Effect of microstructural variables on tensile behaviour of A356 cast aluminium alloy

Abstract

The effects of microstructural variables, including secondary dendrite arm spacing (SDAS), the size of primary α phase, the aspect ratio of eutectic Si particle and the thickness of eutectic wall structure, on tensile behaviour of A356 cast aluminium alloy, were quantitatively identified using linear regression analysis method. For systematic microstructural control of A356 specimen, directional solidification method was used with different solidification rates of 5, 25, 50 and 100 μm s^?1 respectively. The linear regression analysis suggests that each microstructural variable affects tensile strength and tensile elongation of A356 cast aluminium alloy in a similar fashion. The change in tensile behaviour with varying microstructural variables in A356 cast aluminium alloy is discussed based on fractographic and micrographic observations.     

Morphology and mechanical properties of melt-spun and conventionally cast aluminium,AlMg and AlSi alloys before and after hot extrusion

Rapidly solidified aluminium, AlMg (0 to 16.5 at % Mg) and AlSi (0 to 20.2 at % Si) alloys were produced by melt spinning. The AlMg ribbons were single-phase, whereas the AlSi ribbons were dual-phase. In the ribbons of both alloy systems the fineness of the microstructure increased with increasing alloying element content. The melt-spun ribbons were consolidated by hot extrusion. For comparison, conventionally cast alloys of corresponding compositions were extruded analogously. During the extrusion process in AlMg (16.5 at % Mg) and in the AlSi alloys precipitation occurred. The consolidation of the ribbons was markedly influenced by the oxide layer on the ribbon surfaces: in the AlSi consolidates a more intimate contact between the ribbons was apparent than in the aluminium and AlMg consolidates. In the extrudates of the conventionally cast alloys the grains and second-phase particles were much larger than in the consolidates. The observed dependence on alloy composition of hardness, ultimate tensile strength and elongation at fracture of both consolidated ribbons and extrudates of the conventionally cast alloys are discussed in terms of matrix grain size, solute content of the matrix, amount and size of second-phase particles and recrystallization behaviour. For all compositions of the alloys the Vickers hardness of the as-melt-spun ribbons was higher than that of the consolidated products, owing to recrystallization and precipitation provoked by the hot consolidation process. The ultimate tensile strength as well as the elongation at fracture of both consolidated ribbons and extruded conventionally cast alloys did not differ significantly for AlMg. However, due to a finer microstructure and a stronger inter-ribbon bonding, for AlSi alloys with a high silicon content the rapid solidification processing route did yield a product with significantly improved mechanical properties as compared with the extruded conventionally cast alloys.     

Control of mechanical and wear properties of a commercial Al-Si eutectic alloy

The properties examined as a function of microstructural modification were ultimate tensile strength, fracture elongation, Vickers hardness and wear resistance. The microstructural modification was achieved by rapid cooling and additions of small amounts of strontium and lithium master alloys into the eutectic melt. In all experiments the commercial ETIAL 140 alloy was cast instead of a high-purity aluminium-silicon eutectic. This allowed determination of the effect of modification treatment, both on silicon and intermetallic phases. It was found that the slowly cooled and unalloyed castings which contained coarse silicon flakes showed highest wear resistance and lowest ultimate tensile strength, fracture elongation and Vickers hardness values. Rapid cooling and also additions of strontium and lithium master alloys reduced the eutectic interphase spacing and refined the silicon phase. This usually corresponded to a significant increase in all properties except the wear resistance. It was noted, however, that the size of the intermetallic phase particles increased abruptly above 0.04% Sr content which resulted in a sharp reduction in all mechanical properties. Unlike the strontium effect, the lithium addition did not influence the intermetallic size significantly and, therefore, the mechanical properties were not impaired. In addition, the wear resistance also remained relatively unaffected because lithium solid solution hardened the primary aluminium dendrites appeared in the modified alloys.     

