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 boron on the microstructure and mechanical properties of Al-11.6Si-0.4Mg casting alloy modified with strontium

Abstract

The influence of the addition of Al-1B master alloy on the microstructure and mechanical properties of Al-11.6%Si-0.4%Mg alloy modified with 0.030%Sr has been investigated. The mechanical properties and fracture behaviour of the alloys as cast and after T6 heat treatment with three different melt treatments (no treatment; 0.030%Sr modifying treatment; and 0.030%Sr + 0.028%B combined melt treatment) were also compared. Al-1B master alloy has a strong action in refining the dendritic structure in near eutectic Al-Si casting alloys modified with Sr. The Sr+B combined melt treatment can improve considerably the mechanical properties of the alloys, both as cast and after T6 heat treatment. Fracture modes of the alloys with the Sr modifying treatment and the Sr+B combined melt treatment are typically ductile. However, fractographs indicate that the alloy with combined melt treatment suffered greater ductile deformation before fracture. The Sr+B combined melt treatment significantly improves the mechanical properties of near eutectic Al-Si casting alloys.     

Characteristics of α-dendritic and eutectic structures in Sr-treated Al—Si casting alloys

The present study was carried out to determine the effect of alloy composition and solidification conditions on changes in the dendritic and eutectic structures in Al—Si alloys containing strontium. A series of experimental and industrial alloys viz., Al-7% Si, Al-12% Si, 319 and 356 were selected, to cover a variety of alloy freezing ranges. The techniques of thermal analysis, optical microscopy, and SEM/EDX and EPMA analyses were employed to obtain the results presented here. Depression in the eutectic Si temperature in Al-7% Si alloys occurs on addition of alloying elements such as Mg and Cu. Introduction of Sr to these alloys further depresses the eutectic temperature, with a corresponding increase in the volume fraction of the -Al phase. The primary dendrite solidification pattern changes from parallel rows to a branched form, producing an equiaxed type of structure and hence shorter primary dendrite lengths. This is expected to enhance the interdendritic feedability. The lengths of the secondary dendrite arms are controlled by the rejection of solute atoms in front of the growing dendrites during solidification. The higher the alloying content in the alloy (i.e., 319), the smaller the dendrite cell size. The longer solidification time in the 319 alloy also appears to have a considerable influence on the amount of porosity formed in the alloy, in addition to that of Sr.     

Eutectic Solidification in Al-13.0%Si Alloys with Combined Addition of Strontium and Boron

The influence of addition of boron(B) on eutectic solidification in a near-eutectic Al-13.0%Si alloy modified with strontium(Sr) was investigated using thermal analysis and macro/microstructure observation.Addition of B in the Sr-modified alloy leads to a considerable increase in nucleation temperature(TN,the minimum temperature prior to recalescence(TM) and growth temperature(TG).In the Sr-modified alloy,nucleation of eutectic might originate at the heterogeneous sites on the mold wall or in the melt near ...     

Microstructure and mechanical properties of modified and non-modified stir-cast Al-Si hypoeutectic alloys

The microstructure and mechanical properties of modified and non-modified stir-cast commercial aluminium alloys A-S7G03 and A-S4G have been investigated. Stir casting of these alloys resulted in spherical and/or rosette shape primary -phase, and the eutectic silicon was broken into miniature needle morphology. This stir-cast structure slightly improved the mechanical properties in comparison to those of conventionally cast alloys, however the fracture of the stir-cast alloys revealed intergranular brittle fracture. The addition of 0.02% strontium, in the form of Al-5 mass% Sr master alloy, during stir casting modified the eutectic silicon into a very fine spheroidal morphology, while the -phase particle showed the same morphology as the stir-cast alloys. This novel structure resulted in significant improvement of mechanical properties. The elongation of the modified stir-cast alloys was five times greater than that of the non-modified one. A transgranular mode of fracture was observed for the modified stircast alloys, moreover smooth ripple and dimple patterns were observed reflecting the high ductility of the modified stir-cast alloys.     

