Effects of solidification structure on tear resistance of Al–7% Si–0.4% Mg cast alloys

The tear resistance behaviour of Al–7% Si–0.4% Mg cast alloys was examined using Kahn‐type tear test specimens. Tests were performed for two permanent mould casts with an ordinary dendrite structure and a semi‐liquid die cast with a globular cell and fine grain structure. The microstructure of the two permanent mould casts was controlled by the cooling rates and the addition of Ti elements. Tear resistance was evaluated by the ‘pop‐in’ stress, the energies required for crack initiation, UE_i and the crack propagation, UE_p. Special attention was paid to an effective microstructural parameter for tear resistance improvement. Pop‐in, indicating sudden crack extension and arrest, was observed in all specimens. Homogeneous deformation occurs near the notch tip of the semi‐liquid die cast, characterized by a refined grain structure. Refinement of the grain size is more effective than that of the dendrite cell size or eutectic Si particle size to increase the energy for crack initiation. Unit propagation energy, UE_p, can be converted into a critical stress intensity factor, K_c, which in the semi‐liquid die cast was improved due to an increased amount of slant or shear fracture surface.     

Effects of strain rate on mechanical properties and failure mechanism of structural Al–Mg alloys

The effects of strain rate on the mechanical properties and failure mechanism of AA5754 and AA5182 sheets were investigated. Tensile tests were conducted at quasi-static (less than 10⁻¹ s⁻¹) and dynamic (600, 1100 and 1500 s⁻¹) strain rates at room and elevated temperature using an INSTRON machine and Tensile Split Hopkinson Bar (TSHB) apparatus, respectively. Shear band decoration, interrupted tensile tests, electron microscopy, and image analysis techniques were also utilized. The results obtained show that the studied alloys exhibit negative strain rate sensitivity at quasi-static rates, but mild positive sensitivity at dynamic rates. Different failure mechanisms were also observed. Strain localization and shear band formation was found to be a necessary pre-requisite for the development of damage and final failure under quasi-static conditions. In the dynamic strain rate regime however, less shear banding was observed. Void nucleation, growth and coalescence that is characteristic of dynamic tensile fracture appears to be the dominant mode for failure under dynamic conditions.     

The influence of defect and temperature on the fatigue behaviours of Al‐Si‐Cu‐Mg‐Ni alloy

High‐cycle fatigue (HCF) properties of two Al‐Si‐Cu‐Mg‐Ni alloys with different defect sizes named as alloys A (smaller ones) and B (bigger ones) were investigated at 350°C and 425°C, respectively. The results indicate that fatigue strengths of both alloys decrease as the temperature increases. Fatigue cracks originated from pores and oxide films at both temperatures. They propagated preferentially through cracked matrix at 350°C and debonded interface and grain boundary at 425°C. Alloy A exhibits higher fatigue life and fatigue strength than alloy B at 350°C due to its smaller pore sizes. However, it has slightly worse fatigue properties than alloy B at 425°C because the fatigue crack initiation is controlled by oxide film at this temperature and is not affected by its size. This indicates that there is a transition of predominant initiation site from pores to oxide films when the temperature increases. The fatigue strength estimated through defect size is consistent with the experimental results at 350°C, while unsuitable at 425°C.     

Anisotropy of mechanical properties in quaternary Al-Li-Cu-Mg alloys

Considerable anisotropy in the mechanical properties of quaternary Al-Li-Cu-Mg alloys was observed in both plate and sheet products. These alloys showed more than 100% increase in tensile ductility in the test direction oriented at 45–60° to the rolling direction as compared to that in the rolling direction (longitudinal, L). A concomitant decrease in strength was also found. These alloys exhibit superior low cycle fatigue resistance in the long-transverse (LT) direction as compared to the longitudinal (L) direction. Another observation is the occurrence of strength differential (S-D), which is seen to be directional. The trends in S-D are similar under monotonic as well as cyclic loading conditions. The fracture resistance is also highly anisotropic. An attempt is made here to correlate the observed anisotropy in the mechanical behaviour of these alloys with the microstructure and crystallographic texture.     

