The mechanisms of grain refinement in dilute aluminum alloys

The contribution of the peritectic reaction in producing grain refinement in aluminum alloys has been studied in three binary aluminum systems. It appears that titanium has a unique effect compared with zirconium or chromium; there is evidence of a refinement mechanism associated with the peritectic reaction in Al?Ti alloys which was not observed in the other two systems. Additions of boron to Al?Ti and Al?Zr alloys emphasizes the difference in effect of these two transition elements as grain refiners. The significance of the present work is discussed with reference to conflicts apparent in earlier experimental data.     

The grain refinement of zinc-aluminum alloys by titanium

Abstract

The effect of small additions of titanium in the range 0?0.5 wt% on the primary α′ aluminum grain size in Zn–Al alloys containing 7 and 24 wt% Al has been examined. Marked refinement of the α′. phase was associated with the occurrence of small cubic particles in the microstructure. Electron microprobe and electron diffraction analysis showed that the particles were Al₅Ti₂Zn with a simple cubic symmetry, a = 3.99 ± 0.04Å. In the refined alloys the primary aluminum grains were “non-dendritic” and this morphology has been discussed in relation to the effectiveness of the particles as nuclei and the theory of predendritic solidification.

Additions of not less than 0.04 wt% of titanium to Zn-Al alloys containing 7 to 24 wt% Al are sufficient for complete grain refinement of the primary aluminum phase.

Résumé

Les auteurs ont étudié l'influence de faibles additions de titane (0-0.5% poids) sur la taille des grains d'aluminium (α′ primaire) des alliages Zn–Al. Un affinement marqué de la phase α′. est obtenu lorsqu'il y a présence de petites particules cubiques dans la microstructure. Des analyses à la microsonde et par diffraction électronique révèlent que ces particules sont composées de Al₅Ti₂Zn ayant une structure cubique simple (a = 3.99 ± 0.04Å). Dans les alliages á grains affmés, ces derniers sont “non?dentritiques” et cette morphologie est discutée à la lumiére de l'éfficacité des particules qui agissent comme germes et de la théorie de la solidification pré-dendritique.

Des additions d'au moins 0.04% poids de titane aux alliages Zn-Al, contenant de 7 à 24% poids d'aluminium, sont suffisantes pour obtenir un affinement complet du grain de la phase prima ire riche en aluminium.     

铝合金等通道挤压晶粒细化机制

本文讨论铝合金在等通道挤压过程中的晶粒细化机制。发生的晶粒细化主要通过三种机制完成：1)取向分裂诱发形变带;2)应变集中产生的宏观或微观剪切带;3)高角度晶界随应变增加。形变条件和路径、模具几何及材料参数决定形变组织的演化。亚结构和显微剪切带的取向与模具剪切面一致但在原则上与材料的晶体位错滑移系统无关。形变带的晶体取向倾向接近在路径A下稳定织构的取向。在高应变,由于显微组织的压缩和拉长造成的晶界面积增加成为主要晶粒细化机制。变形至一定应变后,形变进入稳态,晶粒细化不再发生。     

脉冲电磁场凝固组织细化和均质化技术研究与应用进展

由于金属凝固过程中选分结晶的作用,不可避免地会出现成分不均匀现象。连铸过程中由于强制冷却,这种成分不均匀现象更为严重。这不仅影响了铸坯和铸锭后续加工性能,并影响了最终产品质量和性能的均匀性和稳定性。研究和生产实践表明,细化凝固组织是解决成分不均匀性的有效手段。脉冲电磁场因能耗低、施加方便、细晶效果显著,近年来得到了广泛关注,有望成为冶金界广泛应用的凝固组织细化和均质化技术。分别介绍脉冲电流、脉冲磁场和脉冲磁致振荡3种脉冲电磁场凝固组织细化和均质化技术的研究现状和应用进展。     

