Microstructure and wear resistance of TiB2/Ni composite coating on pure copper surface by argon arc cladding
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摘要: 通过氩弧熔覆技术在纯铜表面制备TiB2增强 Ni 基复合涂层,以改善其耐磨性能. 将钛粉、硼粉和镍粉在球磨机中充分混合,采用氩弧熔覆技术将纯铜表面预置粉末熔化制备出陶瓷颗粒增强镍基熔覆层. 采用X射线衍射仪、扫描电子显微镜、透射电子显微镜分析涂层的物相及涂层中陶瓷颗粒相的组成、分布及结构,利用显微硬度仪和摩擦磨损试验机测试涂层的显微硬度和耐磨性能. 结果表明,熔覆层物相主要包括γ(Ni, Cu)和TiB2;陶瓷颗粒增强相弥散分布于熔覆层中,其中颗粒相TiB2以六边形存在,熔覆层内部与基体界面处均无缺陷产生;熔覆涂层具有较高的显微硬度,当(Ti+B)质量分数为10%时,涂层显微硬度高达781.3 HV,与纯铜基体对比,熔覆层显微硬度提高约11.7倍;在相同磨损条件下,随(Ti+B)质量分数的增加,熔覆涂层的摩擦系数及磨损失重先减小后增大;氩弧熔覆原位自生TiB2陶瓷颗粒增强镍基熔覆层可显著提高纯铜表面的耐磨性能.Abstract: The TiB2 reinforced Ni-based composite coating is prepared on the surface of pure copper by argon arc cladding technology to improve its wear resistance. The Ti powder, B powder and Ni powder are ball-milled and mixed. The ceramic particle reinforced nickel base coating was fabricated by melting the preset powder on the surface of pure copper by using argon arc cladding. The phase of the coating and the composition, distribution and structure of ceramic particles in the coating were analyzed by X-ray diffractometer, scanning electron microscope and transmission electron microscope. The microhardness and wear properties of the coating were tested by microhardness tester and friction and wear tester. The results showed that the phases of the cladding coating mainly include γ (Ni, Cu) and TiB2. The ceramic particle reinforced phases are uniformly dispersed in the cladding coating. However, the particle phase TiB2 exists in the form of hexagon. There is no defect in the interface between cladding layer and substrate. The cladding coating has high microhardness. When the mass fraction of (Ti+B) is 10%, the microhardness of the coating is as high as 781.3 HV. Compared with the pure copper substrate, the microhardness of the cladding coating is increased by about 11.7 times. With the increase of (Ti+B) mass fraction, the friction coefficient and wear loss of the coating decrease first and then increase under the same wear conditions, respectively. The in-situ synthesized TiB2 particles reinforced nickel base coating can significantly improve the wear resistance of the pure copper surface by argon arc cladding technology.
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Keywords:
- pure copper /
- argon arc cladding /
- TiB2/Ni /
- microhardness /
- wear resistance
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0. 序言
铜合金和铝合金因其高电导率和热导率等优良性能被广泛应用于制冷、电子、汽车等行业的生产制造中. 但由于铜价格昂贵、密度高,而铝合金含量丰富,密度较小且导电及导热性能较高,因此,采用“铝代铜”的方法可以减小成本,减轻工件重量,而铝/铜异种材料焊接铜是“铝代铜”的关键技术,具有重要的研究意义和应用价值[1].
目前,实现铝/铜异种材料焊接的方法有火焰钎焊,扩散焊,电阻焊,电弧焊,激光焊,搅拌摩擦焊等[2-10]. 通过TIG焊实现铝/铜异种材料的焊接具有操作方便,成本较低等优势. 然而,目前铝/铜异种材料TIG焊还是存在较多的问题. 由于铜、铝在焊接时极易氧化,容易在熔池接触处形成大量氧化物,同时,TIG焊热输入较高,导致金属间化合物层较厚,如AlCu,AlCu2,AlCu3,Al2Cu,Al4Cu9等这些金属间化合物的产生会严重影响接头的力学性能和服役可靠性. 但是研究人员发现,在焊接过程中外加磁场具有设备简单,成本低,且对熔池具有搅拌作用,从而达到细化组织、改善接头性能的目的[11-20]. Hicken等人[18]首先研究了外加磁场对焊接电弧的影响,指出电弧收缩并旋转的效果明显. Abralov 等人[19]研究了外加磁场对铝合金组织及性能的影响,结果表明,外加磁场对焊接过程具有电磁搅拌作用,可细化焊缝组织,改善接头的力学性能. Chen等人[20]通过铝钢外加磁场激光焊接,研究表明,熔池中洛伦兹力和热电磁力对焊缝组织影响较大,且接头抗拉强度相对无磁场明显升高.
文中采用TIG熔钎焊对铝/铜异种金属进行对接试验,对比研究有无外加纵向直流磁场下,铝铜熔钎焊接头的宏观形貌和微观组织,研究了磁场对铝/铜异种材料接头界面层组织及力学性能的影响.