The influence of post‐heat treatments on the tensile strength and surface hardness of selectively laser‐melted AlSi10Mg

To improve the mechanical properties of cast aluminium alloys several post‐heat treatments are known. However, these treatments cannot directly be transposed to additively via selective laser melting manufactured aluminium alloys, e. g., aluminium‐silicon‐magnesium (AlSi10Mg). Therefore, this study aims to determine suitable post‐heat treatments to optimise the mechanical properties of SLM‐built AlSi10Mg specimen. The influence of various post‐heat treatment conditions on the material characteristics was examined through hardness and tensile tests. The findings indicate that the Vickers hardness and ultimate tensile strength could not be improved via secondary precipitation hardening, whereas the fracture elongation shows a value which is distinctly higher than the values of a comparable cast alloy. Solution annealing at 525 °C reduces the hardness and the ultimate tensile strength by about 40 % and increases the fracture elongation three times. A subsequent precipitation hardening allows recovery of 80 % of the as‐built hardness, and 90 % of the previous ultimate tensile strength combined with maintaining an improved fracture elongation of about 35 % compared to the respective as‐fabricated condition.     

Microstructure,tensile properties and fracture behaviour of an Al-Cu-Li-Mg-Zr alloy 8090

The addition of lithium to aluminium alloys has the potential for providing a class of high strength alloys with exceptional properties suitable for aerospace applications. One such candidate is 8090, a precipitation hardenable Al-Li-Cu-Mg alloy. Detailed optical microscopical observations were used to analyse the intrinsic microstructural features of the alloy. It is shown that microstructural characteristics have a pronounced influence on tensile properties and fracture behaviour of the alloy in the peak-aged, maximum strength condition. Tensile test results indicate that the alloy has property combinations comparable with other high strength commercial aluminium alloys. The elongation and reduction in area are higher in the transverse direction of the extruded plate. A change in fracture mode was observed with direction of testing. We rationalize such behaviour based on the grain structure of the material, and the nature, distribution and morphology of the second-phase particles. An attempt is made to discuss the kinetics of the fracture process in terms of several competing mechanistic effects involving intrinsic microstructural features, deformation characteristics of the matrix, brittleness of the grain boundary precipitates and grain boundary failure. The role of stress on particle fracture is highlighted.     

冷轧对Al_(10)Cu_(25)Co_(20)Fe_(20)Ni_(25)高熵合金组织结构及力学性能的影响

对铸态Al10Cu25Co20Fe20Ni25高熵合金进行冷轧处理后进行室温拉伸测试,并利用X射线衍射仪(XRD)和扫描电镜(SEM)分别对其相结构、微观组织形貌及拉伸断口进行分析。结果表明:经冷轧工艺处理后,Al10Cu25Co20Fe20Ni25高熵合金硬度最大为285HV,较轧制前提高了51.6%;在变形量为40%时,抗拉强度达到最大值,为638MPa,是铸态合金的2.7倍。拉伸断口分析表明,铸态合金的断裂模式为树枝晶沿晶断裂和韧窝型延性断裂,而冷轧态合金主要为韧窝型延性断裂模式。     

Fatigue properties of friction stir welded particulate reinforced aluminium matrix composites

Few papers have discussed the friction stir welding (FSW) of particulate reinforced aluminium matrix composites and most of them focused on the set-up of the welding process parameters and their effect on microstructure, hardness and tensile behaviour. The aim of this study was to investigate the fatigue resistance of FSW joints on an as-cast particulate reinforced aluminium based composite (AA6061/22 vol.%/Al₂O_3p). The welding process was performed using different process parameters, also investigating their effect on joint microstructure. The mechanical properties of the FSW composites were compared with those of the base material and the results were correlated to the microstructural modifications induced by the FSW process on the aluminium alloy matrix and the ceramic reinforcement. FSW reduced the size of both particle reinforcement and aluminium grains, and also led to a significant increase in interparticle matrix microhardness, for all process parameters. The FSW specimens belonging to a different set of parameters, tested without any post-weld heat treatment, exhibited a very high joint efficiency (ranging from 90% to 99%) with respect to the ultimate tensile strength of the base material. The stress controlled fatigue test showed a high spread both for the base and FSW composites. Statistical analysis disclosed that all FSW specimens belonging to different process parameters showed apparently slightly worse fatigue behaviour than that of the base composite. Statistical processing applied to the different welding parameters revealed that all the welded specimens belonged to the same population. Therefore it can be concluded that the parameters used produced joints with similar microstructure and comparable fatigue behaviour. The slight difference in the fatigue behaviour of the FSW specimens whose process parameters differed form those of the unwelded composite was explained by the different microstructural homogeneity in the transition from the base to the FSW zone.     

