Hardness and microstructural developments of spray formed aluminium alloys with high Mg2Si content during hot extrusion and heat treatment

Abstract

Spray forming offers the possibility of producing alloys with very fine, homogeneous microstructures. Even materials with high contents of intermetallic precipitates, which cannot be produced by casting because of the high solidification rates required, can be distributed homogeneously. Alloying aluminium with high contents of Mg and Si (>20 wt-%Mg₂Si) gives an increase in stiffness plus a significant reduction in density, but a very fine distribution of the Mg₂Si particles in the aluminium matrix is required. Therefore, such alloys are commonly produced by spray forming. Post-spraying processes such as forming and heat treatment are generally carried out to optimise properties. To examine the microstructure and hardness as a result of subsequent processing, aluminium alloys with high Mg₂Si content (22–30 wt-%) have been produced under a variety of spray forming conditions. The duration and temperature of heating before extrusion were varied. In addition, some specimens were preheated without extrusion. The influence of subsequent heat treatment was investigated by varying the age hardening parameters. Hardness measurements were conducted and the distribution and size of the precipitates were evaluated by light microscopy. Image analysis was used to study the coarsening behaviour of primary Mg₂Si. The results indicate that the subsequent processing conditions have a strong influence on the microstructure and hardness of the material. Further, a significant dependence of coarsening rate during subsequent processing on the initial state of the material after spray forming was observed. Knowledge of correlations between process parameters and microstructural development offers the possibility of optimising the hot extrusion and heat treatment parameters for high Mg₂Si containing aluminium alloys.     

Change in microstructure of Al–Si–Cu casting alloys during high temperature solution treatment

Abstract

The solution treatment in Al–Si system casting alloys is usually performed to obtain supersaturated solid solution and spheroidising Si particles. It can be inferred that a high temperature solution treatment enhances mechanical properties without any special apparatuses or techniques. However, it is well known that the solution treatment close to an eutectic temperature causes local melting. In this study, the change in microstructure of Al–Si–Cu casting alloys, which have been solution treated at temperatures ranging from 773 to 824 K, have been investigated from a viewpoint of Cu concentration and the distributions of micropores and locally melt regions due to eutectic reaction. Tensile and hardness tests were carried out to discuss the relationship between mechanical properties and microstructures. In addition to a surface observation, an internal microstructural observation was carried out using the high resolution X-ray computed tomography. The burnt regions during the high temperature solution treatment were identified to be Cu rich. Porosity increased with increasing the solution treatment temperature. The porosity in the sample solution treated above a binary eutectic temperature was confirmed to be >0˙2 vol.-%. The Cu concentration in the α-phase increased below the binary eutectic temperature.     

Effect of material and test parameters on wear behaviour of eutectic Al–Si alloy (LM13)

Abstract

This paper investigates the effects of contact pressure, sliding speed and alloy condition on dry sliding wear behaviour of LM13 (Al–12Si–0˙8Cu–1˙0Ni–0˙6Mg) alloy. A pin on disc type wear testing machine was used to study the wear behaviour of alloy in untreated, melt and heat treated condition. Dry sliding wear tests were performed at two contact pressures (0˙31 and 0˙93 MPa) and four sliding speeds (0˙2, 0˙5, 0˙9 and 2˙0 m s^–1). The wear rate decreased with the increase in sliding speed irrespective of contact pressure and alloy condition used in this work. The untreated alloy exhibited higher wear rate than melt and heat treated alloy. The refinement of eutectic Si by melt treatment and spheroidisation of eutectic Si after heat treatment of LM13 alloy have been attributed to the reduction in wear. Melt treatment and heat treatment of LM13 alloy increased the hardness and tensile strength, while increase in ductility was marginal.     

Friction stir welding of dissimilar A319 and A356 aluminium cast alloys

Abstract

In the present investigation, the microstructure and mechanical characteristics of dissimilar A319 and A356 cast Al alloys plates joined by friction stir welding (FSW) were evaluated. The effect of tool rotational and welding speeds as well as the post-weld heat treatment (PWHT) on such properties was investigated. Post-weld heat treatment was carried out at a solutionising temperature of 540°C for 12 h followed by aging at 155°C for 6 h. For the as welded specimens, the welded zone (WZ) exhibited higher hardness values when compared with the A319 and A356 parent alloys. The peak hardness at the WZ was found to increase by increasing the tool rotational speed and/or reducing the welding speed. In contrast, the post-weld heat treated (PWHTed) specimens exhibited lower hardness values at the WZ than the parent alloys. For PWHTed specimens, the peak hardness at the WZ was found to decrease by increasing the tool rotational speed and/or reducing the welding speed. Tensile tests results demonstrate that, for the as welded specimens, the tensile fracture took place on A356 side where the hardness was minimal. While for PWHTed specimens, the fracture took place at the WZ. Increasing the tool rotational speed reducing both tensile and yield strengths, but increases the ductility of the joint.     

