Semisolid casting with ultrasonically melt-treated billets of Al-7mass%Si alloys

The demand for high performance cast aluminum alloy components is often disturbed by increasing impurity elements,such as iron accumulated from recycled scraps.It is strongly required that coarse plate-like iron compound of β-Al5FeSi turns into harmless form without the need for applying refining additives or expensive virgin ingots.The microstructural modification of Al-7mass%Si alloy billets with different iron contents was examined by applying ultrasonic vibration during the solidification.Ultrasonically melt-treated billets were thixocast right after induction heating up to the semisolid temperature of 583 oC,the microstructure and tensile properties were evaluated in the thixocast components.Globular primary α-Al is required to fill up a thin cavity in thixocasting,so that the microstructural modification by ultrasonic melt-treatment was firstly confirmed in the billets.With ultrasonic melt-treatment in the temperature range of 630 oC to 605 oC,the primary α-Al transforms itself from dendrite into fine globular in morphology.The coarse plate-like β-Al5FeSi compound becomes markedly finer compared with those in non-treated billets.Semisolid soaking up to 583 oC,does not appreciably affect the size of β-Al5FeSi compounds;however,it affects the solid primary α-Al morphology to be more globular,which is convenient for thixocasting.After thixocasting with preheated billets,eutectic silicon plates are extremely refined due to the rapid solidification arising from low casting temperature.The tensile strength of thixocast samples with different iron contents does not change much even at 2mass% of iron,when thixocast with ultrasonically melt-treated billets.However,thixocast Al-7mass%Si-2mass%Fe alloy with non-treated billets exhibits an inferior strength of 80MPa,compared with 180MPa with ultrasonically melt-treated billets.The elongation is also improved by about a factor of two in thixocastings with ultrasonically melt-treated billets for all iron contents of Al-7mass%Si alloys,for example,the elongation of 11% in thixocast of Al-7mass%Si-0.5mass%Fe alloy with ultrasonically melt-treated billets,5% in that with non-treated billets.

超声熔体处理Al-7%Si合金坯料的半固态铸造

由于从循环利用的废料中不断累积增加Fe元素等杂质元素,所以铸造铝合金零部件高性能的要求往往很难实现。要求合金中粗大板条状的含铁化合物β-Al5FeSi在不使用变质剂和昂贵的纯铝锭条件下转变成无害的形貌结构。在凝固过程中通过利用超声振动来检测分析其对具有不同铁含量的Al-7%Si合金坯料的显微组织的变质效果。在感应加热至半固态温度583℃后,立即对超声熔体处理的坯料进行触变铸造,然后对触变铸造的铸件进行微观组织和拉伸性能检测分析。在触变铸造时,要求球形的初生α-Al相充填合金组织中的微小孔洞中,以便首先确定在坯料中通过超声熔体处理使合金显微组织产生变质效果。在605~630℃温度范围内进行超声熔体处理时,初生α-Al相从树枝状晶转变为细小的圆球形形貌。与未进行超声熔体处理的坯料相比,坯料超声熔体处理后合金中的粗大板条状β-Al5FeSi化合物相变得明显细小。升至583℃后进行半固态铸造不会明显改变β-Al5FeSi化合物的尺寸大小,可是却会使初生α-Al相的形貌变得更圆整,这有利于触变铸造。经预先加热的坯料进行触变铸造后,由于低的铸造温度,产生了快速凝固效果,使得合金中的共晶硅层片变得特别细小。在对超声熔体处理的坯料进行触变铸造时,具有不同铁含量的触变试样的抗拉强度变化并不大,即使合金含铁量达到2%。然而,用未处理的Al-7%Si-2%Fe合金坯料进行触变铸造时,其强度较差,仅为80 MPa,而用超声熔体处理的同种合金坯料,其抗拉强度达180 MPa。用超声熔体处理的不同含铁量的Al-7%Si合金坯料进行触变铸造时,其伸长率也提高了约一倍,例如触变铸造超声熔体处理的Al-7%Si-0.5%Fe合金坯料时,试样的伸长率为11%,而未进行超声熔体处理的同种合金触变铸造试件,其伸长率只有5%。