The tensile behaviour of rapidly solidified magnesium alloys

In this paper, the microstructure and tensile behaviour of three rapidly solidified magnesium alloys is reported. The alloy's composition, i.e. neodymium content, is observed to have an influence on tensile properties and fracture behaviour. The elastic modulus, yield strength and ultimate tensile strength of the alloy increase with an increase in neodymium content. The ductility of the alloys decreased marginally with an increase in neodymium content. The tensile fracture characteristics of the alloys is highlighted in the light of alloy composition and microstructural effects.     

The microstructural control of cast and mechanical properties of zinc-aluminium alloys

The zinc-aluminium alloys containing 8, 12, and 27% aluminium are finding increasing applications in the casting industry. These alloys are stronger than most aluminium alloys. In addition, they possess high wear resistance and bearing properties. However, surface sinks and shrinkage defects are observed on the bottom faces of such castings, contrary to general foundry practice. In the present investigation, this problem observed in the Zn-8%Al, Zn-12%Al, Zn-27%Al alloys was tackled by controlling various casting parameters and also by additions of the master alloys of strontium and lithium into the molten alloys. It was found that the underside shrinkage problem was influenced by the aluminium content of the alloy, melt superheat, casting size and cooling conditions. The strontium and lithium additions were found to be beneficial in reducing the underside shrinkage problem. The ultimate tensile strength, fracture elongation and Vickers hardness were all increased with aluminium concentration and lithium addition. It was found also that the most problematical Zn-27%Al alloy, which provided the highest mechanical properties, was very suitable for the squeeze-casting technique. The mechanical properties were increased sharply in these squeeze-cast bars.     

Grain size effects on the mechanical properties of some squeeze cast light alloys

Mechanical property-grain size relationships have been examined for squeeze cast Al-4.5% Cu alloy, for an aluminium alloy with a composition corresponding to wrought 7010, and for a magnesium alloy AZ91. The general trend of the results obtained showed that the tensile properties and the fatigue strength improved as grain size decreased and the reverse was found to be the case for the fatigue crack propagation resistance and fracture energy of these castings. However, the results also showed that no simple common relationship existed between grain size and the tensile properties of the different alloys. The results are discussed in respect of their microstructures.     

Tensile and creep properties of Al–7Si–0.3Mg alloy with Zr and Er addition

The combined and singular effects of Zr and Er addition on the microstructure, tensile and impression creep behaviour of the cast A356 alloy were investigated. The Zr and Er refined the α-Al dendrites and secondary dendrite arm spacing (SDAS) and modified the eutectic morphology. The simultaneous addition of Zr and Er exhibited the smallest α-Al dendrites and SDAS with a fibrous eutectic morphology, resulting in the highest yield (～118?MPa) and tensile (～190?MPa) strength, and the largest creep resistance. The stress exponent, and the activation energy, were within the range of 6.28–7.22 and between 121.6 and 148.2?kJ?mol^?1, respectively. The lattice self-diffusion climb-controlled creep was the dominant creep mechanism. The constitutive creep equation was developed for each alloy.     

Effect of microstructure and overaging on the tensile behavior at room and elevated temperature of C355-T6 cast aluminum alloy

The present study was focused on the microstructural and mechanical characterization of the Al–Si–Cu–Mg C355 alloy, at room and elevated temperature. In order to evaluate the influence of microstructural coarseness on mechanical behavior, samples with different Secondary Dendrite Arm Spacing (SDAS) (20–25 μm for fine microstructure and 50–70 μm for coarse microstructure), were produced through controlled casting conditions. The tensile behavior of the alloy was evaluated at T6 condition and at T6 with subsequent high temperature exposure (41 h at 210 °C, i.e. overaging), both at room and elevated temperature (200 °C). Microstructural investigations were performed through optical and electron microscopy.The results confirmed the important role of microstructure on the tensile behavior of C355 alloy. Ultimate tensile strength and elongation to failure strongly increased with the decrease of SDAS. Larger SDAS, related to lower solidification rates, modify microstructural features, such as eutectic Si morphology and size of the intermetallic phases, which in turn influence elongation to failure. Overaging before tensile testing induced coarsening of the strengthening precipitates, as observed by STEM analyses, with consequent reduction of the tensile strength of the alloy, regardless of SDAS. A more sensible decrease of tensile properties was registered at 200 °C testing temperature.     

