Influence of the mesoscale porous structure features on the macroscale mechanical properties of sintered silver nanoparticles
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摘要:
为研究烧结纳米银材料细观孔隙结构对宏观力学性能的影响,首先利用高斯滤波算法和分位数切割函数生成具有不同孔隙率(0.1,0.2和0.3)的代表性微元(RVE). 通过对RVE施加周期性边界条件,获得其单轴拉伸力学性能,使用Abaqus软件建立了由烧结纳米银颗粒制成的搭接接头的宏观模型模拟剪切试验,烧结层的材料属性与预测的RVE的弹塑性应力-应变曲线保持一致. 结果表明,随着孔隙率的减小,RVE模型的弹性模量和屈服强度增大;值得注意的是,随着应变的增大,塑性变形最后阶段的应力呈现较大的下降趋势,使得材料更容易受到损伤. 通过比较宏观模型的剪切模拟,可以观察到孔隙率的变化对烧结纳米银颗粒的剪切变形有显著影响,具体而言,随着孔隙率的增加,孔隙部位更容易出现裂纹并扩展,形成多个孔隙的贯通裂纹,从而导致烧结银的抗剪强度降低.
Abstract:In order to investigate the influence of the mesoscale porous structure of sintered silver nanoparticles on the macroscale mechanical properties, representative volume elements (RVEs) with different porosities (0.1, 0.2 and 0.3) are firstly generated by using the Gaussian filtering algorithm and the cutting quantile functions. The uniaxially tensile mechanical properties of the RVEs are obtained by applying periodic boundary conditions. A macroscale model of the lap joint made of sintered silver nanoparticles is then established using Abaqus software to simulate the shear test. The material properties of the sintered layer are consistent with the predicted elastoplastic stress–strain curves of the RVE. The findings reveal that as the porosity decreases, the elastic modulus and yield strength of the RVE model increase. However, it is worth noting that the stress at the final stage of the plastic deformation demonstrates a significant decreasing trend as strain increases, rendering the material more susceptible to damage. Furthermore, through a comparison of shear simulation results of the macroscale model, it can be observed that porosity variations have a notable impact on the shear deformation of sintered silver nanoparticles. Specifically, as the porosity increases, the likelihood of crack initiation and propagation in porous regions rises. This leads to the coalescence of cracks among multiple pores, consequently resulting in the reduction of shear strength of the sintered silver nanoparticles.
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Keywords:
- sintered silver nanoparticles /
- porous structure /
- RVE /
- elastoplastic response
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0. 序言
在“双碳”目标背景下,新能源电动汽车的需求日益增加,而锂电池也作为新能源电动汽车动力电池首选材料[1-3]. 锂电池的铝极柱与铜基电路板的连接是动力电池蓄电系统制备的必要环节,如何获得低电阻、高强度等优异性能的铝铜焊接接头是实现动力电池长寿命、高可靠性的重要保障[4].
铝与铜在物理性能及化学性能方面因存在较大差异而使其焊接性很差[5-8],无法满足使用要求,多采用在铝极柱与铜电路板间搭建镍箔“桥”的方式,通过实现镍/铜、镍/铝分别连接的形式替代铝铜的直接连接. 一般而言,镍箔与铜基板的连接常采用回流焊的方式,通过锡钎料的熔化及与镍、铜界面分别形成金属间化合物的形式获得镍/铜的有效连接[9-11]. 由于回流焊在钎料熔点以上加热时间相对较长,且电路板需随焊点同时加热,要求设备容量大,存在产品生产周期长,加工不灵活, 且生产效率低等问题[12]. 激光作为一种新型热源,具有加热冷却速度快、灵活性高等优点[13-16],有望解决回流焊存在的不足.
Nishikawa等人[17]利用激光作为热源在铜基板上进行钎焊,结果表明,激光作用下界面处金属间化合物厚度极小(<1 μm),且接头的冲击可靠性优于传统回流焊接头;Lee等人[18]发现利用激光钎焊在较短时间内可获得锡钎料与铜的有效连接.
