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.

Improvement in mechanical properties of hypereutectic Al-Si-Cu alloys through sonosolidified slurry

For the wider applications,it is necessary to improve the ductility as well as the strength and wear-resistance of hypereutectic AlSi-Cu alloys,which are typical light-weight wear-resistant materials.An increase in the amounts of primary silicon particles causes the modified wear-resistance of hypereutectic Al-Si-Cu alloys,but leads to the poor strength and ductility.It is known that dual phase steels composed of hetero-structure have succeeded in bringing contradictory mechanical properties of high strength and ductility concurrently.In order to apply the idea of hetero-structure to hypereutectic Al-Si-Cu alloys for the achievement of high strength and ductility along with wear resistance,ultrasonic irradiation of the molten metal during the solidification,which is called sono-solidification,was carried out from its molten state to just above the eutectic temperature.The sono-solidified Al-17Si-4Cu alloy is composed of hetero-structure,which are,hard primary silicon particles,soft non-equilibrium a-Al phase and the eutectic region.Rheo-casting was performed at just above the eutectic temperature with sono-solidified slurry to shape a disk specimen.After the rheo-casting with modified sonosolidified slurry held for 45 s at 570 oC,the quantitative optical microscope observation exhibits that the microstructure is composed of 18area%of hard primary silicon particles and 57area%of soft a-Al phase.In contrast,there exist only 5 area%of primary silicon particles and no a-Al phase in rheo-cast specimen with normally solidified slurry.Hence the tensile tests of T6 treated rheo-cast specimens with modified sono-solidified slurry exhibit improved strength and 5%of elongation,regardless of having more than 3 times higher amounts of primary silicon particles compared to that of rheo-cast specimen with normally solidified slurry.

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%。

The Effect of Nitrogen Gas Flow Rate on the Cr-Containing DLC (Diamond-Like Carbon) Coating by AEGD Hybrid-CVD Coating Process

The development of electric and hybrid automobiles has gained momentum with the growth of interest in the field of miniaturization of electrode materials. In particular, technologies that improve the electrical property of stainless steel, while maintaining corrosion resistance, are gaining interest in terms of maintaining specific resistivity. The study on metal doping in diamond-like carbon coating is currently in progress to enhance the characteristics of conductivity and corrosion resistance with excellent properties such as corrosion resistance and lubrication coating. It is the process of using Cr arc with DLC coating to actuate AEGD. The change of I(D)/I(G) (Graphite peak (G) and disordered bond peak (D)) ratio and G-peak position in Cr-containing DLC film causes graphitization and thus lowers the basic electric resistance. Simultaneous input of nitrogen gas leads to deposition of CrN by a specific ratio of Cr and N in the DLC coating, and the nitrogen atoms replace hydrogen in bonding to increase the sp3 bond structure in the DLC film, in which CrN is not deposited, to result in specific resistivity of a specific value or less.

The Phenomenon of High Hardness Values on the S-Phase Layer of Austenitic Stainless Steel via Screen Plasma Nitriding Process

The purpose of this study is to improve the surface properties of austenitic stainless steel using the double-folded electrode screen plasma nitriding (SPN) process. In general, the S-phase is well-known for its excellent properties such as improved hardness and wear resistance along with sustained corrosion resistance. The concentrated nitrogen via SPN process was injected to form S-phase with time at 713 K. This study was carried out under the conditions of 44 at% of nitrogen injection, which was higher than 25 at% known as the condition of no precipitation of S-phase formed by the SPN process, and 20 K higher than the maximum temperature without precipitation phase. The hardness analysis of stainless steel sample treated by the SPN process at 713 K showed a much higher value than the typical nitriding hardness at a depth of lower nitrogen than the maximum nitrogen concentration. The SPN 20 hr treated specimen showed the average value of 2339 HV while 40 hr showed the average value of 2215 HV. The result is attributed to the concentrated nitrogen formed in the SPN process reacting with the alloying elements contained in the base material to form fine precipitates, thus producing a synergy effect of the extreme hardening effect;that is, the movement of precipitates and dislocations due to the GP-zone (Guinier-Preston zone).

Trend of Nitriding on Chromium-Molybdenum Steel via Low Temperature Screen Plasma Technology

Low temperature screen plasma technology, a high plasma density, through using a low energy supply, shows excellent effects on a low alloy chromium-molybdenum steel for plastic molds because it does not show a compound layer and a high surface hardness without a deterioration in matrix hardness. For interest about hardening depth, both the screen plasma nitriding and plasma nitro-carburizing processes were tested including nitrogen, hydrogen and a methane mixed gas environmental at 653 K, 713 K. The optical emission spectroscopy (OES) has been analyzed during screen plasma nitriding (SPN) and a nitro-carburizing process (SPNC) was proceeded at 713 K and the same pressure. I find it difficult to dissociate nitrogen molecules perfectly with neutral nitrogen atoms via the DC-plasma nitriding process. Therefore, the SPN and SPNC process have shown a high density of plasma species even though low temperature plasma conditions have a high peak intensity of Hβ and Hγ in the results of the analysis by OES. The hardness value was measured with the micro-Vickers hardness tester after the SPN, SPNC process and the chemical composition of nitriding layers were traced by GDOES. The screen nitriding layer via the screen plasma technology has shown excellent properties with a thickness depth of about 850 ~ 900 HV without the deterioration of matrix hardness value.

Effect of Formation of Concentrated Stainless Steel Layer by Thermochemical Reaction and Addition of Hydrocarbon Gas on Sintered Part

As the environmental load has recently increased, the use of sintered stainless steel for automobile parts is increasing to help weight reducing, high performance and external exposure. Although the low priced pre-mixed sintered stainless steel powder parts are used instead of the high priced pre-alloyed powder parts, there have been problems of poor corrosion resistance and high price because the parts are sintered at low temperature due to the change of final part size. This paper describes the alloying process of producing parts having high hardness and corrosion resistance through expanded high concentration chromium on the surface only of sintered steel, which is relatively easier to sinter, using the pack-chromizing technology to improve hardness and corrosion resistance to solve the problem. Notable is the coating where the activated-chromium formed during the pack-chromizing process remains in the coating layer can lower the friction coefficient of the coated layer to up to 0.1. On the one hand, when the hydrocarbon gas was injected so as to promote the chromium-iron mixed carbide formed, the friction coefficient is increased to 0.4 with high hardness values. The thickness of the chromium alloying layer on the specimen can vary at the same temperature and same phase of the coating layer depending on which chromium resource materials (i.e. chromium or chromium-iron mixed powder) is used.