Compression behaviour of semisolid YL112 die casting aluminium alloy following isothermal heat treatment

Abstract

The semisolid compression deformation behaviour of YL112 die casting aluminium alloy with the non-dendritic and dendritic structures has been compared in tests using a Gleeble-1500 thermomechanical simulator. The non-dendritic structure was obtained by isothermal treatment at 570°C for 120 min. In tests up to compressive strains of 0·8, the semisolid compression stress of the alloy with non-dendritic structure initially increases rapidly with increasing strain, then decreases, before reaching a plateau value. Under different deformation temperatures and deformation rates, the maximum compressive stresses are obtained in all cases at strain values of ～0·07. The semisolid deformation stress decreases with increasing deformation temperature and increases with increasing deformation rate. The compression behaviour of the two types of alloy differs. The semisolid compression stress of the alloy with dendritic structure is higher than that of the alloy with non-dendritic structure; and for strains >0·07, the semisolid compression stress increases and decreases with increasing strain for alloys with dendritic and non-dendritic structures respectively.     

Effect of melt treatment on impact toughness of 356 type alloys

The effects of melt treatment in particular the types of grain refiners as well as Sr modification on the impact properties A356·2 alloys in as cast and heated treat conditions have been investigated. The results show grain refining is very effective parameter in improving the alloy toughness when the alloy is well modified. The best values for total absorbed energy of T6-tempered alloys are achieved after using 0·04%Ti in the form of Al–5%Ti–1%B and Al–10%Ti master alloys. A significant degradation in impact properties is observed due to the Sr–B interaction in some cases leading to the formation of Sr₆B compound. The improvements in toughness are mainly due to the change in silicon particle morphology and to the dissolution and fragmentation of intermetallics during the solution heat treatment process at 540°C for 8 h prior to quenching and artificial aging.     

Enhancing wettability of SiC in semisolid A356 aluminium matrix composites

Abstract

Stir casting plus cooling plate technique has been used for the fabrication of Al matrix composites based on alloy 356. Improvement of the wettability of SiC particles was carried out, employing the oxidisation of SiC particles, the use of wetting agents by adding magnesium into the matrix and the coating of SiC particles using a sol–gel technique. The introduction of SiC particles into partially solidified alloy with high viscosity prevents the particles from floating and agglomerating. Unoxidised SiC particles are mostly detached from the Al matrix during the grinding suggesting poor adhesion and poor wettability between the matrix and the particles. Oxidised SiC particles and sol–gel silica coated SiC particles indicate good binding between composite and matrix. The use of magnesium promoted wettability of SiC with A356 alloy. The eutectic silicon phase formed on the surface of SiC particles during solidification may be due to nucleation effects provided by the particles.     

Semisolid heat treatment of chromium cast iron

Abstract

The effect of semisolid heat treatment on the microstructures of three chromium white cast irons has been studied. The primary objective was to examine the effect of semisolid heat treatment on the microstructures with specific emphasis on revealing the variations related to the carbide morphology of the selected materials. It is found that graphitization of carbide in a 1·5 wt.% chromium cast iron is enhanced during isothermal heat treatment in the semisolid state. The eutectic carbide in high chromium cast irons is fully or partially modified into discrete particles during heat treatment in the semisolid state.     

Effect of hydrogen content,melt cleanliness and solidification conditions on tensile properties of A356 alloy

Abstract

In this study, the influence of hydrogen content, melt cleanliness and solidification conditions on the porosity distribution and mechanical properties have been investigated in a laboratory scale step mould casting with an A356 alloy. Three hydrogen levels, namely 0·10, 0·20 and 0·40 mL/100 g, were reached by treatment of the melt by Ar degassing, Ar–10H₂ and Ar water vapour mixture respectively. The hydrogen content of the melt was continuously measured. For each hydrogen level, two castings were produced, namely with and without filter. Porosity was increased with increasing hydrogen content but there was no significant effect of filtration. Moreover, similar tensile properties were achieved with different hydrogen levels and different porosity levels.     

