金属凝固组织的细化方法和机理-脉冲电场

1、 脉冲电流在脉冲电流在细化细化金属凝固金属凝固组织中的作用组织中的作用12级材料加工二级材料加工二梁柱元梁柱元1.前言前言2.研究动态及现状研究动态及现状3.作用机理作用机理4.结语结语 晶粒细化是控制金属材料组织的最重要、最基本的方法,其不但能提高材料的强度和塑性, 还能显著提高其力学性能。而把脉冲电流引入其中，是凝固控制技术的重要进展。脉冲电流能有效地细化金属凝固组织，而且环保、经济、潜力巨大，已成为当前研究的热点,受到研究者们的广泛关注。脉冲电流，即是方向不变,强度随时间周期性改变的电流,也叫脉动电流。 1984 年，印度学者A.K.Misra在三元合金Pb -Sb - Sn 凝固过程中

2、施加直流电。结果发现凝固后的组织得到了细化。从此开创了电流控制细化凝固组织的新技术。1990 年，M.Nakada 等首次使用脉冲电流作用于Sn - Pb 合金的凝固过程，发现凝固组织从粗大的树枝状组织变为等轴晶组织，而且电压越高等轴晶组织比例越大。 Barnak等对Sn60%-Pb40%合金进行了施加电脉冲实验。 其研究结果表明, 随着电流密度的增加和频率的减小，共晶团尺寸也大幅减小。 Barnak.J.P等人的研究还表明随电流密度增加,过冷度T也随之增加（如右图所示）。 脉冲电流对金属凝固组织的研究是从低温合金逐渐向高温合金过渡的。早期研究中，主要集中在Pb - Sn 合金。 而M.Nak

3、ada 等在使用脉冲电流研究Sn- 15%Pb 合金的凝固过程中，发现凝固组织不再出现柱状晶，而变成球状等轴晶。 近年来，研究人员将脉冲电流作用于凝固温度较高的纯Al、Al - Si、Al - Cu 合金及碳钢、铸铁、不锈钢等高温合金的凝固过程。 上海大学翟启杰教授领导的课题组在对纯Al 施加脉冲电流处理后，凝固组织发生显著细化（如下图所示）。 用脉冲电流处理凝固温度不同的各种合金，发现金属或合金凝固温度越高，要获得相同的晶粒细化效果就需要更高的脉冲电流参数。 目前，对于脉冲电流细化金属凝固组织的机制，众多研究者的见解存在较大差异。箍缩效应焦耳热效应结晶潜热释放理论原子团簇理论当前主要的观点

4、当金属熔体中有电脉冲通过时, 脉冲电场与其自身变化所产生的磁场会对金属熔体产生洛伦兹力，大小为 F=Bj，方向指向熔体的中心。这个力会对熔体产生反复的压缩和舒张。 同时,由于B与j在熔体内部不同位置而有所不同,因而在金属内部会产生收缩力梯度,各单位流团之间会产生流速差。 依据牛顿第二定律 ,由于流速差的存在, 在熔体内部产生剪切力。 这种运动的作用：这种运动的作用： （1）抑制了金属熔体中型壁处晶核的长大,甚至可能在熔体与型壁处形成一个间隙层。 （2）使熔体中已形成的枝晶碎裂。 （3）使熔体迅速失去过热,增加过冷度,提高形核率,故而得到细化的凝固组织。 熔融金属仍是一个电阻率为e的导体,当电流

5、通过时,电子与原子晶格发生碰撞,产生热能,即焦耳热效应Q=ej2,从而导致熔体温度变化:T=j2e/c 式中熔体密度; c比热容; 通电时间 产生两大作用：产生两大作用： （1）对凝固体系来讲,焦耳热相当于内热源,它使熔体整体冷速下降,过冷度减小。 （2）对一个固液共存的凝固体系,固相金属的电阻率远小于同材料液体电阻率,故电流优先通过固相体系,因而固相体系产生的焦耳热效应远大于液相,这就可能导致固相重熔现象发生,降低了固液界面温度梯度,从而导致凝固体体系接近同时凝固,结晶出比较均匀的凝固组织。 在合金凝固过程中通入电脉冲,和焦耳热效应有着类似效果,促使了结晶潜热的释放,理论依据为: H=H0-