Effect of Squeeze Cast Process Parameters on Fluidity of Hypereutectic Al-Si Alloy

The effects of processing variables on the fluidity of hypereutectic Al-Si alloy melt during squeeze casting were investigated.The maximum fluidity of Al-16.0%Si alloy during squeeze casting was obtained under the applied pressure of 30 MPa.The fluidity increased with superheat.The fluidity increased with silicon content in the range from 12.0% to 20.0%.That was decreased respectively by eutectic modification and primary silicon refinement.     

太阳能热发电中换热管石墨防护套环的制备与研究

太阳能热发电中的Al-12.07%Si(质量分数,下同)储能材料在高温下有很强的腐蚀作用。采用石墨套环包覆20G钢换热管,使换热管与熔融铝硅合金隔离,石墨良好的抗腐蚀性将保护换热管不受熔融铝硅合金的腐蚀。     

Poisoning effect of bismuth on modification behaviour of strontium in LM25 alloy

Nucleation and growth, temperature measurements and microstructure observations of silicon phase are presented for strontium modified Al–7%Si (LM25) cast alloy treated with bismuth. The results show that addition of bismuth in strontium modified alloys may have a poisoning effect resulting in lost modification of the silicon phase. With increasing Bi/Sr ratio, thermal analysis measurements showed that the eutectic growth temperature increased remarkably to 573°C and recalescence decreased to 0·2°C and the morphology of silicon displayed the same flake-like structure as in the unmodified alloys. Microstructural observation showed that a minimum Bi/Sr ratio of 1·2 which is equivalent to a Sr/Bi ratio of 0·43 is required for effective strontium modification and neutralization of the poisoning effect of bismuth.     

Degassing of aluminum alloys during re-melting

The evolution of porosity during re-melting of near-eutectic casting Al alloys has been investigated in-situ using X-ray micro-focus radiography. During re-melting process, gas bubbles float out of the melt quickly when the liquid melt interface passes through them. Based on the degassing phenomenon from the real-time observation, several re-melting experiments were carried out with pure aluminum and a hypoeutectic Al-7%Si-0.4%Mg (A356) in addition to the near-eutectic Al-13%Si alloys. The results clearly show that the area fraction of porosity in the final Al castings decreases dramatically after re-melting one or two times, of which the mechanism is discussed in this paper.     

Recent developments in hypoeutectic Al-Si A-S4G alloy

The effect of adding Sr in the form of the Al-5 Sr master alloy to commercial A-S4G and high purity Al-4Si alloys on the modification process has been investigated. The volume fraction of the eutectic matrix decreased by modification due to the movement of the eutectic point to the higher Si content side. The tensile properties, especially elongation, have been increased by modification. The elongation of the A-S4G alloy is increased from a value of 0.9 to 15% by modification. Additionally, the elongation of the modified high purity alloy reached a value of 34%. The fracture path of the modified alloys circumvents the -phase while it is not yet known if it propagates intergranularly or transgranularly through the eutectic matrix. The fracture surface revealed dimple and smooth ripple patterns reflecting the high ductility of the modified alloys.     

Al-Si共晶系连续定向凝固自生复合材料的组织与性能

本文采用新型连续定向凝固(CDS)装置研究了Al-Si合金(成分为12.7%～16.6%Si)的可采用工艺范围、共生区及其连续定向凝固材料的组织与性能。实验结果表明,Al-Si共晶系合金通过连续定向凝固可以获得短纤维的自生复合材料.在凝固速度为0.02～2.0mm/s的范围内,随着凝固速度的增加,Al-Si共晶系合金的共晶硅形态经历了片/纤维转变,抗拉强度及延伸率随凝固速度的增加而增大。本文还分析了Si纤维的形成机制。      

Microstructures and modification performance of melt-spun Al-10 Sr alloy

The microstructures of Al-10 Sr alloy quenched at various wheel speeds have been investigated using X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The experimental results show that the microstructures of melt-spun Al-10 Sr alloy are quite different from those of ingot-like alloy consisting of coarse plate-like Al₄Sr embedded in the eutectic matrix. For lower wheel speeds of 500 and 1000 rpm, the microstructure is composed of nanoscale particle-like Al₄Sr embedded in -Al solid solution. With wheel speeds exceeding 1500 rpm, the formation of primary Al₄Sr phase is completely suppressed and the eutectic microstructure consisting of alternate dark and bright zones is obtained. Eutectic Al₄Sr in dark zones is stripe-like and irregular blocky, while regular stripe-like in bright zones, about 10–20 nm in width. Furthermore, modification tests indicate that the modification performance of melt-spun Al-10 Sr alloy is much better than that of ingot-like alloy, reducing the amount of Sr addition and greatly shortening the incubation time. The reasons for the formation of microstructures and the improvement of modification performance of melt-spun Al-10 Sr master alloy have also been discussed.     