Influence of grain refinement on hot tearing in B206 and A319 aluminum alloys

Aluminum-copper (Al-Cu) and aluminum-silicon-copper (Al-Si-Cu) alloys are among the most common aluminum casting alloys. Aluminum alloy B206 is a relatively new Al-Cu alloy with high strength and ductility at room and elevated temperatures, while A319 is an Al-Si-Cu alloy with good strength and excellent wear resistance. However, despite their advantages, when these alloys are cast via the permanent mold casting (PMC) process, they show a high susceptibility to hot tearing. Grain refinement has shown promise as a means to reducing hot tears in aluminum alloys. In this study, Ti-B grain refiner was used to investigate the effect of grain refinement on hot tearing in B206 and A319 aluminum alloys during permanent mold casting. The results suggest that Ti-B additions significantly reduced hot tearing in B206 and A319. Grain sizes were also seen to reduce significantly in both alloys with addition of Ti-B grain refiner. However, Ti-B grain refiner had a diverse effect on alloy grain morphology, as a dendritic morphology in B206 was transformed to a more globular one, while in A319, the grain structure remained dendritic.     

The mechanism of grain refinement during solidification of Zn-Ti base alloys

Through cooling curve determinations, metallography, and electron microprobe analysis the mechanism of grain refinement in Zn-Ti and Zn-Ti-Cu alloys was determined. It is shown that zinc-titanium-oxide particles in the melt act as nucleants. Through probe analysis and considerations of matching of crystallographic planes it appears that the nuclei are the spinel Zn₂TiO₄. Additions of lead to Zn-Ti melts drastically reduces the effectiveness of the nucleant.     

Study of the mechanism of grain refinement of aluminum after additions of Ti- and B-containing master alloys

A highly sensitive thermal analysis technique has been used to study the mechanisms of grain refinement in high-purity aluminum. Additions of Al-Ti-B master alloys were made both below and above the peritectic concentration in reference to the Al-rich corner of the binary Al-Ti phase diagram (0.15 pct Ti in solution). The experiments were conducted at various times after the addition of grain refiner. From the results, except for formation of TiB₂, no effect of boron on the Al-rich portion of the binary Al-Ti phase diagram can be observed. With hypoperitectic additions of Al-Ti-B master alloys, TiB₂ particles are the most frequent nucleant for aluminum grains. Also, when Al-5Ti-lB additions are made, nucleation frequently occurs above the equilibrium liquidus temperature. From a thermodynamic point of view, this phenomenon can occur only if regions of the melt (which contain bondes and nucleate new grains) have a higher Ti concentration than is present in the bulk of the liquid. A mechanism has been proposed to account for this observation. When hyperperitectic additions of grain refiner were made, a metastable formation of Al solid was often observed to occur at 2 to 5 deg above the equilibrium peritectic temperature. Other researchers have made this observation and proposed that a metastable aluminide phase was formed, even though no X-ray evidence of this phase was found. The experiments reported here show that the metastable nucleation occurs on boride particles when cooling from high temperature, which allow high (metastable) quantities of dissolved Ti to be retained in portions of the melt.     

The use of gas-generating mold coatings for the grain refinement of aluminum

Experiments are described in which an effervescent mold coating containing hexachloroethane has been used to promote the dynamic grain refinement of commercial purity aluminum ingots. The influence of the amount of hexa chloroethane in the mold coating, the degree of superheat of the cast metal and the nature of the mold material was assessed. Comparison has been made with unrefined ingots and with ingots grain refined using conventional inoculation procedures. It is concluded that low levels of hexa chloroethane (10 to 20 wt pct in the mold coating) can produce effective refinement provided conditions exist favorable for the survival of dendrite fragments or nucleating particles generated by the remelting associated with turbulence in the liquid or chemical and/or mechanical interactions between the effervescing gas and the mold coating. The effectiveness of refinement is enhanced by high rates of heat transfer and low pouring superheats. Visiting Professor in the Department of Mechanical Engineering, State University of Campinas.     

Grain refinement in magnetically stirred GTA welds of aluminum alloys

The mechanisms of grain refinement have been examined for magnetically stirred gas tungsten arc (GTA) welds completely penetrating thin sheets of several aluminum alloys. Grain refinement in unstirred welds may be brought about by adding sufficient titanium to produce heterogeneous nucleation by Ti-rich particles. In some alloys magnetic stirring is shown to extend the range of welding conditions which produce a partially equiaxed structure, and to widen the equiaxed fraction of partially equiaxed welds. This is attributed to magnetic stirring lowering the temperature gradient, allowing nucleation and growth of Al-rich grains further ahead of the columnar interface growing in from the fusion boundaries. In alloys with low Ti levels, magnetic stirring may cause refinement by sweeping grains from the partially molten zone ahead of the advancing solidification interface. This mechanism requires that the partially molten zone be sufficiently wide, and that the grain size in this zone remain small.     

