by Digital Image Correlation (DIC) was used to determine local weld properties. The results obtained enabled to conclude that the dissimilar welds strength is strongly dependent on the presence of the well-known hooking defect and that the hooking characteristics are strongly conditioned by base materials properties/positioning. By placing the AA 6082-T6 alloy, as top plate, in contact with the tool shoulder, superior weld properties are achieved independently of the tool geometry. It is also concluded that the use of unthreaded conical pin tools, with a low shoulder/pin diameter relation, is the most suitable solution for the production of welds with similar strengths for advancing and retreating sides.     

Effect of coherent and incoherent precipitates upon the stress and strain fields of 6xxx aluminium alloys: a numerical analysis

This paper investigates the stress and strain fields induced by the coherent/incoherent precipitates of type Mg_xSi_y in the 6xxx series aluminium alloys using numerical means. During welding of 6xxx series alloys, the heat-affected zone experiences high temperature heating and cooling cycles and hence results in the non-uniform stress and strain states both at macro- and micro-scales. In the present work, the macro-scale finite element (FE) simulations were carried out to calculate and compare the bulk material properties with those of experimental findings. Since the microstructure of 6xxx alloys comprises mostly of soft α-matrix and coherent/incoherent Mg_xSi_y precipitates, the properties of the individual constituents were used to perform micro-scale FE simulations. Temperature dependent theoretical yield strengths for the precipitates were also determined. The simulated micro-stress and strain fields showed strong dependency over the nature (coherent/incoherent), shape morphology (needle-like, random etc.) and distribution of precipitates. Additionally, unlike coherent precipitates, the plastic deformation of the matrix due to incoherent impurities was found to be highly non-uniform and localized. FE analysis was also performed to characterize the effect of grain size upon the stress state of material.     

Material flow and mechanical behaviour of dissimilar AA2024-T3 and AA7075-T6 aluminium alloys friction stir welds

The scope of this investigation is to evaluate the effect of joining parameters on the mechanical properties, microstructural features and material flow of dissimilar aluminium alloys (3 mm-thick AA2024-T3 and AA7075-T6 sheets) joints produced by friction stir welding. Mechanical performance has been investigated in terms of hardness and tensile testing. Material flow using the stop action technique has also been investigated in order to understand the main features of the mixing process. No onion ring formation has been observed; the boundary between both base materials at the stir zone is clearly delineated, i.e., no material mixing is observed. A non-stable rotational flow inside the threads has been identified due to the formation of a cavity on the rear of the pin. Microstructural observation has revealed the development of a recrystallised fine-grained stir zone, with two different grain sizes resulting from the two different base materials.     

Microstructural changes accompanying repair welding in 5xxx aluminium alloys and their effect on the mechanical properties

At the shipyards, the aluminium alloy 5083 is welded with a multi-pass sequence using the metal inert gas technique. If, while checking the weld integrity either after welding or during service, defects are detected in the vicinity of the weldment, repairs are usually employed to extend the service life.

The repair method involves removal of the upper passes, depending on the thickness and re-welding under the same conditions.

Purpose of this paper is to examine the microstructural changes accompanying repair welding, define their effect on properties of primary importance and set, if possible, an upper limit as far as the number of repairs is concerned.     

Influence of residual fatigue damage on the fracture toughness parameters of an AA6082-T6 aluminium alloy

The present investigation has been carried out in order to study the influence of the previous accumulated fatigue damage induced during high cycle fatigue (HCF), on the fracture toughness parameters of an AA6082-T6 aluminium alloy. The results show that previous fatigue damage accumulated in HCF does not affect the tensile static mechanical properties of the material, but gives rise to a significant debit of the toughness properties on this aluminium alloy. The fracture toughness results have shown that the crack opening displacement at a crack extension of 0.2 mm (COD_0.2) decreases in the range of ∼18 to 36% whereas the value of the non-linear fracture mechanics parameter  J _0.2, decreases in the range of ∼11 to 25% at applied maximum stresses of 200 and 275 MPa, respectively. Optical microscopy observations conducted on the surface of the specimens subjected to HCF damage indicate the existence of microcracks ∼15 to 25 μm long nucleated along the grain boundaries of the material. Also, the scanning electron microscopy (SEM) observations of the fracture surfaces after the tearing tests show the predominance of a ductile fracture mechanism for the material prior to residual fatigue damage, whereas a mixed ductile–brittle fracture mechanism and the presence of flat facets were observed on the fracture surfaces of the specimens with a fatigue damage of 0.70.     

Influence of dynamic recrystallisation on hot ductility of aluminium killed mild steel

Abstract

Torsion tests at strain rates of 10^?3, 0·16, and 1 s^?1 and at temperatures between 850 and 1100°C have been carried out on an aluminium killed mild steel after either heating directly to the testing temperature or soaking for 30 min at 1200°C, then cooling the specimen to the testing temperature. During direct heating, a dispersion of small AlN particles was present and after soaking a coarser distribution of AlN particles was observed. Compared with the latter, the former microstructure seriously retarded the completion of dynamic recrystallisation. The effect of the two different particle distributions was examined for the following parameters: peak and steady state stress, peak strain, and dynamically recrystallised grain size. The retardation of completion of dynamic recrystallisation has the additional effect of impairing the hot ductility.