热处理对A356铝合金组织结构和力学性能的影响

用两种不同的热处理制度对稀土和锶综合细化变质的A356合金进行处理,一种是长时间标准处理制度T6（535℃固溶4h＋150℃时效15h）,另一种是短时间的热处理制度ST（550℃固溶2h＋170℃时效2h）。采用光学显微镜、扫描电镜及室温拉伸实验等手段分析热处理制度对A356合金微观组织和拉伸力学性能的影响。结果表明：在550℃下固溶2h可以获得Mg、Si过饱和且分布均匀的α（Al）固溶体,并使共晶硅相球化;再经170℃人工时效2h后,可以达到传统T6处理的时效析出效果。拉伸实验结果表明,A356铝合金经传统T6处理得到了最高的拉伸强度和断裂伸长率;通过ST短时热处理后,其拉伸强度、屈服强度及伸长率分别可以达到T6处理时的90%,95%和80%。     

Effect of melt filtration on porosity formation in Sr-modified A356 aluminium alloy

A major concern in Sr-modification is the increased tendency for microporosity formation in the alloy. The effects of melt filtration and Sr-modification on porosity content have been investigated. A significant effect of melt filtration on the reduction of porosity in Sr-modified samples is that the inclusion content of Sr- modified melts is higher than that in an unmodified melt. In an attempt to understand the types of inclusions in Sr-modified A356 melts that cause pore formation, samples have been studied by scanning electron microscopy. It was found that the exposure time of the melt in an oxidising atmosphere affects the oxidation behaviour and the inclusion types of Sr-modified melts. In the cases where the melt was not held for a long time in the oxidising atmosphere, Al₂O₃ films and particles played a significant role, while the Sr- containing oxides did not take part in pore formation. At relatively short holding times, the surface oxide layer of a melt of A356 alloy consisted of Al₂O₃ and Al₂O₃.MgO but was found to contain no detectable SrO. However, intermetallic compound Al₂Si₂Sr was found to be present and it is tentatively proposed that it has role in pore formation. When the melt was held for a long time in an oxidising atmosphere, more than 50% of the surface oxide layer consisted of SrO-containing spinels. IJCMR/460     

Influence of static magnetic fields on the protective ability of phosphate coatings on aluminium alloys

Abstract

Phosphate coatings on aluminium are used to improve the corrosion resistance of aluminium before the latter is painted. Phosphate coatings deposited in phosphate solution treated with a magnetic field prior to deposition can take on new properties. A magnetic field can either improve or deteriorate the properties of the phosphate coating formed, depending on the composition of aluminium.     

Evaluation of Young's modulus of high stiffness aluminium die cast alloys using nanoindentation technique

Abstract

A new correction method which is appropriate for the evaluation of Young's modulus of crystalline compounds has been examined and established to correct the size effect. Using a nanoindentation test and the established correction method, Young's moduli of primary Si phase, Al–(Fe,Mn)–Si and Al–Ni compound phases in the hypereutectic Al–Si die cast alloys were evaluated to be 182±9, 208±29 and 170±4 GPa respectively. The rule of mixtures for Young's modulus and the area fraction of each phase were applied to evaluate the Young's modulus of aluminium die cast alloys. The Young's modulus calculated by the rule of mixtures is in good agreement with the value obtained by a tensile test. The combination of the rule of mixtures and nanoindentation test is an effective approach to the precise evaluation of the Young's modulus of aluminium die cast alloys having complicated microstructures.     

Effect of stirring on behaviour of double oxide film defects in A356 aluminium melt

The effect of stirring of melt on the behaviour of double oxide film (bifilm) defects in A356 melt was investigated using a reduced pressure test (RPT) technique. The melt was poured from a height into a crucible to introduce bifilms into the melt. The melt was then either remained stagnant or mechanically stirred, and RPT samples were taken from the melt at 8 min intervals. The RPT samples were then characterised by determination of their porosity parameters and examination of the internal surfaces of the pores using a scanning electron microscope. The results showed that stirring of the melt facilitated the diffusion of H in the melt into the atmosphere of the defects and hence accelerated the removal of the defects from the melt by floatation. This effect was attributed to the increase in the stress induced to the defects, which increased the rate of formation of cracks on their layers.     

