动态复合细化变质对A356铝合金显微组织的影响

为弥补Al-10Sr中间合金对A356铝合金变质处理的不足,采用自制的Al-5Ti-1B-1RE中间合金与A1-10Sr中间合金对A356铝合金进行动态复合细化变质处理,研究变质处理后合金的显微组织,并与理论计算结果进行了比较。结果表明:采用JJ-1型精密增力电动搅拌器对熔体进行强力搅拌、振动,动态复合细化变质不仅能使共晶硅相由粗大的板片状转变为细密的颗粒状,并在α-Al边界均匀析出,而且使α-Al相明显细化,力学性能显著提高,与约翰逊-梅尔方程理论对组织晶粒尺寸控制研究结果相一致;同时A356铝合金熔体吸气倾向显著减轻,与热力学近似计算方程和斯托克斯定律对除气机制进行定量计算研究结果相一致。     

Preparation of Al–Ti–C–Sr master alloys and their refining efficiency on A356 alloy

Al–Ti–C–Sr master alloys with various amounts of Sr were prepared through a method of liquid solidification reactions. The as-prepared Al–Ti–C–Sr master alloys were then used as grain refiners to modify A356 alloy. The microstructures of the Al–5Ti–0.25C–2Sr, Al–5Ti–0.25C–8Sr alloys and modified A356 alloy were investigated. The results showed that the Al–5Ti–0.25C–2Sr alloy consisted of phases of α-Al, lath-shaped or tiny blocky TiAl₃, granular TiC, and blocky or rim AlTiSr, while the Al–5Ti–0.25C–8Sr alloy contained an irregular blocky Al₄Sr phase besides the above-mentioned phases. Satisfactory grain refining and modifying effects were obtained by the addition of Al–Ti–C–Sr alloys (0.5 wt.%) to the A356 alloy. Meanwhile, the sizes of the α-Al dendrites / SDAS(40 µm) decreased to 32.7 µm (or 30 µm).The morphology of eutectic silicon was changed from needle-/platelike form to fibrous/globular form. The grain refinement and modification effects of Al–Ti–C–Sr alloys on A356 alloys were mutually promoted. Compared with the Al–5Ti–0.25C–2Sr alloy, the Al–5Ti–0.25C–8Sr alloy possessed higher efficiency in grain refinement and modification of the A356 alloys.     

Influence of melt treatments on sliding wear behavior of Al–7Si and Al–7Si–2.5Cu cast alloys

The microstructures and dry sliding wear behavior of Al–7Si and Al–7Si–2.5Cu cast alloys were studied after various melt treatments like grain refinement and modification. Results indicate that combined grain refined and modified Al–7Si–2.5Cu cast alloys have microstructures consisting of uniformly distributed α-Al grains, eutectic Al– silicon and fine CuAl₂ particles in the interdendritic region. These alloys exhibited better wear resistance in the cast condition compared with the same alloy subjected to only grain refinement or modification. The improved wear resistances of Al–7Si–2.5Cu cast alloys are related to the refinement of the aluminum grain size, uniform distribution of eutectic Al-silicon and fine CuAl₂ particles in the interdendritic region resulting from combined refinement and modification. This paper attempts to investigate the influence of the microstructural changes in the Al–7Si and Al–7Si–2.5Cu cast alloys by grain refinement, modification and combined action of both on the sliding wear behavior.     

Microstructural features of as-cast A356 alloy inoculated with Sr, Sb modifiers and Al-Ti-C grain refiner simultaneously

This article focuses on the microstructural refinement of as-cast A356 alloy, obtained by melt inoculation of the same. The inoculants used are Sr (modifier), Sb (modifier) and Al-Ti-C (grain refiner). Microstructural characterization of A356 alloy reveals that the Secondary Dendritic Arm Spacing (SDAS) of α-Al dendrites and size/morphology of eutectic silicon decrease by treating the melt with pre-defined amounts of Sr, Sb and Al-5Ti-2C grain refiner. Microstructure of the inoculated as-cast A356 alloy shows that, modified eutectic-silicon exhibits fine fibrous morphology at shorter melt holding, while the same exhibits fine lamellar morphology on longer melt holding.     

