中英文翻译-铅元素对AZ91镁合金形成过程的影响-精品

Effect of Pb addition on ageing behavior of AZ91 magnesium alloy

Abstract

The ageing behavior of AZ91 alloy containing 2wt.% lead is investigated. The results show that addition of Pb suppresses the discontinuous precipitation and thereby slows down the ageing kinetics. Even though the peak hardness obtained for both alloys is almost same, the peak hardness reaches after 2400min for Pb added alloy against 900min for the base alloy. The presence of Pb in solid solution might have inhibited the diffusion of Al and Mg atoms, which is essential for the formation, and growth of discontinuous precipitation in this alloy.

Keywords: AZ91 magnesium alloy, Pb addition, Ageing behavior, Microstructure,

DTA

1. Introduction

Magnesium-based alloys are increasingly used in the auto-mobile industries. The reasons for this lie principally in their improved speci?c properties compared with aluminum alloys or steels and light weight (30% less than Al, and 75% less than steels) [1]. Among the most common commercial magnesium alloys, AZ91 is endowed with one of the best combinations of castability, mechanical strength and ductility [2]. Presence of aluminum provides mechanical and casting properties as well as corrosion resistance, whereas zinc contributes to solid solution strengthening. The maximum solid solubility of alu-minum in Mg is 12.7wt.% at 437 ?C and decreasing to about 2wt.% at room temperature. Hence castings (sand and perma-nent mould castings) of this alloy are heat-treated to obtain better mechanical properties and studies are available in the lit-erature to understand the ageing behavior of this alloy [3–6].During solution treatment at 410℃, the Mg17Al12—βphase redissolves in to the Mg matrix and forms a homogenized solid solution. On subsequent ageing at lower temperature, aluminum precipitates out in two forms: i.e. discontinuous and contin-uous precipitates. Discontinuous precipitation is the cellular growth of alternating layers of phase (Mg17Al12) and near equilibrium magnesium matrix at high angle grain boundary[2,7,8].Growth of the discontinuous precipitation regions ceases relatively early in the precipitation process. Continuous pre-cipitation forms in the remaining regions of the matrix that are not already occupied by discontinuous precipitates [7].It is demonstrated that the continuous precipitates are responsi-ble for age hardening, whereas the discontinuous precipitates are detrimental to age

hardening [6]. Moreover, the discon-tinuous precipitates formed during high temperature exposure(creep) reduce the creep properties considerably [9]. Dynamic discontinuous precipitates occur at the grain boundaries from the supersaturated eutectic solid solution lead to the sliding of grain boundaries and weaken the material [9]. Hence sup-pressing this dynamic discontinuous precipitation during high temperature exposure is inevitable to improve its high temper-ature properties. It is also reported that addition of some of the trace elements like gold and rare earth suppresses the discontin-uous precipitation during ageing [10,11]. But these elements are costlier.

The preliminary study on the Pb addition to Mg–7Al has reported that addition of Pb has the tendency to suppress the discontinuous precipitation [12]. Besides, Pb is cheaper mate-rial compared to gold and rare earth. So, in this study a detailed investigation on the effect of Pb addition on the ageing behavior of gravity。 Permanent mold cast AZ91 alloy has been carried out.，Even though, the Pb vapor is hazardous to health; addition of small amount with proper care will not be serious one. The aging behavior has been examined using Brunei hardness and differential thermal analysis, combined with microstructure observation developed during heat treatment by optical microscopy and scan-ning electron microscopy. 2. Experimental 2.1. Casting

The AZ91 magnesium alloy used for this experiment was prepared by melting together the required quantities of com-mercially available pure Mg, pure Al, pure Zn and Al–10Mn master alloy. Melting was carried out in a steel crucible in the resistance furnace under proper ?ux cover. When the melt tem-perature was 700 ?C, 2wt.% pure Pb was added in to the melt.The melt was then re?ned at 720 ?C and held for 20min for the oxide particles to settle. The re?ning of the melt was car-ried out by stirring the molten melt after adding the re?ning ?ux in to the melt. The melt was poured in to a preheated(250℃) die coated rectangular metallic mold having dimen-sions 240mm×210mm×40mm. Sulphur dusting was done during pouring to avoid burning. 2.2. Heat treatment

Cylindrical samples of size 15mm diameter and 15mm height machined form the castings of the base AZ91 alloy and Pb added AZ91 alloy were solution treated at 410 ℃ for 48h in a muffled furnace. To avoid oxidation of Mg alloy samples during

solution treatment, a mixture of 20% coke mixed with 80% foundry sand bed was kept inside the oven separately to absolve the oxygen. The experimental results showed that this simple method effectively reduced the oxidation of the spec-miens. These samples were then quenched in cold water and subsequently aged at 200 ℃ up to 50 h for further investigations on the ageing kinetics. 2.3. Differential thermal analysis (DTA)

Pieces weighing 20 mg were cut from the as cast and solution the discontinuous precipitates at grain boundaries. The supper-treated samples and DTA measurements were performed using sign of Mg17Al12 formation is easily understandable with XRD SETSYS16/18, SETARAM machine. Pure Mg piece was used analysis performed on the samples. Table 1 gives the relative as a reference. The DTA experiment was carried out under argon intensity values of major representative Bragg reflection of the atmosphere with a heating rate of 10C/min. Temperature versus heat flow curve was obtained to understand the ageing behavior of these alloys. 2.4. Micro structural characterization

The machined samples were initially ground using different grades of polishing paper, and then polished with 0.25m diamond paste. Polished samples were etched by an etchant (solution of 5 ml acetic acid, 10ml H2O, 6g picric acid and 100 ml ethanol). Optical microscopic studies were carried out using Leica DMRX Microscope with Q-Win image Analysis software. The distribution of Pb in the alloy was examined in a JEOL, JSE 35c scanning electron microscope(SEM) attached with an energy dispersive spec-stereoscope (EDS).