Tensile behaviour in 30Ni-30Cu-40Mn-based alloys for a dental application

It was possible to examine the tensile behaviour in experimental 30Ni-30Cu-40Mn-based alloys which were modified by alloying additions of aluminium, indium and tin. Namely, the experimental alloys were developed on the basis of crack formation in a commercial nickel-based alloy and microstructural features in nickel-based alloys investigated. The addition was done by substituting only the manganese content (40 wt%) to 35 and 40 wt%. The results indicated that both changes of tensile strength and elongation were obtained with castability values above 94%. Comparison of tensile properties in experimental Ni-Cu-Mn-based alloys studied here showed that the addition of aluminium to the alloys was appropriate to obtain results similar to those for commercial alloys, indicating that the refining of dendrite arm spacing was obtained by aluminium addition.     

Microstructure and mechanical properties of novel Al-Y-Sc alloys with high thermal stability and electrical conductivity

The microstructure and mechanical properties of novel Al-Y-Sc alloys with high thermal stability and electrical conductivity were investigated.Eutectic Al3 Y-phase particles of size 100-200 nm were detected in the as-cast microstructure of the alloys.Al3 Y-phase particles provided a higher hardness to as cast alloys than homogenized alloys in the temperature range of 370-440℃.L12 precipitates of the Al3(ScxYy) phase were nucleated homogenously within the aluminium matrix and heterogeneously on the dislocations during annealing at 400℃.The average size of the L12 precipitates was 11±2 nm after annealing for 1 h,and 25-30 nm after annealing for 5 h,which led to a decrease in the hardness of the Al-0.2 Y-0.2 Sc alloy to15 HV.The recrystallization temperature exceeded 350℃and 450℃for the Al-0.2 Y-0.05 Sc and Al-0.2 Y-0.2 Sc alloys,respectively.The investigated alloys demonstrated good thermal stability of the hardness and tensile properties after annealing the rolled alloys at 200 and 300℃,due to fixing of the dislocations and grain boundaries by L12 precipitates and eutectic Al3 Y-phase particles.The good combination of strength,plasticity,and electrical conductivity of the investigated Al-0.2 Y-0.2 Sc alloys make it a promising candidate for electrical conductors.The alloys exhibited a yield stress of 177-183 MPa,ultimate tensile stress of 199-202 MPa,elongation of 15.2-15.8%,and electrical conductivity of 60.8%-61.5% IACS.     

Effect of process parameters on the strength of resistance spot welds in 6082-T6 aluminium alloy

In this study the microstructural and mechanical behaviour of resistance spot welds (RSW) done on aluminium alloy 6082-T6 sheets, welded at different welding parameters, is examined. Microstructural examinations and hardness evaluations were carried out in order to determine the influence of welding parameters on the quality of the welds. The welded joints were subjected to static tensile-shear tests in order to determine their strength and failure mode. The increase in weld current and duration increased the nugget size and the weld strength. Beyond a critical nugget diameter the failure mode changed from interfacial to pullout. Taking into consideration the sheet thickness and the mechanical properties of the weld, a simple model is proposed to predict the critical nugget diameter required to produce pull-out failure mode in undermatched welds in heat-treatable aluminium alloys.     

Hot tensile and fatigue behaviour of zinc–aluminum alloys produced by gravity and squeeze casting

The tensile and fatigue properties of zinc–aluminum alloys (ZA-8, ZA-12 and ZA-27) in squeeze and gravity cast forms have been investigated. Tensile tests were conducted at ambient and elevated temperatures up to 150 °C. At low temperatures, the ultimate tensile strength and yielding strength of the squeeze cast alloys have been found to be superior those of the gravity-cast alloys, as the temperature increased they decreased. In the same way, Brinell hardness of the squeeze cast alloys were obtained at higher values than gravity castings. The fatigue tests were performed at a constant speed of 400 rev/min and under a number of stress levels ranging from 100 to 150 MPa. The fatigue behaviour results of the ZA alloys were similar to obtained from the tensile testing. The squeeze cast alloys exhibited good fatigue resistance in proportion to the gravity castings. Metallography examinations showed that the microstructure of the castings differed according to the method of casting used. It was considered that the mechanical properties of the alloys were affected from these micro-structural changes.     