Effect of Al-5Ti-1B on the microstructure of near-eutectic Al-13.0%Si alloys modified with Sr

The addition of Sr in Al-Si alloys as a modifier causes a transition of the morphology of eutectic silicon from coarse plate-like or acicular to fine fibrous. However, it may also lead to the formation of long columnar dendritic -Al phase. Al-5Ti-1B master alloy is often used as a grain refiner to achieve the fine equaixed grains in aluminum and aluminum alloys. The aim of the present study is to highlight the effect of Al-5Ti-1B master alloy additions on the microstructure of near-eutectic Al-Si alloys modified with Sr. When the addition of Al-5Ti-1B master alloy was below 0.82 mass%, the dendritic -Al phase changed from long columnar to equiaxed, and there were no noticeable changes of the morphology and size of eutectic silicon, while the size of eutectic cells decreased slightly. However, when the addition was above 0.82 mass%, the deleterious influence of Al-5Ti-1B master alloy on the modification effect of Sr emerged and with further increases in addition level a fully unmodified microstructure was finally produced. The results indicate that the effective Sr in the melt decreases with increased addition of Al-5Ti-1B. The poisoning event of Al-5Ti-1B master alloy on the modification of Sr is supposed to be related with the interaction between Sr and Ti.     

Influence of melt treatments and turning inserts on cutting force and surface integrity in turning of Al–7Si and Al–7Si–2.5Cu cast alloys

The microstructures, machinability and surface characteristics of Al–7Si and Al–7Si–2.5Cu cast alloys were studied after various melt treatments like grain refinement and modification. Results indicate that combined grain refined and modified Al–7Si–2.5Cu cast alloys have microstructures consisting of uniformly distributed α-Al grains, eutectic Al–silicon and fine CuAl₂ particles in the interdendritic region. These alloys exhibited better machinability and surface characteristics in the cast condition compared with the same alloy subjected to only grain refinement or modification. Performances of the turning inserts (Un-coated, PVD and Polished CVD diamond coated) were evaluated in machining Al–7Si and Al–7Si–2.5Cu cast alloys under dry environment using a lathe. The Polished CVD diamond coated insert outperformed the Un-coated or PVD-coated cutting inserts which suffered from sizeable edge buildup leading to higher cutting force and poor surface finish. The Polished CVD diamond coated insert shows a very small steady wear without flaking of the diamond film during cutting. This paper attempts to investigate the influence of grain refinement, modification and combined action of both on the microstrutural changes in the Al–7Si and Al–7Si–2.5Cu cast alloys and their machinability and surface finish when different turning inserts used.     

变质细化和热处理对挤压铸造成形A356铝合金构件性能的影响

用Al-10Sr变质剂和Al-5Ti-B细化剂处理A356铝合金熔体,并结合挤压铸造和T6热处理工艺,研究变质细化与热处理对A356铝合金挤压铸造件的组织和性能的影响规律。结果表明,随着Al-10Sr变质剂加入量的增加,共晶Si的形貌由片状和长杆状变为颗粒状和蠕虫状,α-Al的晶粒尺寸先减少后增大。当Al-10Sr的加入量(质量分数)为0.3%时,挤压铸造成形件的最优抗拉强度、屈服强度和延伸率分别为221.3 MPa、104.5 MPa和10.3%。Al-10Sr变质能提高形核率、细化α-Al晶粒尺寸和改变共晶硅形貌,使铸造件的力学性能提高。随着A-5Ti-B的增加,晶粒尺寸先降后增,力学性能先增后降。Al-5Ti-B的加入量为0.6%时,最优抗拉强度、屈服强度和延伸率分别为215.6 MPa、106.6 MPa和9.0%。T6热处理(固溶540℃/4 h+时效190℃/4 h)使屈服强度和抗拉强度显著提高和延伸率降低。经过0.6% 的Al-5Ti-B细化处理,T6处理挤压铸造件的最优的抗拉强度、屈服强度和延伸率分别为297.5 MPa、239.3 MPa和8.0%。共晶硅的球化和细化、成形件成分的均匀化以及Mg₂Si强化相在基体中弥散析出,是热处理后构件力学性能提高的主要原因。     

Comparation about efficiency of Al–10Sr and Mg–10Sr master alloys to grain refinement of AZ31 magnesium alloy

In the article, the effects of Al–10Sr and Mg–10Sr master alloys on the grain refinement of AZ31 magnesium alloy, are compared and analyzed. The results indicate that adding Al–10Sr or Mg–10Sr master alloys to AZ31 magnesium alloy could effectively reduce its grain size, but the refinement efficiency of Mg–10Sr master alloys is higher than that of the Al–10Sr master alloys. In addition, for a given melt holding time, the refinement efficiency of the two master alloys respectively increase with Sr adding amount increasing from 0 to 0.1 wt%, and the increasing laws are similar. For a given Sr adding amount, the refinement efficiency of Al–10Sr mater alloy gradually increases with the melt holding time increasing from 20 to 80 min, but its changing is not obvious for the Mg–10Sr mater alloy. The difference of refinement efficiency for the Al–10Sr and Mg–10Sr master alloys might be related to the dissolution modes and rates of Al₄Sr and Mg₁₇Sr₂ phases in the melt of AZ31 magnesium alloy.     

