Influence of silicon and magnesium on grain refinement in aluminium alloys

Abstract

Grain refinement in Al–Si alloys with silicon contents in the range of 0·2–30 wt-% has been studied in detail with conventional as well as higher level additions of a Al–5Ti–1B master alloy. A poisoning effect was observed with Al–Si alloys containing ≥7 wt-%Si and the extent of poisoning increased with an increase in the silicon content. Silicon improves the grain refining behaviour of aluminium when added in small quantities (0·2%). Magnesium can counteract the poisoning effect of silicon. The optimum level of magnesium required to overcome the poisoning effect depends on the silicon content of the alloy. Higher level additions of a grain refiner could overcome the poisoning effect of silicon and the level required to achieve good grain refinement is a function of the silicon and magnesium contents of the alloy. The present paper also reports the influence of degasser and melt temperature on the grain refining response of Al–Si alloys.     

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.     

Effects and mechanisms of grain refinement in aluminium alloys

Grain refinement plays a crucial role in improving characteristics and properties of cast and wrought aluminium alloys. Generally Al-Ti and Al-Ti-B master alloys are added to the aluminium alloys to grain refine the solidified product. The mechanism of grain refinement is of considerable controversy in the scientific literature. The nucleant effects i.e. which particle and its characteristics nucleate α-Al, has been the subject of intensive research. Lately the solute effect i.e. the effect of dissolved titanium on grain refinement, has come into forefront of grain refinement research. The present paper attempts to review the literature on the nucleant effects and solute effects on grain refinement and addresses the importance of dissolved titanium in promoting nucleation of α-Al on nucleant particles.     

Effect of solute content on the grain refinement of binary alloys

Data are presented for the grain refining of AI-Si and Pb-Sb alloy systems. These show a minimum grain size near the maximum solubility limit. An explanation in terms of the solidification interval is presented. In general, the optimum grain refining is predicted to occur at the solubility limit since t is at this concentration that the solidification times are longest.     

Recent advances in grain refinement of light metals and alloys

Grain refinement leads, in general, to a decreased tendency to hot tearing, a more dispersed and refined porosity distribution, and improved directional feeding characteristics during solidification. Reduced as-cast grain size can also lead to improved mechanical properties and wrought processing by reducing the recrystallized grain size and achieving a fully recrystallized microstructure. It is now well established that the two key factors controlling grain refinement are the nucleant particles including their potency, size distribution and particle number density, and the rate of development of growth restriction, Q, generated by the alloy chemistry which establishes the undercooling needed to trigger nucleation events and facilitates their survival. The theories underpinning our current understanding of nucleation and grain formation are presented. The application of the latest theories to the light alloys of Al, Mg and Ti is explored as well as their applicability to a range of casting and solidification environments. In addition, processing by the application of physical processes such as external fields and additive manufacturing is discussed. To conclude, the current challenges for the development of reliable grain refining technologies for difficult to refine alloy systems are presented.     

Modeling of grain refinement in aluminum and copper subjected to cutting

Plane-strain orthogonal cutting has recently been exploited as a means to refine the microstructure of metallic materials from tens of micrometers or greater to a few hundred nanometers. While experimental work has produced a significant body of knowledge with regard to microstructure and properties of machined materials, only a handful of studies can be found in the literature to discuss the microstructural evolution mechanism and to predict grain refinement during cutting. In this paper, dislocation density-based material models are developed to model grain size refinement and grain misorientation during cutting of Al 6061 T6 and OFHC Cu under various cutting conditions. It is shown that the developed CEL finite element model embedded with the dislocation material models captures the essential features of the deformation field and grain refinement mechanism during cutting. The model predicts the grains in the machined chips are refined from an initial size of 50–100 μm to about 100–200 nm for Al 6061 T6 and OFHC Cu at a low cutting speed of about 0.02 m/s with negative rake angle tools. It is shown that a small applied strain, high cutting speed or high cutting temperature will all contribute to a coarser elongated grain structure during cutting.     

Effects of grain refinement and boron treatment on electrical conductivity and mechanical properties of AA1070 aluminum

Aluminum has been used as an alternative to copper for the production of electrical grade conductors. However, good electrical conductivity with excellent mechanical properties is hard to realize. Grain refinement can improve plastic deformation and mechanical properties. Boron treatment is advantageous to the improvement of electrical conductivity. So it is promising to achieve good mechanical properties and electrical conductivity by the interaction of grain refinement and boron treatment. In our work, the effect of grain refinement and boron treatment on electrical conductivity and mechanical properties of AA1070 aluminum was studied. The ideal grain refiner is Al–5Ti–0.8B–0.2C master alloy and with 0.2% addition, the electrical conductivity keeps at 60.7% IACS. Besides, the effect of different boron additions on electrical conductivity of AA1070 aluminum was studied. With 1%Al–6B addition, its electrical conductivity can reach 64% IACS, improved by 5.3%. With 0.2%Al–6B and 0.5%Al–5Ti–0.8B–0.2C additions, the electrical conductivity can reach 63.2% IACS, ultimate tensile strength at room temperature (UTS_25 °C) is 85 MPa, and elongation (Ɛ) is 58%. Compared with AA1070 aluminum without addition, the ultimate tensile strength and elongation have been improved by 26.9% and 9.4% separately.     