铅元素对AZ91镁合金形成过程的影响

摘要

据调查，AZ91镁合金在形成的过程中至少包含2%的微量元素。该结果显示：铅物质能抑制化学过程的急剧进行，从而降低化学反应的速度，尽管在同一时间内获得两种合金元素是非常难的。我们只能在400分钟时间内获得铅元素，900分钟时间内获得基本合金。固体铅的存在能抑制铅镁原子的扩散，它的形成是有必要的并且可以抑制反应的急剧形成。

关键词：AZ91镁合金，铅元素，化学反应，微型机构，数学分析

1前言

目前，镁合金大量应用于自动控制工业中。多半原因是他们制定了具体的改进方案，这种产品比铅箔或钢制品的质量要轻(比铅轻30%，比钢轻75%)。在这许多普通商业的镁合金中，AZ91被认为是最好的化合物，因为它有较强的机械性能及韧性。铅箔的出现使机械和铸造产品具有抗腐蚀性能，同时锌的加入使其强度大大得到加强。固体中含量最大的为镁元素，在437℃时可达到总量的12.7%；在室温下也可以增加2%左右。因此，这种合金的铸件被经过高温加热处理后将研究出机械性能更好、有宝贵价值的资料供人们来参考。经过加热到410℃形成Mg17Al12—β，我们需要重新解决镁铸模可以形成同种固体的问题。目前进行的镁合金的化学镀镍方法主要有：浸锌法和直接化学镀法。浸锌法是在含焦磷酸盐的锌盐溶液中浸渍后,经氰化镀铜打底,然后进行化学镀。直接化学镀法的预处理包括:碱性除油、酸性浸蚀和活化阶段。酸性浸蚀一般采用铬酸盐酸洗;活化采用HF溶液。碱洗前处理是为了镁合金表面的油脂。在碱性溶液中,镁合金表面会形成氧化物或氢氧化物,酸洗的目的就是为了去除在镁合金表面形成的疏松的膜。氢氟酸活化是为了在镁合金表面形成比较致密的氟化镁薄膜,形成均一电势的表面[9]。因此，在高温高压下这种剧烈的反应是不可避免的，可以想办法改善它高温下的产物。它也说明了许多要素的总的特点，像黄金以及一些稀有金属也是在间接反应的过程中形成，不过它们是比较昂贵的。

据报道，已经初步研究出铅作用于AZ91镁合金后，铅元素倾向于抑制间接反应，另外铅与黄金和稀有金属相比比较便宜，因此一个详细的论述表明铅加入反应后的重要性。如果长时间的使用AZ91合金，甚至铅可以危机到人的身心健康，不过少量的使用可能没有那么严重，这个试验的过程正在被调查研究。经过困难微型分析，结合显微组织进一步观察热处理的过程中依靠光学显微镜和电子

显示仪器。

2 试验材料和方法

2.1 试验材料

施镀基体材料为AZ91镁合金试片,其化学成分为: 0. 2% ~0. 6% Si、0. 45% ~0. 9%Mg、0. 35% Fe、0. 1% Cu,余量为镁，反应温度为700℃。所采用的主要药剂为:氢氧化钠、硝酸、氧化锌、酒石酸钾钠、三氯化铁、硫酸镍、次亚磷酸钠、焦磷酸钠、柠檬酸三钠、氯化铵、硼酸、三乙醇胺等,均为分析纯。 2.2 热处理

首先将AZ91基础合金和AZ91镁合金放入到15mm×15mm圆柱形的、温度达到410℃的密闭容器里进行加热48小时。为了避免镁合金样本在加热时溶解，可采用加入20%的焦炭和80%的铸砂混合物，这样可以避免氧元素与镁发生反应。试验结果显示这种简单的发法可以明显的减少对实验不利的氧化作用。 2.3 数学分析

将一块20mg的反应物投入到反应熔炉中观察间接反应的情况。需加热的样本结合数据分析在XRD SETSYS16/18, SETARAM机器的帮助下将很快形成产物Mg17Al12。纯净的镁片用来在样本上做实验。 下面给出表1仅作为参考。在氩元素的作用下进行了DAT实验，并且很有研究价值。并且温度曲线的变化对化学反应的进行有一定的影响。 2.4 显微结构显示

在不同挤压比下对AZ91镁合金铸锭进行了热挤压,然后对部分热挤压变形AZ91镁合金进行了人工时效处理,通过显微组织观察及力学行能测试,探讨了热挤压及后续人工时效处理所引起的AZ91镁合金的显微组织演化,确定了经过不同挤压比热挤压及人工时效处理的AZ91镁合金在不同试验温度状态下的抗拉强度σ_b、屈服强度σ_(0.2)及断裂延伸率δ,并利用扫描电子显微镜对拉伸断口形貌进行了观察,进而确定了热挤压变形AZ91镁合金的拉伸断裂特征。