Cleavage crystallography of liquid metal embrittled aluminum alloys

Failure in aluminum alloys subjected to liquid metal embrittlement (LME) typically occurs by either intergranular (IG) fracture or transgranular (TG) cleavage. In this study, the crystallography of liquid metal-induced TG cleavage in six aluminum alloys having a variety of microstructures has been determinedvia Laue X-ray back reflection. The cleavage crystallography was independent of alloy microstructure, and the cleavage plane was (100) in all cases. It was further determined that the cleavage crystallography was not influenced by alloy texture. Examination of the fracture surfaces indicated that there was not a unique direction of crack propagation. In addition, the existence of (100) cleavage on alloy 2024 fracture surfaces was inferred by comparison of secondary cleavage crack intersection geometry on the 2024 surfaces with the geometry of secondary cleavage crack intersections on the test alloys.     

Influence of convection on feathery grain formation in aluminum alloys

The influence of convection on the formation of feathery grains, i.e., of columnar grains made of twinned dendrites growing along 〈110〉 directions, in AA1050 aluminum alloys has been studied. Round billets have been semicontinuously cast in a mold equipped with lateral liquid feeding. The fluid flow pattern in the liquid sump has been modeled using Fidap software. Feathery grains have been observed in the region opposite the mold entrance, i.e., in regions where the change of the velocity field (shearing rate) is the highest. Electron back scattered diffraction (EBSD) maps of two feathery regions, which were symmetric with respect to the liquid flow pattern, showed clear symmetry relationships. Furthermore, the 〈110〉 secondary dendrite arms had grown in directions opposite to the fluid flow. This experimental evidence brings more experimental support to the mechanism of feathery grain formation proposed earlier by Henry et al.     

双源超声辅助铸造相位差对铝合金细晶效果的影响

建立双源超声铝合金铸造熔池模型,利用fluent软件模拟相同频率与不同频率下相位差对熔池声场的影响。仿真结果显示,相同频率下,相位差显著影响熔池声场的分布,随相位差增大,熔池空化域变小;不同频率下,相位差对熔池声场的分布无影响。通过不同相位差双源超声铸造试验发现,同频率振动下,相位差对双源超声铸造边部及超声辐射区的晶粒细化效果影响很小,心部晶粒细化效果随相位差变大效果变差;不同振动频率作用下,相位差对铸锭细晶效果无影响,心部晶粒细化效果与同频率相位差为90?时接近。同频率相位差0?超声作用下,铸锭心部晶粒尺寸较常规不同频率双源超声作用下心部晶粒尺寸大幅减小。     

Effect of grain refinement on the fluidity of two commercial Al-Si foundry alloys

The effect of grain refinement on the fluidity of AlSi7Mg and AlSi11Mg has been investigated by spiral tests. Two different types of grain refiners have been evaluated. An AlTi5Bl master alloy was added to different Ti contents. Since the commercial alloys had a high initial content of titanium, model alloys were made to investigate the fluidity at low grain refiner additions. Commercial alloys grain refined only by boron additions have also been investigated. The results from the fluidity measurements have been verified by measuring the dendrite coherency point of the different cast alloys. Although different, the two methods show similar trends. The spirals from each fraction grain refiner cast were subsequently investigated metallographically at the tip of the spirals and at a reference point a distance behind, but no obvious difference in structure was observed. For both alloys, an increase in fluidity is observed as the content of grain refiner increases above 0.12 pct Ti, while the fluidity is impaired with increased grain refinement below 0.12 pct Ti. The alloys grain refined with ~0.015 pct B show the highest fraction solid at dendrite coherency, the smallest grain size, and the best fluidity.     

Influence of grain refinement on some mechanical properties of non ferrous cast alloys

The influence of the grain refining method on some mechanical properties of non ferrous casting, by analyzing published data, is presented. Grain refining methods are grouped into: cooling rate control; nucleant agents addition; and mechanical agitation. Grain size decrease influence on the reported mechanical properties can be grouped into: decrease; improvement; and no observed influence. This analysis shows that there is a relationship between the grain refining method and the mechanical behavior of the studied alloys.