Effects of solutes on grain refinement of selected wrought aluminium alloys

Abstract

An analysis of the effects of alloy additions on grain refinement in a series of model and commercial aluminium alloy compositions is reported. The data of Birch and Fisher published earlier describing grain refinement in 32 systems, including AA3004, AA5083, AA6063, and AA7050, are considered in terms of the supercooling effect of each alloy addition. A simple model describing grain size in terms of additive supercooling effects of individual alloy additions is proposed which fits the data reasonably well. Deviations from simple additive behaviour are evident in systems where strong intermetallic interactions occur. Interaction coefficients among solutes are invoked to explain these deviations. Individual alloying effects on grain refinement are treated in terms of constitutional supercooling parameters, and behaviour is shown to be similar to that described in earlier studies. Zirconium is shown to have a general poisoning effect in a range of alloy compositions.     

Effect of grain orientation on aluminum relocation at incipient melt conditions

Aluminum is commonly used for structural applications in the aerospace industry because of its high strength in relation to its weight. It is necessary to understand the mechanical response of aluminum structures at elevated temperatures such as those experienced in a fire. Aluminum alloys exhibit many complicated behaviors that require further research and understanding, such as aluminum combustion, oxide skin formation and creep behavior. This paper discusses the effect of grain orientation on aluminum deformation subjected to heating at incipient melt conditions. Experiments were conducted by applying a vertical compressive force to aluminum alloy 7075 block test specimens. Compression testing was done on test specimens with the applied load on the long transverse and short transverse orientations. Results showed that the grain orientation significantly influences aluminum’s strength and mode of failure.     

Effect of ultrasonic vibration on the grain refinement and SiC particle distribution in Zn-based composite filler metal

Refining the matrix microstructure of the composite is an effective method to avoid severe reinforcement particle pushing by the advancing solid–liquid interface during solidification. The effect of the ultrasonic vibration, which was injected into the melt at various stages of solidification, on the grain refinement and particle distribution in a Zn-based solidified composite filler metal was investigated. Perfect grain refinement was obtained with the application of continuous ultrasonic vibration. However, severe particle pushing by the sound radiation pressure was observed, resulting in serious particle segregation. Uniform distribution of SiC particles as well as grain refinement was obtained when proper intermittent ultrasonic treatment was applied.     

Determination of grain size in uranium-chromium alloys by ultrasonic attenuation

Ultrasonic attenuation was measured in cylindrical samples of uranium-chromium alloys used for nuclear fuel elements. The grain structure of the samples was previously refined by proper heat-treatment and the grain size in the range of 50 to 200 μm was determined by conventional metallographic methods. The attenuation coefficient,γ, was correlated with the mean grain diameter,ˉD, for three ultrasonic frequencies: 4, 6 and 12 MHz. The experimental results were compared with existing theories and good agreement was found with the curves based on Merkulov's analysis for cubic metals. A more practical result of this work was the development of a non-destructive testing procedure for grain-size determination in nuclear fuel elements cast from these alloys. The highest frequency (12 MHz) is normally used as it is more sensitive in the lower grain-size limit (50 to 100 μm) and only in border-line cases would one switch to the other frequencies (6 and 4 MHz) which are more applicable to the higher grain-size limit (100 to 200 μm).     

Effect of acicular ferrite formation on grain refinement in the coarse-grained region of heat-affected zone

The microstructure of acicular ferrite and its formation for the grain refinement of coarse-grained region of heat-affected zone of high strength low-alloy bainite steels were studied using three-dimensional reconstruction technique. Crystallographic grain size was analyzed by means of electron backscatter diffraction. It was revealed that the microstructure in the coarse-grained region of the heat-affected zone consisted of predominantly bainite packets and a small proportion of acicular ferrite. Acicular ferrite was of lath or plate-like rather than needle or rod-like morphology. Tempering of the coarse-grained region of heat-affected zone showed that the acicular ferrite was more stable than the bainite, indicating that the acicular ferrite was formed prior to bainite. The acicular ferrite laths or plates divided the prior austenite grains into smaller and separate regions, and confining the bainite transformed at lower temperatures in the smaller regions and hence leading to the grain refinement in the coarse-grained region of the heat-affected zone.     

Influence of intermetallic inclusions on the endurance of aluminum alloys

The results of experimental and theoretical investigations of the influence of intermetallic inclusions on the fatigue limit of secondary aluminum alloys are presented. We study AK8M3 secondary silumin. The influence of the content of iron in the alloy on the fatigue limit and intermetallic inclusions is analyzed. It is shown that the size and volume fraction of inclusions increase and the fatigue limit decreases as the content of iron increases. It is experimentally demonstrated that the treatment of alloy with a special modifying complex [2] leads to the globularization of intermetallic inclusions and increases the fatigue limit. We propose analytic dependences for the determination of the influence of intermetallic phases on the fatigue limit of heterogeneous aluminum alloys. By comparing the experimental and numerical values of the fatigue limit of AK8M3 alloy, we reveal their satisfactory agreement.     