1. 试验
对厚度为2 mm的T2紫铜板和2.5 mm的2A16铝合金板进行TIG对接试验,采用直径为2 mm的Zn-2%Al药芯焊丝(药芯为CsAlF4)作为填充金属. 选择型号为WSM-315B直流逆变焊机,氩气作为保护气体. 将励磁线圈固定在焊枪上,使其与焊枪同轴,使励磁线圈底部与待焊工件之间距离始终保持在5 mm,试验装置示意图如图1所示. 经过前期预试验后,选定最佳焊接工艺参数为:焊接电流I = 95 A,焊接电压U = 16 V,焊接速度v = 140 mm/min,选定直流磁场参数为:磁场强度B = 0 ~ 20 mT(每5 mT为一个递增单位). 将焊后试样用线切割机沿垂直于焊缝方向截取,采用XQ-1型热镶嵌机制作成大小为ϕ30 mm镶嵌块,对其进行打磨、抛光. 采用扫描电子显微镜(SEM,scanning electron microscope)和能谱仪(EDS,energy dispersive spectrometer)分析纵向直流磁场对接头的宏观形貌和界面金属间化合物(IMC,intermetallic compound layer)的影响,使用WDW-100型电子万能拉伸试验机对接头进行拉伸试验,拉伸速率为1 mm/min,拉伸试样尺寸如图2所示,每组参数拉伸3组试样,将得到的3组参数求其平均值,算出对应的抗拉强度.
2. 结果与讨论
2.1 组织分析
图3为铝铜TIG熔钎焊接头的宏观组织照片,由图3可知,接头主要由五部分组成,分别是铜母材、焊缝区、铝母材以及铜侧钎缝区和铝侧熔合区. 在铜侧界面处:母材上半部分较为弯曲,表明此处铜母材有部分熔化或溶解现象,下半部分较为平直,表明此处母材基本不熔化,从而在铜侧形成类钎焊接头. 在铝侧母材处可明显看出其熔化量较多,并出现明显的熔合线,在铝侧形成熔化焊接头,使得整个接头表现出熔钎焊的特征.
铜侧钎焊区的IMC层是影响接头性能的关键:一方面IMC层是钎缝界面的连接层,对接头起连接作用;另一方面IMC层由金属间化合物组成,是接头的薄弱点. 不同试验条件下铜侧IMC层微观组织照片如图4所示. 由图4可知:有无磁场下,铜侧钎焊区处的IMC层均明显有两层金属间化合物(用Ⅰ层和Ⅱ层表示),其中Ⅰ层靠近铜母材,且与铜母材衬度相近并呈平行层状,而在Ⅰ层外侧存在一层锯齿状的灰色组织(Ⅱ层). 然而,未加磁场时,IMC层整体较为平直,当外加磁场时,IMC层变得不平整. 这是因为通过磁场对电弧形态的改变以及熔池液态金属的快速运动改变温度梯度以及散热方向,加速元素扩散,导致金属间化合物的生长方向发生改变,使IMC层变得不平整. 同时,IMC层生长方向的变化调节了无磁场作用下接头纯粹的界面拉伸行为,导致应力结构由单纯变得复杂,促使界面层与焊缝之间的结合面积增加,起到了一定的“机械咬合”作用,一定程度上提升了接头的性能.
不同磁场强度下IMC层厚度变化如图5所示. 可知,随着磁场强度从0 ~ 20 mT增加,IMC层平均厚度总体呈现先降低后升高的趋势. 当B = 0时,IMC层的平均厚度为32.8 μm;当B = 10 mT时,IMC层明显变薄,平均厚度骤降至14.6 μm. 这是因为直流磁场中旋转的电弧带动熔池表面液态金属做定向回转运动,增加了对铜侧母材的冲刷面积,使IMC层厚度明显减小,在一定程度上提升接头的力学性能. 当B = 20 mT时,由于磁阻现象,电弧扩张及熔池液态金属流动情况减弱,IMC层的平均厚度逐渐增加.