磁场作用下铝合金凝固过程的电阻变化

为了确定磁场对铝合金凝固过程的影响,利用双电桥测试了Al-21%Cu、Al-22Si合金分别在无磁场、直流磁场和交流磁场作用下从液态到固态的电阻变化.分析了Al-Cu合金与Al-Si合金电阻-温度曲线的差别,研究了磁场对合金凝固过程中液、固相线温度及微观结构的影响.结果表明,Al-21%Cu合金的电阻-温度曲线呈单调下降趋势,且有两个明显的转折点,而Al-22%Si合金的电阻-温度曲线不再是单调下降,两个转折点也很不明显,在直流磁场作用下,Al-21%Cu、Al-22%Si合金的液相线、固相线温度均有所降低,在交流磁场作用下,两种铝合金的液、固相线温度均有所升高.在Al-Si合金凝固过程中施加外磁场时,抑制了初生β-Si的生成.     

Structural modification of Al-Si eutectic alloy by Sr and its effect on tensile and fracture characteristics

Modification of the commercial A-S13 alloy and high purity Al-Si eutectic alloy has been successfully achieved using Al-5 Sr master alloy. The refined structure showed more fine and uniformly distributed eutectic silicon in the high purity alloy. The incidence of Al-dendrites due to modification was related to the movement of the eutectic point to the higher-silicon side. The chill-cast alloys showed better refined structure and higher mechanical properties in comparison with the sand-cast alloys. The modified chill-cast alloy exhibited the best mechanical properties. The ultimate. tensile strength of which reached 280 M Pa and the elongation was 9.2%. The fracture patterns changed from well faceted brittle appearance to smooth silky and dimple-like for the normal and modified alloys respectively. This reflects the change from brittle to ductile behaviour after modification. A model for the fracture mechanism was proposed to explain the observed higher ductility for the modified alloy.     

High strength Al–Zn–Mg–Cu–Ni–Si alloy with improved casting properties

Abstract

The casting properties of high strength Al-7Zn-7Mg-1Cu-3Ni-3Si(wt-%) alloy are described. Compared with common Al-Zn-Mg-Cu alloys, an improvement of casting properties has been achieved by adding elements (Ni, Mg, Si) that form eutectic phases, thus reducing the solidification interval of the alloy. A comparison of thermal cooling curves, castability and hot tearing tendency has been carried out for three alloys: Al-7Zn-2Mg-1Cu (structure consists mainly of solid solution), quasi-ternary eutectic alloy Al-7Zn-7Mg-1Cu-3Ni-3Si and the common casting alloy Al-10Si. In addition, the effect of melt protection against oxidation on castability has been evaluated. It is shown that the casting properties of the protected quasi-ternary eutectic alloy are significantly better than those of the common Al-7Zn-2Mg-1Cu alloy and that they achieve a level close to that of Al-10Si alloy.     

Effect of alloying elements on the segregation and dissolution of CuAl2 phase in Al-Si-Cu 319 alloys