Insights into the dual effects of Ti on the grain refinement and mechanical properties of hypoeutectic Al-Si alloys

Hypoeutectic Al-Si alloys are becoming increasingly popular in automotive and aerospace engineering fields due to their excellent overall performance,and grain refinement is regarded as an important way to improve casting and mechanical properties.Titanium(Ti)is a basic element for grain refinement;thus,a certain amount of Ti is often included in Al-Si alloys.In the present work,the changes in the grain re-finement,mechanical,and casting properties of Al-Si alloys with different Ti concentration levels under various grain refinement conditions were systematically investigated.The specific roles of Ti in the het-erogeneous nucleation of α-Al grains were summarized,and the formation mechanism of Ti-rich zones in Al-Si alloys was revealed.Excess Ti concentration could not efficiently reduce the grain size of Al-Si alloys and eventually resulted in inferior mechanical and casting qualities;hence,the recommended Ti concentration level for the aluminum alloy grades of A356 and A357 is ≤ 0.1 wt％.Furthermore,an opti-mized technique for the grain refinement of hypoeutectic Al-Si alloys was presented.A small amount of an Al-TCB master alloy was introduced to achieve the best grain refinement and mechanical properties in a trace Ti environment.The addition of 0.5 wt％of the Al-TCB master alloy at the Ti concentration level of 0.06 wt％increased the ultimate tensile strength,elongation,and quality index of the Al-7Si-0.45Mg alloy to 328.8±5.0 MPa,14.4％±0.6％,and 970.7±33.1 MPa,respectively.     

第二相粒子对ECAP挤压的2A12铝合金晶粒细化的影响

采用等径角挤压技术对2A12铝合金在室温下进行挤压,成功制备了亚微米尺度的块体铝合金材料.挤压前材料的平均晶粒尺寸约5μm,两次挤压后,平均晶粒尺寸细化至200nm左右.合金中的Al2Cu相在挤压过程中由针状变成了块状颗粒,而Al2CuMg相在挤压过程中晶粒大小基本不变.研究发现,硬颗粒Al.2CuMg对基体α-Al有剪切和割裂作用,可以促进基体的晶粒细化过程,并初步给出了晶粒细化的模型.     

Influence of grain size on tensile properties of Al-Mg alloys

Abstract

Grain size refinement is an important strengthening mechanism in Al-Mg 5000 series alloys, which have a relatively large Hall-Petch slope compared with other Al alloys. In addition, the high work hardening rate exhibited by Al-Mg alloys provides excellent formability. This paper investigates the influence of grain size on the flow stress over a range of strains, and in several different Al-Mg alloys. It is found that the Hall-Petch slope decreases after yield, indicating that the large grain size effect is primarily associated with initiating plasticity in these alloys. Beyond yield the slope decreases to a value equivalent to other, non-Mg containing alloys, and shows no clear dependence on strain. The intercept stress from the Hall-Petch plots at different strains is non-linear with ? 1/2 for alloys containing up to 3 wt-%Mg, which indicates that the free slip distance is strain dependent in these alloys. In an Al-5 wt-%Mg alloy the intercept stress is linear with ? 1/2, indicating that solute atoms are controlling the free slip distance. If Mn is added to the Al-5 wt-%Mg alloy, as it is in commercial alloys, it has little influence on the grain size dependence, but it does increase the frictional stress at the highest Mn level of 0.7 wt-%.     

Probabilistic modeling of fatigue related microstructural parameters in aluminum alloys

This paper presents the results of probabilistic modeling of the fatigue related microstructural parameters in unclad 2024-T351 aluminum sheets. The statistical distributions of the constituent particle size, which were obtained from metallographic measurements from polished surfaces, were determined by graphical goodness-of-fit tests. The distributions of the crack-nucleating particle sizes were determined using the data measured from various fatigue fracture surfaces. Initially, an extreme value theory based model was investigated to correlate the overall particle distribution with its fatigue subsets. Furthermore, a new Monte Carlo simulation was developed to determine the fatigue subsets using the microstructural parameters such as particle size, grain size, and grain orientation distributions, in association with qualitative criteria on fatigue crack nucleation and growth mechanisms.     

铝及铝合金表面处理研究进展

铝合金耐磨性差、特殊条件下耐蚀性差的缺点限制了它的进一步利用,对铝合金进行表面处理长期以来一直是扩大铝合金使用范围地行之有效的方法.文章综述了铝合金的各种表面处理方法,比较了它们的优缺点,指出表面氧化是铝合金表面处理的主流,复合处理、纳米化处理将是今后铝合金表面处理的主要研究方向.