MST/997     

Micromechanics of room and high temperature fracture in 6xxx Al alloys

The micromechanics of ductile fracture has made enormous progress in recent years. This approach, which was mostly developed in the context of structural integrity analysis, is becoming a key tool for materials scientists to optimize materials fracture properties and forming operations. Micromechanical models allow quantitatively linking fracture properties, microstructure features at multiple lengths scales, and manufacturing conditions. After briefly reviewing the state of the art, this paper illustrates the application of the micromechanics-based methodology by presenting the results of an investigation on the damage resistance of 6xxx Al produced by extrusion.The presence of coarse, elongated, particles is the key microstructural feature affecting the fracture behaviour of 6xxx Al. The detrimental elongated β-type particles are transformed into rounded α-type particles by heat treatment. In situ tensile tests revealed that, at ambient temperature, the α particles and the β particles oriented with the long axis perpendicular to the main loading direction undergo interface decohesion, while the β particles oriented perpendicular to the loading direction break into several fragments. At high temperatures, only interface decohesion is observed. Uniaxial tensile tests on notched and smooth round bars were performed on two different alloys, at different temperatures ranging between 20 °C and 600 °C, under different loading rates, while systematically varying the content in β versus α particles. The ductility increases with decreasing amount of β particles, increasing temperature and strain rates, and decreasing stress triaxiality.A viscoplastic extension of the Gurson model has been developed for capturing the complex hierarchy of damage mechanisms, coupled with viscoplastic and stress state effects. Three populations of voids are modelled while accounting for the different void nucleation mechanisms leading to different initial void aspect ratio. Proper modelling of the initial void aspect ratio and of its evolution with void growth was the key to predict the effect of the β → α conversion on ductility. The void coalescence criterion takes into account the presence of secondary voids resulting from particle fragmentation. The characteristics of particles entering the model were all measured experimentally. The temperature and rate dependent flow properties of the matrix material have been obtained by inverse modelling. The only fitting parameters are the critical stresses for void nucleation. The model is validated by comparing the predictions to the experimental data involving different relative proportion of α and β particles, temperature, loading rate and stress triaxiality. This type of model opens the path for an “alloy by design” strategy which relates end-use properties to upstream manufacturing operations.     

Microstructure and texture evolution during accumulative roll bonding of aluminium alloys AA2219/AA5086 composite laminates

Accumulative roll bonding of two aluminium alloys, AA2219 and AA5086 was carried out up to 8 passes. During the course of ARB, the deformation inhomogeneity between the two alloy layers results in interfacial instability after the 4th pass, necking of the AA5086 layers after the 6th pass and fracture along the necked regions after the 7th and 8th pass. The EBSD analysis shows deformation bands along the interfaces after 8 passes of ARB. The ARB-processed materials predominantly show characteristic deformation texture components. The weak texture after the 2nd pass results from the combination of a weakly-textured starting AA2219 layer and a strongly-textured starting AA5086 layer. A strong deformation texture forms due to the high imposed strain after a higher number of ARB passes. Subgrain formation and related shear banding induces copper/S components in the case of the small elongated grains, while planar slip leads to the formation of brass component in the large elongated grains.     

Influence of microstructures on press bonding of 5083 aluminium alloys

Abstract

Solid state bonding of superplastic 5083 (SP5083) and 5083–H323 aluminium alloys has been studied. Press bonding was employed to bond the aluminium alloys in air. Temperature, time, deformation, and grain size were used as process variables whose effects were assessed in terms of bond strengths. The measured shear strengths were strongly influenced by the bonding parameters and alloy microstructures. The strengths of highly deformed specimens were very sensitive to the bonding temperature, and the combined effects of bonding temperature and holding time was very dependent on the microstructural conditions. Using alloy sheets with a finer grain structure resulted in higher bond strengths.     

Analysis of high temperature plastic behaviour and its relation with weldability in friction stir welding for aluminium alloys AA5083-H111 and AA6082-T6

The influence of the plastic behaviour of two aluminium alloys, very popular in welding construction, on friction stir weldability, is analysed in this work. The two base materials, a non-heat-treatable (AA5083-H111) and a heat-treatable aluminium (AA6082-T6) alloy, are characterised by markedly different strengthening mechanisms and microstructural evolution at increasing temperatures. Their plastic behaviour, under different testing conditions, was analysed and compared. The two base materials were also welded under varied friction stir welding (FSW) conditions in order to characterise their weldability. The relation between weldability, material flow during FSW and the plastic behaviour of the base materials, at different temperatures, was analysed. It was found that the AA6082 alloy, which displays intense flow softening during tensile loading at high temperatures, and is sensitive to dynamic precipitation and overageing under intense non-uniform deformation, displays good weldability in FSW. Under the same welding conditions, the AA5083 alloy, which in quasi-static conditions displays steady flow behaviour at increasing temperatures, and is sensitive to moderate hardening at high strain rates, displays poor weldability.