High‐temperature low‐cycle fatigue behaviour of a cast Al–12Si–CuNiMg alloy

The low‐cycle fatigue behaviour of a cast Al–12Si–CuNiMg alloy, with a high content of Si, is investigated at 200, 350 and 400 °C. The fatigue test results show that the alloy exhibits symmetrical hysteresis loops, moderate cyclic softening and higher fatigue resistance at higher temperature. The fracture surface analysis reveals that more tear ridges are formed at higher temperature, which strongly affect the fatigue resistance. Furthermore, evaluation of the material fatigue resistance using an energy‐based Halford–Marrow model indicates that the material's ability to absorb and dissipate plastic strain energy is enhanced as temperature increases.     

Microstructure‐texture‐fracture toughness property correlation in annealed Al‐6Mg alloy with minor scandium and zirconium additions

The effect of minor addition of scandium and zirconium on the fracture toughness (FT) behaviour of aluminium‐6 wt% magnesium alloy is studied. Texture measurement and transmission electron microscopy have revealed that the evolution of texture in cast alloys after annealing is decided by the morphology and character of the precipitates. It is further demonstrated that the said minor addition influences the FT behaviour of Al‐6Mg alloy by manipulating in‐plane anisotropy as decided by the precipitate morphology. Textural situation in annealed state has also been related to the FT behaviour.     

Investigation on microstructure and mechanical properties of Al-Zn-Mg-Cu alloys with various Zn/Mg ratios

The effects of Zn/Mg ratios on microstructure and mechanical properties of Al-Zn-Mg-Cu alloys aged at 150℃ have been investigated by using tensile tests,optical metallography,scanning electron microscopy,transmission electron microscopy and atom probe tomography analyses.With increasing Zn/Mg ratios,the ageing process is significantly accelerated and the time to peak ageing is reduced.T'phase predomi-nates in alloys of lower Zn/Mg ratios while η'phase predominates in alloys with a Zn/Mg ratio over 2.86.Co-existence of T'phase and η'phase with a large number density is beneficial to the high strength of alloys.Such precipitates together with narrow precipitate free zones cause a brittle intergranular fracture.A strength model has been established to predict the co-strengthening effect of T'phase and η'phase in Al-Zn-Mg-Cu alloys,including the factors of the grain boundary,solid solution and precipitation.     

Closure mechanisms in Al–Si–Mg cast alloys and long-crack to small-crack corrections

Crack growth behavior of small and long cracks has been a subject of much research; the differences observed in the growth response were largely attributed to crack closure. In this study, cast Al–Si–Mg alloys of various residual stress levels and Si content/morphology were investigated, and the dominant closure mechanisms were identified. The near-threshold fatigue crack growth behavior of the alloys is controlled by two types of closure: residual stress-induced (due to residual stresses introduced during heat treatment) and microstructure/roughness-induced (due to variations in Si content/morphology). These closure mechanisms were individually discussed and closure corrective techniques were applied to long crack growth results. The near-threshold closure corrected long crack growth data compare favorably with small crack growth data.     

Mechanical properties of friction stir butt‐welded Al‐5086 H32 plate

Al‐5086 H32 plates with a thickness of 3 mm were friction stir butt‐welded using different welding speeds at a tool rotational speed of 1600 rpm. The effect of welding speed on the weld performance of the joints was investigated by conducting optical microscopy, microhardness measurements and mechanical tests (i.e. tensile and bend tests). The effect of heat input during friction stir welding on the microstructure, and thus mechanical properties, of cold‐rolled Al‐ 5086 plates was also determined. The experimental results indicated that the maximum tensile strength of the joints, which is about 75 % that of the base plate, was obtained with a traverse speed of 200 mm/min at the tool rotational speed used, e.g. 1600 rpm, and the maximum bending angle of the joints can reach 180^o. The maximum ductility performance of the joints was, on the other hand, relatively low, e.g. about 20 %. These results are not unexpected due to the loss of the cold‐work strengthening in the weld region as a result of the heat input during welding, and thus the confined plasticity within the stirred zone owing to strength undermatching. Higher joint performances can also be achieved by increasing the penetration depth of the stirring probe in butt‐friction stir welding of Al‐5086 H32 plates.     