Influence of zirconium on grain refining efficiency of Al–Ti–C master alloys

The influence of Zirconium on the grain refinement performance of Al–Ti–C master alloys and the effect mechanism has been studied in this paper. The experimental results show that Zr not only results in poisoning the Al–Ti–B master alloy, but also poisons the Al–Ti–C master alloys. The poisoning effect is more obvious at higher melting temperature. When 0.12%Zr is added into the melt, the grain refinement performance of Al–5Ti–0.4C refiner with 0.2% addition level absolutely disappears at 800 °C. The experimental results also show that it is difficult to refine the commercial purity Al containing 0.15%Zr by Al–5Ti–0.4C master alloy. Further experiments show that the Zr element can interact with both TiAl₃ and TiC phases. If both of them are present, Zr preferentially reacts with TiAl₃ phase.     

Effect of melt treatment on microstructure and impact properties of Al-7Si and Al-7Si-2.5Cu cast alloys

The microstructures and impact toughness of Al-7Si and Al-7Si-2.5Cu cast alloys were studied after various melt treatments like grain refinement and modification. The results indicate that combined grain refined and modified Al-7Si-2.5Cu alloys have microstructures consisting of uniformly distributed α-Al grains, interdendritic network of fine eutectic silicon and fine CuAl₂ particles in the interdendritic region. These alloys exhibited improved impact toughness in as cast condition when compared to those treated by individual addition of grain refiner or modifier. The improved impact toughness of Al-7Si-2.5Cu alloys are related to breakage of the large aluminum grains and uniform distribution of eutectic silicon and fine CuAl₂ particles in the interdendritic region resulting from combined refinement and modification. This paper attempts to investigate the influence of microstructural changes in the Al-7Si and Al-7Si-2.5Cu cast alloys by grain refinement, modification and combined action of both on the impact toughness.     

Structures and properties of DIN661 alloy with fine-crystalline additives

The structures and properties of DIN661 alloy with the addition of 1 wt.% of 40% La–60% Ce and with 0.3–0.4 wt.% of Al-based rapidly solidified additives together with SHS Al–Ti mater alloy were compared. Experiments were carried out under industrial conditions. The treatment with the rare earth additives resulted in considerable grain refinement but with worse mechanical properties because of the presence of large crystals of intermetallic phases, primary silicon, α-Al dendrites and greater gas porosity. Less quantities of rapidly solidified additives and SHS Al–Ti mater alloy provided refinement of these phases and better mechanical properties.     

Semisolid processing of near-eutectic and hypereutectic Al–Si–Cu alloys

Semisolid processing of near-eutectic and hypereutectic versions of alloy 380 offers to overcome the problems encountered in casting hypereutectic Al–Si alloys and was thus explored in the present work. Experimental near-eutectic and hypereutectic Al–Si–Cu alloys obtained by adding elemental silicon to the 380 alloy were melted and were cooled to within 5 to 15 °C of their liquidus points before they were poured into a permanent mould in order to produce non-dendritic feedstock for thixoforming. This low superheat casting (LSC) process largely replaced α-Al dendrites with relatively smaller α-Al rosettes in all alloys. The slugs machined from the LSC ingots thus obtained were thixoformed after they were heated in situ in the semisolid range, between 568 and 573 °C, for 5 min in a laboratory press. Semisolid soaking sufficed to produce the required globular structure even when some dendritic features were retained in the starting feedstock. The hardness of the thixoformed parts which ranged between 84 and 96 HB have increased to 121–131 HB after the T6 heat treatment, implying a considerable age-hardening potential. The T6 treatment also improved the morphology of the eutectic silicon with potential benefits regarding the ductility of the thixoformed part.     

Microstructure and mechanical property developments in Al–12Si gravity die castings after Ti and/or Sr additions

This study evaluates the influence that modification and refinement have on the mechanical properties of the eutectic Al–Si alloy and on the quantitative and qualitative correlations with the microstructure. A general improvement in the mechanical properties of the alloys was observed after the additions. The best mechanical values were obtained when both Ti and Sr were present, in particular for the experimental composition with the higher Ti/Sr ratio. The results also indicate the development of a ductility trough for the alloy with lower Ti/Sr ratio, indicating a poisoning effect of Ti over Sr.     