Predictive equations of the tensile properties based on alloy hardness and microstructure for an A356 gravity die cast cylinder head

One of the main problems in the design of complex Al–Si cast components is the wide variety of mechanical properties in different regions of the castings which is due to the wide range of solidification microstructures, related to the local solidification conditions. There are many papers available on the widely used A356/A357 Al–Si–Mg alloys, however, most experimental data on their tensile or fatigue properties are generally obtained from specimens cast separately or produced under controlled laboratory conditions, that are extremely different from those of industrially cast components. Moreover, most of these data often relate the mechanical properties to only one microstructural parameter, such as solidification defects or secondary dendrite arm spacing, and do not take their simultaneous effect into consideration. For all these reasons, the main problem, in the design phase, is the lack of knowledge of the true local mechanical properties in complex-shaped castings, which often means a conservative approach is necessary, with a consequent increase in thickness and therefore in weight. The aim of this research was to study a complex A356 gravity die cast cylinder head, in order to verify the range of variability of the main microstructural parameters and tensile properties, using specimens directly machined from the casting. The component was heat treated at the T6 condition, and the effect of the delay between quenching and aging on the alloy hardness was also evaluated. Simple experimental equations have been proposed, able to successfully predict the local tensile properties in the casting, when only the most important microstructural parameters and alloy hardness are known. These equations allow the designer to predict the local tensile behaviour without any tensile tests; moreover they can also link the post-processing results of the casting simulation software to the pre-processing phase of the structural ones, with an approach of co-engineered design.     

CRACK INITIATION AND SMALL FATIGUE CRACK GROWTH BEHAVIOUR OF SQUEEZE-CAST Al-Si ALUMINIUM ALLOYS

Abstract— Fatigue strength, crack initiation and small crack growth behaviour in two kinds of squeeze-cast aluminium alloys, AC8A-T6 and AC4C-T6 were investigated using smooth specimens subjected to rotatary-bending fatigue at room temperature. Fatigue resistance of these alloys was almost the same as that of the wrought aluminium alloys because of their fine microstructure and of the decrease in defect size due to squeeze-casting. Fatigue crack initiation sites were at the eutectic silicon particles on the surface of specimens or at internal microporosity in the specimens. Crack initiation life, defined as a crack length of 50 μm on the specimen surface, was successfully estimated from an evaluation of initiation sites using fracture mechanics and the statistics of extrema. Small fatigue crack growth in the two kinds of alloys obeys the relation proposed by Nisitani et al. , namely that d(2c)/d N = C (σ_a/σ_B)ⁿ· (2 c ), where C is a constant and σ_B is the ultimate tensile strength. It is pointed out that an improvement in fatigue strength of cast aluminium alloys can be expected by refining the eutectic silicon rather than by an increase in static strength.     

Optimization of microstructural and mechanical properties of friction stir welding using the cellular automaton and Taguchi method

In this study, microstructural and mechanical properties of friction stir welding (FSW) of AA1100 is optimized using Taguchi L9 orthogonal design of experiments. First, in order to study the microstructural properties of the weld, microstructure evolution of the weld zone is simulated with the cellular automaton (CA) method coupling the modified Laasraoui–Jonas (LJ) model. Then, the microstructural simulation results were validated by obtained experimental results. Good agreements between the simulation and the experimental results were observed. Then, tensile and hardness test were done to investigate the mechanical properties of the weld. The design parameters considered in the experiment were rotational speed, traverse speed, and shoulder diameter. The optimum process parameters were determined with reference to grain size (GS), ultimate tensile strength (UTS) and hardness. The predicted optimal value of grain size, ultimate tensile strength and hardness was validated by conducting the confirmation test using optimum parameters. Analysis of variance was done in order to determine the most dominant factors in friction stir welding.     

Effect of solidification conditions on microstructure,mechanical and wear properties of Mg–5Al–3Ca–0.12Sr magnesium alloy

In the present study, the microstructure, mechanical and wear properties of AXJ530 alloy under different solidification condition were investigated. AXJ530 alloys were cast in a multi-step permanent mould casting (PMC) with five different cooling rates, and also in high pressure die casting (HPDC). The effect of cooling rate was determined for the room temperature mechanical properties and the dry sliding wear resistance of the AXJ530 alloys. The results showed that grain size of AXJ530 alloy was refined and thinner lamellar eutectic phase formed at higher cooling rate. It was concluded that these changes led to the observed concurrent increases in ultimate tensile strength (σ_uts), yield strength (σ_0.2) and elongation (δ) of the AXJ530 alloy. The relationship between grain size and yield strength/hardness agreed with Hall–Patch behavior. The dry sliding wear rate of the PMC specimens decreased with increasing of cooling rate, but micro-porosity/inclusion in the HPDC specimen decreased its wear resistance properties. Abrasion was determined to be the dominant wear mechanism for the AXJ530 alloys.