Effect of melt flow on dendritic growth in AlSi7-based alloys during directional solidification

Abstract

High-strength automotive components are often made of AlSi7-based alloys. A very challenging problem with aluminium casting is the influence of melt flow during solidification, because it affects the microstructure formation and therefore the material properties. The scope of this paper is to investigate the effect of forced melt flow on the evolution of the dendritic microstructure in a binary AlSi7 alloy during directional solidification. Global modelling using the software CrysMAS provides typical flow patterns and velocities. These values are used as boundary condition for the flow in the phase field code MICRESS, which allows the numerical simulation of dendritic array solidification in 2D with applied flow. From solidification experiments in a gradient furnace with applied rotating magnetic field the dendrite shapes are determined. It is found consistently that intense melt flow leads to asymmetric dendrite shapes and the growth behaviour of the dendrite arms is directly correlated with the flow direction.     

Effect of united refining and modification on mechanical properties of A356 aluminium alloys

The refining and modification effect of Ti （from GRA/）, B, Sr and RE （cerium-riched mixtures of rare earth） on the mechanical properties ofA356 aluminum alloys under T5 and T6 treatment condition were studied by OM, SEM, EDAX, etc. It is found that the addition of RE to A356 alloys containing Ti and/or B and Sr makes strength and elongation increase in T6 treatment, but make elongation decrease in T5 treatment, at the same time, the long axis of α（Al） grain structure decreases and the mean diameters of silicon particles increase with RE additions increasing. Grain refining with 0.01%Ti plus 0.03% B makes the dendrite a（Al） grain structure transform into equiaxed structure, resulting in obvious increase of elongation percentage. The mean diameters of silicon particles in T5 treatment are smaller than that in T6 treatment. Roundness of silicon particles in T5 treatment is higher than that in T6 treatment. A356 alloys modified and refined with Ti, B and Sr obtain the best mechanical properties in T5 treatment, however, the alloys with Ti, B, RE and Sr additions obtain the best mechanical properties in T6 treatment.     

Influence of duplex melting process on melt quality and solidification of grey cast iron

Abstract

The quality of molten iron produced by two melting processes, the medium frequency induction furnace melting process and the shortcut duplex melting process with blast and medium frequency induction furnaces, has been studied comparatively. The shortcut melting process contributes to a higher undercooling tendency of base iron, and the extent of this tendency depends on other processing parameters such as ratio of charges, fluctuation in composition, superheating temperature and holding time, etc. When the shortcut melting process is used in the production of grey cast iron, there are no essential differences, with subsequent proper inoculation, in the microstructure and mechanical properties of castings compared with the conventional medium frequency induction melting process. Inoculation is a very effective method to alleviate and/or eliminate the negative effect when the shortcut melting process is used.     

In situ observation of solidification phenomena in Al–Cu and Fe–Si–Al alloys

Abstract

Synchrotron radiation enables the observation of solidification in metallic alloys. In situ observations of solidification for Al–Cu alloys (5, 10 and 15 wt-%Cu) are reported. Nucleation and fragmentation of dendrite arms were often observed in the 15 and 10%Cu alloys when unidirectional solidification was performed from the planar interface. In contrast, nucleation and fragmentation were rarely observed in the 5%Cu alloys. The nucleation ahead of the solidifying front and the fragmentation in the mushy region strongly depended on alloy composition. This paper also presents in situ observation of solidification of Fe–10Si–0·5Al (at-%) alloys. The dendritic growth of δ-Fe was clearly observed using this technique. The development of X-ray imaging techniques enables the solidification of various conventional cast alloys such as Al, Ni and Fe alloys to be observed and will be increasingly used to investigate solidification phenomena.     

熔体温度处理对不同成分铝合金凝固组织和性能的影响

运用熔体温度处理工艺 ,研究了A35 6铝合金中Cu ,Fe ,Mg成分的变化对凝固组织及性能的影响。组织观察和性能测试结果表明 ,熔体温度处理工艺能够明显抑制富Cu、富Mg、富Fe等金属间化合物相的析出 ,改善和减小了富Cu、富Mg、富Fe相析出形貌和尺寸 ,显著地提高合金力学性能。用团簇理论分析和解释了有关的实验现象和结果。     

Effect of low temperature melt on solidification structure of A356 alloy with melt thermal treatment

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