The effect of microstructure on the mechanical properties of two-phase titanium alloys

This article presents the results of investigations of microstructure and mechanical properties of two-phase +β titanium alloys with different volume fraction of the β-phase. Microstructure of the specimens was examined using an optical microscope. Fracture surfaces were observed by SEM technique. The influence of the microstructure and phase composition on the mechanical properties of the alloys was studied. Static tensile tests, hardness tests and fatigue investigations were performed. It was noticed that the volume fraction and chemical composition of the β-phase has a significant effect on mechanical properties and cracking process during fatigue.     

Bruchzähigkeitsverhalten von Magnesium‐Druckgusslegierungen

Fracture Toughness of Pressure Die Cast Magnesium Alloys The increasing use of pressure die cast alloys for the manufacturing of automotive components requires materials values, which describe the mechanical behaviour comprehensively. There is especially a lack of data for the design of safety relevant components. In this paper fracture mechanics investigations are presented to assess the crack resistance of the alloys AZ91, AM50 and AE42 on the basis of the multispecimen test method. The investigations occurred on SENB and CT‐specimens taken from pressure die cast plates of the same dimensions. The results reveal a clear correlation between the microstructure, especially the concentration and the distribution of intermetallic phases, and the crack resistance. Because of the cast‐induced defects like microshrinkage and gas inclusions the good intrinsic properties of the alloys can not be completely exploited.     

Effect of Mg and Sr-modification on the mechanical properties of 319-type aluminum cast alloys subjected to artificial aging

Aluminum-based 319-type cast alloys are commonly used in the automotive industry to manufacture cylinder heads and engine blocks. These applications require good mechanical properties and in order to achieve them through precipitation hardening, artificial aging treatments are applied to the products. The standard artificial aging treatment for alloy 319, as defined by the T6 heat treatment temper, consists in solution heat-treating the product for 8 h at 495 °C, water quenching at 60 °C, and then artificially aging at 155 °C for 2–5 h.

The present paper reports on aging heat treatments that were performed on four Al–Si–Cu–Mg 319-type alloys: 319 base alloy, Sr-modified 319 alloy, 319 alloy containing 0.4 wt% Mg, and the Sr-modified 319 + 0.4 wt% Mg alloy. This investigation was carried out in order to examine the effect of Sr-modification and additions of Mg on the microhardness, tensile strength and impact properties of 319-type alloys over a range of aging temperatures and times (150–240 °C, for periods of 2–8 h).

The results show that the best combination of properties is found in the Sr-modified alloy containing 0.4 wt% Mg (i.e. alloy 319 + Mg + Sr). Also, the optimum artificial aging temperature changes when Mg is present in the alloy.     

Microstructure and mechanical properties of laser metal deposited AlSi10Mg alloys

Thin-walled specimens with more than 150 layers were deposited by laser metal deposition without cracks and concaving deformation. The microstructure in each layer could be divided into three zones according to the morphology. The homogeneity deteriorated with the rising of the input. The tensile strength dropped 29.7% when the porosity increased from 0.53% to 1.88%. Controlling the oxygen under 0.5% and optimising the heat input, the as-built tensile strength reached 360?MPa. The fracture elongation was enhanced from 3.9% to 12.7% when the heat input was increased from 480 to 1200?W. The decrease of the secondary dendrite arm spacing and the change of fracture mechanism is the main reason leading to the strengthening of the mechanical properties.     