Influence of additions of Zr, Ti–B, Sr, and Si as well as of mold temperature on the hot-tearing susceptibility of an experimental Al–2% Cu–1%?Si alloy

This study was undertaken to investigate the effects of chemical composition and mold temperature (MT) on the hot-tearing susceptibility (HTS) of an experimental Al–2% Cu–1% Si alloy using a constrained rod casting mold. The HTS results were then compared with 206 (Al–5 wt% Cu) alloys containing the same additions. In general, the Al–2% Cu–1% Si based alloys exhibited higher resistance to hot-tearing than did the 206-based alloys. It was found that an elevated MT is beneficial in reducing the HTS of the Al–2% Cu–1% Si and 206 alloys in that the HTS value decreased from over 21 to less than 5, as the MT was increased from 250 to 450 °C. Increasing the Si content reduced the HTS of the Al–2% Cu–1% Si alloy considerably; this reduction may be attributed to an increase in the volume fraction of eutectic in the structure. The addition of Sr caused deterioration in the hot-tearing resistance of the base alloy due to the formation of Sr-oxides and an extension of the freezing range of the alloy. The refinement of the grain structure obtained with the Zr–Ti–B addition decreased the severity of hot-tearing as a result of an increase in the number of intergranular liquid films per unit volume and a delay in reaching the coherency point. It was also observed that α-Fe intermetallic particles may impede the propagation of hot-tearing cracks. The Al–2% Cu–1% Si alloy with 1 wt% Si addition was judged to be the best composition in view of its low HTS.     

Impact toughness in Al–12Si and Al–12Si–3Cu cast alloys—Part 1: Effect of process variables and microstructure

The charpy impact energy of Al–12Si and Al–12Si–3Cu cast alloys was measured in terms of the total absorbed energy. The standard charpy specimens 10×10×55 mm with a 2 mm V-notch were prepared from the castings. Effect of process variables and microstructural changes on the impact toughness of Al–12Si and Al–12Si–3Cu cast alloys was investigated. The results indicate that combined grain refined and modified Al–12Si–3Cu cast alloys have microstructures consisting of uniformly distributed α-Al dendrites, eutectic Al–Si and fine CuAl₂ particles in the interdendritic region. These alloys exhibited better impact toughness in the cast condition compared with the same alloy subjected to only grain refinement or modification.     

Er对原位ZrB2/A356.2复合材料组织与性能的影响

采用原位合成法制备ZrB_2/A356.2复合材料,研究了引入ZrB_2对A356.2铸态微观组织的影响以及不同Er含量对复合材料铸态微观组织及力学性能的影响。结果表明,引入ZrB_2后α-Al晶粒和粗大共晶Si发生略微细化,添加Er后α-Al晶粒细化明显,并且Er对共晶Si有很好的变质效果,α-Al由粗大树枝晶变为蔷薇状、球状,共晶Si从粗大的针片状变为细小的短棒状、球状,有效提高了复合材料的力学性能。当Er添加量为0.10%(质量分数)时,其细化变质效果最为明显,复合材料力学性能提高最为显著。     

Study on the grain refinement of A356 alloy by Al–3 wt-% VN master alloy

ABSTRACT

A novel Al–3?wt-% VN master alloy, mainly consisting of α-Al and VN phases, was successfully prepared by stir casting. The grain refinement performance of the master alloy on A356 alloy was then investigated. The results showed that the α-Al grain size of A356 alloy refined by Al–3?wt-% VN master alloy was 350?±?95?µm while that of A356 alloy treated by traditional Al–5Ti–B master alloy was 570?±?105?µm. Moreover, for A356 alloy with Al–3?wt-% VN addition, the good grain refining efficiency did not fade significantly within 30?min. The effectiveness of grain refinement might be attributed to VN particles, which acted as the heterogeneous nuclei of α-Al grains. Owing to the refinement strengthening, the yield strength, ultimate tensile strength and elongation of A356 alloy were improved.     

An investigation on hot-cracking mechanism of Sr addition into Mg–6Al–0.5Mn alloy

The hot-tearing susceptibility of Mg–6Al–0.5Mn alloy decreased with the increasing Sr content (≤0.2 wt%). The related mechanism is described as follows: Sr refines the grain and reduces the tendency of divorced eutectic; Sr decreases the eutectic temperature and results in higher intergranular strength. Both of the above reasons could improve the filling capacity of the melt, resulting in the best hot-tearing-resistant property of the alloy.     

Response to thermal exposure of the mechanically alloyed Al–Ti/C powders

Al–TiC_p composites have been extensively studied in recent years not only because of their attributes as wear-resistant structural materials but also on account of their potential as very efficient grain refiners. Al–Ti–C alloys of various compositions have already been commercialized as grain refining master alloys and have long been in use in aluminium foundries world wide. The present work was undertaken to investigate the possibility of manufacturing Al–TiC_p grain refiner master alloy tablets by the mechanical alloying route. Carbon was mechanically alloyed into Al–Ti alloy powder grains via high energy ball milling. The Al–Ti/C powder blend thus obtained was heat treated to promote the precipitation of TiC. Al₄C₃ was the first phase to form inside the powder grains upon thermal exposure. The Al₄C₃ particles which were too small to be identified with the optical microscope until 750 °C, have grown until 850 °C where they have started to react with Al₃Ti to produce TiC. A very fine dispersion of TiC particles was thus generated inside the powder particles while the Al₃Ti phase has almost vanished.