6、Ht+H1 H合金液最终热能; H0凝固开始时的热能; Ht 冷却过程中传出的热能; H1 结晶过程释放的结晶潜热。 分析：分析： H1的增加,将导致合金热能的增加,且金属熔液在电脉冲处理时释放的结晶潜热量大于未加电脉冲时的潜热释放量,从而使冷速下降,导致形核率增大,最终组织细化。 但这一理论是在假设Ht 不变条件下讨论的,并未涉及到因电脉冲产生的起伏效应而引起的Ht的变化,以及H1与Ht的定性、定量关系。故这一理论还需深入探讨。 初步研究表明:在液相线以上温度较低的熔体中，存在许多类固相原子团簇,随温度升高,熔体中类固相原子团簇不断熔解和分散,最后熔体的结构将变成没有团簇的无序均匀结构。 当

7、脉冲电流作用于低温熔体时,所产生的瞬时电磁力也将作用于熔体的原子团簇,当电感应磁力足够大时,一方面能破坏原子团簇尺寸,增加团簇数量,另一方面阻碍了小团簇的电迁移效应而引起团簇凝聚。 当熔体温度开始降低时,融体中被破坏,尺寸更小的团簇有可能作为凝固结晶核心形核长大,这样导致最终凝固组织的细化。根据团簇理论,随团簇尺寸的变化,团簇的物理和化学性质随包含的原子数目而变化,这将最终导致试样一些性能的变化。 脉冲电流处理对于细化金属凝固组织有着显著作用。虽然其机理还有待深入研究,当前该技术还难以直接应用于生产,但这一技术具有广阔的前景和巨大的潜力,在可以预测的 将来,随着研究的不断进 行,脉冲电流处理技

8、术定 能得到深入发展,广泛应用,造福人类。 Grain refinement of pure Al with different electric current pulse modes Jianhong Ma, Jie Li, Yulai Gao, Qijie Zhai In the previous researches, the methods to apply ECP could be classified into two types. two electrodes were inserted into the top surface of the melt; the electro

9、des were placed at each end of the mould. In this paper, the solidification structure refinement of pure Al with ECP at different locations in the mould was investigated. To understand the grain refinement mechanism during solidification by applying ECP at other positions in the mould. To find the b

10、est match to improve the refining efficiency. IIIIIIIV 1 pulse generator; 2 digital oscillograph; 3 copper strip; 4 electrodes; 5 sand mould; 6 sample. Pure Al (99.7 wt.%) was melted in an electric resistance furnace and heated to 1113 K.After holding at 1113 K for 30 min, the molten metal was poure

11、d into two sand moulds at the same time. one sample was treated with an ECP while the other was untreated. The ECP treatment was continued for 3.5 min. The cooled samples were cut longitudinally for metallographic examination To evaluate the refining efficiency by different ECP modes,the area fracti

12、on of fine grains in all slices were measured. As shown in the picture, the boundaries of the fine equiaxed zone and the whole longitudinal section were sketched firstly, then accounting their area S1 and S0 by software. As a result, the area fraction P could be obtained(P = S1/S0).The picture showe

13、d the typical solidification structure of pure Al, i.e. the columnar crystal dominant in the samples without exerting ECP. The solidification structures after applying ECP at different modes were presented in the picture. It is clear that thesolidification structure of each sample can be divided int

14、o equiaxed and columnar grainregion. With Mode I and Mode II, the upper part of the samples was mainly consists of large columnar crystals, yet the lower part of the samples obtained fine equiaxed grains,and the area fraction of fine grains was about 38.3% and 51.8% respectively. In the case of Mode

15、 III, the area of coarse columnar crystal tends to be reduced while the region of the equiaxed grains expanded more widely than that with Modes I and II. The area fraction of fine grains in Mode III was achieved to 65.6%. With Mode IV, the region of the equiaxed grains expanded further, and there ar

16、e some fine equiaxed grains even near the top surface. The highest proportion (82%) of fine equiaxed grain area was obtained in Mode IV. Namely the electromagnetic force of ECP continuously make the heterogeneous nuclei formed on the mould wall fall off and move into the liquid metal, leading to the

17、 multiplication of nuclei and resulting the solidification structure refinement . However, the nuclei multiplication source should be different due to various ECP modes . A s shown in the picture, there would be different ECP affected region that decided the nuclei multiplication source. With Mode I

18、V, the highest proportion of equiaxed grain area was achieved to 82%. It suggested that the new crystal nuclei were dissociated from the whole mould wall. Thus it generated more new nuclei than that with modes I II III. Moreover, the ECP can generate Joule heat during solidification process, and such heat effect will delay the formation of solidification shell and therefore promote the nucleation on the mould wall. With Mo