The hypoeutectic 319 aluminum alloy (Al-7%Si-3.5%Cu) was used in the present study to investigate the effect of diverse alloying elements on the dissolution of the copper phase (CuAl₂) during solution heat treatment. Elements such as Sr, Fe and P were added to the base alloy individually and in various combinations. The cooling curves of these alloys were obtained by solidifying the alloy melts in a preheated graphite mold (600°C, cooling rate 0.8°C/s). From these the first derivate curves were plotted and used to determine the effect of the additives on the precipitation temperature of the Al-CuAl₂ eutectic reaction. Microstructural examination was carried out using optical microscopy, image analysis, and electron probe microanalysis (EPMA), with energy dispersive X-ray (EDX) and wavelength dispersive spectroscopic (WDS) analysis facilities. Samples from different alloys were solution heat treated at 505°C for various times up to 100 hours. The results explicitly reveal that solution heat treatment plays a critical role on the dissolution of the CuAl₂ phase. It is found that Sr leads to segregation of the CuAl₂ phase away from the Al-Si eutectic regions, which slows down its dissolution during solution heat treatment. The -Al₅FeSi phase platelets act as preferred precipitation sites for the copper phase and hence lessen the degree of segregation. Thus, addition of Fe can accelerate the copper phase dissolution. However, phosphorus addition has a negative effect on CuAl₂ dissolution due to (i) its solubility in the CuAl₂ phase particles, and (ii) the formation of (Al,P)O₂ oxide particles which act as nucleation sites for the precipitation of the block-like CuAl₂ phase. It retards the complete dissolution of this copper phase even after 100 hr solution treatment. In the case when phosphorus and iron are added together, the negative effect of phosphorus can be neutralized to some extent.     

Comparative study on the effect of iron and silicon addition on the microstructure and mechanical properties of aluminium-lithium powder atomized alloys

The present work was conducted on a wide series of Al-Si, Al-Li, Al-Li-Si and Al-Li-Fe alloys produced by centrifugal atomization. Solidification in Al-Si powders occurs by heterogeneous nucleation at the interface with the atmosphere, followed by propagation of the solidification front towards the particle centre. Addition of 3% Li to the Al-Si alloys shifts the ternary eutectic point to less than 4% Si. The eutectic structure is a mixture of aluminium and AlLiSi phases. For Al-2.5% Li-1.3% Fe, in addition to -Al₃Li phase precipitates, Al₃Fe (needle-like) and Al₆Fe (globular) phase precipitates are present. The volume fraction of Al₃Fe dispersoids is not affected by the presence of the grain boundaries, and is enough to strengthen the precipitation-free zones.     

半固态铝硅合金中共晶硅的形貌

利用扫描电镜研究了未变质和锶变质半固态铝硅合金中共晶硅的形貌,实验结果表明,未变质半固态合金中晶硅的形貌较常规铸态合金有较硅变化,从成簇的细长片状过渡到以粗短片状或碎块状为主,锶变质半固态合金中的共晶硅仍以细长片状为主,但有一部分为粗短片状或碎块状,上述结果与旋转磁场引起的熔体流动对凝固过程的影响有关。     

Understanding Melt Flow Behavior for Al-Si Alloys Processed Through Vertical Centrifugal Casting

The objective of this article is to investigate the appearance, microstructure, and hardness of Al-Si alloys Al-12Si and Al-17Si in vertical centrifugal casting process. During rotation of the mold, molten metal flow affects the formation of uniform cylinder. In this study, flow of molten metal for Al-Si alloys at different rotational speeds is focused. It is found that for Al-17Si alloy a uniform cast tube is observed for 1000 rpm, whereas for Al-12Si it is at 1200 rpm; above and below these speeds, irregular cast tubes are formed. Finally, fine structured grain size with high hardness value is found in a uniform cast tube in comparison with others.     

Ca在铝合金中的作用

Ca在铝合金中的作用具有两面性,一方面表现出抑制P变质、降低流动性、增加氧化物夹杂等有害作用,另一方面体现出改善Al-Si合金中β-Fe相形态、细化初晶硅、变质共晶硅等有益影响。首先综述了Ca对铝合金显微组织、铸造性能、变质作用等的影响;其次系统归纳了Ca与Si、Fe、P、Sr、Cu等元素的二元交互作用规律,其中Ca-Si、Ca-Fe、Ca-Sr、Ca-Cu等元素二元交互均表现出协同作用,而Ca-P的抑制作用可以通过添加B或C2Cl6来消除;最后,开发含Ca的二元、多元联合变质剂,深化Ca对铝合金微合金化作用机理以及再生循环过程中Ca对其他元素的影响等都是今后重点研究的思路与方向。