The effect of grain size and composition on the fracture toughness of Ti–Al–Nb alloys undergoing stress-induced martensitic transformation

The effect of grain size and composition on the fracture toughness of Ti–Al–Nb alloys in β solution-treated condition was investigated. The fracture toughness of the alloys was found to increase with an increase in grain size initially, reach a maximum and subsequently decrease with further increase in grain size. This trend was attributed primarily to the effect of grain size on the enhancement of fracture toughness due to stress-induced martensitic transformation (SIMT) at the crack tip, which in turn can be related to the effect of grain size on trigger stress for SIMT. Alloys containing higher Al and Nb showed a higher toughness for the same grain size, which was also explained in terms of effect of composition on the trigger stress.     

Spray formed and rolled aluminium‐magnesium‐scandium alloys with high scandium content

Aluminium‐magnesium‐scandium alloys offer good weldability, high corrosion resistance, high thermal stability and the potential for high strength by precipitation hardening. A problem of aluminium‐scandium alloys is the low solubility of about 0.3 mass‐% scandium when using conventional casting methods. The solution of scandium can be raised by higher cooling rates during solidification. This was realised by spray forming of Al‐4.5Mg‐0.7Sc alloys as flat deposits. Further cooling rates after solidification should also be high to prevent coarse precipitation of secondary Al₃Sc. Therefore a cooling device was designed for the spray formed flat deposits. The flat deposits were rolled at elevated temperatures to close the porosity from spray forming. Microstructures, aging behaviour and tensile properties of the rolled sheets were investigated. Strength enhancements of about 100 MPa compared to conventional Al‐Mg‐Sc alloys were achieved.     

Subzero tensile properties of 7010 aluminium alloy and Ti-6A1-4V and IMI550 titanium alloys in sheet form

Tensile properties are reported for Al-6Zn-2.5Mg-1.7Cu-0.12Zr (7010), Ti-6Al-4V and Ti-4Al-4Mo-2Sn-0.5Si (IMI550) alloy sheet, 1.7 mm thick tested at 293, 223, 173 and 77 K. The strength of these alloys increased and the reduction of area decreased with decreasing test temperature. The Young's Modulus (E), 0.1% proof stress (σ_0.1) and true tensile strength ($\text{σ}{}_{\mathrm{TS}}$) were related to temperature T in degrees absolute (in the range 293-173 K for E and 293-77 K for σ_0.1 and $\text{σ}$_TS) by     

Effect of heat treatments on tensile properties and microstructures of wrought Mg−Zn−Zr-RE alloys

The effect of heat treatments on the tensile properties and microstructures of wrought Mg–Zn–Zr-Rare earth alloys MB25 and MB26 have been studied in this paper. It was shown that the homogenization of the cast ingots decreased the strength of the extruded bars because some grain boundary phases were dissolved during the process of this treatment, while the ageing treatment of the extruded bars increased the strength due to the dispersive precipitation of MgZn₂ phase. The quenching + ageing treatments of the extruded bars decreases the strength and plasticity because of the growing up of the grains.Abbreviations ED electron diffraction - GB grain boundary - RE rare earth - TEM transmission electron microscope     

Effect of pores and acicular eutectic silicon particles on the performance of Al–Si–Mg (AS7G03) casting

Excellent castability, corrosion resistance and high specific strength has made cast Al–Si–Mg alloys a suitable candidate material for various aerospace application. Aluminium alloy casting AS7G03, belonging to Al–Si–Mg series of cast alloy in Y23 condition, is being used as outlet adaptor of liquid propellant tank for Indian space programme. During developmental stage, one of the castings namely oxidizer tank outlet adaptor failed and parted in to two pieces during the proof pressure test (PPT) at 22 bar.This paper brings out the details of investigation and correlates the effect of pores and acicular unmodified silicon particle on the performance of the material.