图6a,6b分别是图4a,4b虚线框内区域的放大SEM照片. 表1则是对图6a,6b中各点进行的能谱分析. 由图6a可知,无磁场时,Cu部分熔化或溶解,由于二元化合物稳定性较高,出现如AlCu,Al2Cu的脆性金属间化合物,导致界面出现气孔等缺陷,使接头性能下降. A点位于Ⅰ层,其Cu,Al原子占比约为1∶1;B点和C点均位于Ⅱ层,B点的Cu,Al原子占比接近1∶2,C点主要成分为Zn和Al原子,故推测Ⅰ层为AlCu化合物,Ⅱ层为Al2Cu和Zn-Al共晶相. 与无磁场相比,在磁场作用下,IMC层中化合物的种类和厚度均发生明显变化. 当外加纵向直流磁场时,由图6b可知,A点位于Ⅰ层,由Al,Cu,Zn原子占比可推测为AlCu+CuZn,B点和C点均位于Ⅱ层,且两者的原子占比相同,则由Al,Cu,Zn的原子占比推测为Al4.2Cu3.2Zn0.7,观察可知这些Al4.2Cu3.2Zn0.7三元化合物主要由两部分组成,一部分呈层状紧邻Ⅰ层,另一部分呈树枝状向焊缝区生长,这些枝状组织细长且数量较多. 形成树枝状组织有两方面原因. 首先,在于磁场导致电弧扩张,熔池表面的温度梯度降低,使得作用在熔池表面的热量分布较为均匀,熔池在高温作用时间减少,促进了Al4.2Cu3.2Zn0.7相的生成. 其次,是因为熔池中带电流体带动液态金属做同向的回转流动. 熔池流体流速增加,对铜界面冲刷作用增强,使得更多的Cu和Zn元素溶解扩散到液态熔池中,当熔池凝固后就会在界面层处形成一定厚度的金属间化合物层以及焊缝中细长的枝状金属间化合物. 结合图4可知,相比于无磁场,当B = 10 mT时,Cu侧Ⅰ层AlCu化合物的厚度变薄,Cu侧Ⅱ层的Al2Cu消失. 表明Al4.2Cu3.2Zn0.7三元化合物的出现抑制了硬脆的AlCu和Al2Cu生长,接头性能明显改善.
2.2 抗拉强度分析
图7为不同磁场强度对接头抗拉强度的影响曲线. 由图7可知,随着磁场强度从0 ~ 20 mT增加,接头抗拉强度呈现先增加后减小的趋势. 无磁场时,接头的抗拉强度最低,为89.7 MPa. 因为IMC层的形状平直,接头只是纯粹的界面拉伸,与IMC层结合并不紧密;且此时IMC层平均厚度较厚,IMC层中的AlCu和Al2Cu都是脆性化合物. 当B = 10 mT时,接头抗拉强度最高,为110.8 MPa,与无磁场作用相比提升了约24%. 原因是磁场强度为10 mT时,磁场对铜侧母材的冲刷作用增强,使得IMC层厚度明显减小,磁场电弧的扩张和熔池流动的改变使更多的Cu和Zn元素溶解扩散到液态熔池中,当熔池凝固后就会在IMC层处形成Al4.2Cu3.2Zn0.7三元金属间化合物,它的出现抑制了硬脆的Al2Cu与AlCu生长. Al4.2Cu3.2Zn0.7三元化合物的枝状分布可以起到一定的“机械咬合”效果,提高了接头承载力,使得抗拉强度升高.
3. 结 论
(1)相比于无磁场,在纵向直流磁场的作用下,Cu侧IMC层的形状、厚度和化合物种类均发生变化:平均厚度明显变薄,由32.8 μm降至14.6 μm;形状由平直变为弯曲,起到“机械咬合”作用;Cu侧IMC层Al4.2Cu3.2Zn0.7三元化合物的出现抑制了硬脆的AlCu与Al2Cu化合物的生长,提高了接头性能.
(2)添加直流磁场后,接头的抗拉强度均比无磁场时高,且接头抗拉强度随着磁场强度的增加呈现先增大后减小的趋势. 当焊接电流I = 95 A,焊接电压U = 16 V,焊接速度v = 140 mm/min,磁场强度B = 10 mT时,接头抗拉强度最高,达到110.8 MPa,比无磁场提高了约24%.
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图 2 熔覆涂层横截面形貌
Figure 2. Cross-section morphology of cladding coating. (a) cross-section morphology of 10%(Ti+B) coating; (b) tissue morphology of identified area 1 in Fig. 2a
图 3 不同(Ti+B)质量分数熔覆涂层的SEM截面形貌
Figure 3. SEM cross-sectional morphology of cladding coatings with different mass fraction of (Ti+B). (a) 5%(Ti+B) coating; (b) 10%(Ti+B) coating; (c) 15%(Ti+B) coating; (d) tissue morphology of identified area 1 in Fig. 3c
表 1 氩弧熔覆涂层材料配比
Table 1 Argon arc cladding coating material ratio
组别 (Ti+B)质量分数w1(%) Ni质量分数w2(%) Ti粉质量m1/g B粉质量m2/g Ni粉质量m3/g 1 5.0 95.0 0.688 8 0.311 2 19.00 2 10.0 90.0 1.377 5 0.622 5 18.00 3 15.0 85.0 2.001 5 0.988 5 17.00 表 2 氩弧熔覆工艺参数
Table 2 Argon arc cladding process parameters
熔覆电流
I/A熔覆电压
U/V熔覆速度
v/(mm·min−1)氩气流量
Q/(L·min−1)氩气纯度
wp(%)150 15.5 120 12 99.99 -
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