文中选用镍、铜箔片作为研究对象,采用回流焊及半导体激光钎焊技术对其进行连接,对比研究两种焊接方式下接头形貌、接头界面显微组织的形成机制,并对两种焊接接头的力学性能进行评价,从而提出利用激光钎焊替代回流焊的可行性技术.
1. 试验方法
试验材料为纯镍(22 mm × 8 mm × 0.3 mm)和纯铜(50 mm × 10 mm × 0.1 mm)箔片,焊接前利用乙醇清理母材表面的油污,再用超声波清洗后吹干待用,焊接示意图如图1所示. 回流焊采用北京七星天禹电子有限公司生产的TYR108N-C回流焊机,加热方式为红外线 + 热风对流方式,预热138 s(90 ~ 215 ℃),保温30 s(215 ~ 238 ℃),焊接时间248 s(238 ~ 250 ℃),冷却时间为180 s. 激光器为武汉锐科光纤激光技术股份有限公司生产的RFL-A2000D激光器,波长为915 nm,最大激光功率为2 000 W. 采用尺寸为10 mm × 2 mm的矩形光斑由左至右扫描,扫描速度为10 mm/s,激光钎焊功率为400 W,离焦量和焦距分别为0和300 mm. 两种焊接方式均采用Sn-3.5Ag-0.5Cu钎料,厚度为100 μm,且未使用钎剂.
为避免取样过程对焊点界面破坏,试验首先采用冷镶嵌(环氧树脂配合固化剂)方法对焊接试样进行镶嵌,然后用400号砂纸打磨至焊点,再用800号、1000号、1500号、2000号、3000号及5000号砂纸对金相样品由粗到细逐一打磨,而后使用粒度50 nm的SiO2悬浮液对试样进行纳米级抛光,以达到镜面效果,利用腐蚀液(2.5% HNO3 + 1% HCl + 96.5% C2H5OH)对焊接接头横截面进行化学腐蚀8 ~ 10 s,采用日本日立公司(HITACH)生产的SU5000扫描电子显微镜观察焊接接头的横截面形貌,采用牛津Oxford EDS X-MAX 20能谱仪对焊接接头界面组织进行成分分析,采用微型拉力测试仪测量焊接接头的最大剪切力,为了保证力学性能测试的准确性,取3次测试后的平均值.
2. 结果与讨论
2.1 接头成形及焊缝组织
图2为焊后镍/铜接头的横截面形貌. 从图中可以看出,两种焊接方式所获得的焊缝均连续,界面处成形良好,均无明显孔洞及裂纹缺陷. 由此可见,回流焊、激光钎焊在所选择工艺条件下均可实现镍-铜的有效异质连接.
利用扫描电镜对焊缝的显微组织进行观察可知,两种焊接方式下接头两侧界面均呈现明显的扩散反应层,且锡-铜侧界面厚度均明显大于锡-镍侧. 对比而言,回流焊缝中的显微组织分布均匀,而激光钎焊焊缝内的显微组织则呈非连续分布,焊缝中心处出现等轴晶状凝固态组织. EDS结果表明,回流焊缝中心处的元素组成为97.5Sn-2Cu-1.5Ni(原子分数,%),可推断是锡的固溶体. 对激光钎焊焊缝处显微组织进行EDS分析可知,焊缝组织由两部分组成,其一是由成分为2.9%Ni-50.1Sn-45.8Cu-1.2Ag(原子分数,%)组成的等轴晶状组织,可推断为是(Cu, Ni)6Sn5金属间化合物;其二则是等轴晶附近的富锡相,根据EDS结果可推断为是锡的固溶体(图3).
在回流焊过程中,锡钎料在高温下熔化,此时镍、铜箔金属元素向熔融的锡钎料中发生扩散.分析表明,铜原子在熔融钎料内的扩散速度远大于镍原子的扩散速度[19-22]. 由于回流焊的加热和冷却时间相对较长,铜原子有充分的时间发生扩散并迁移至镍箔处,因此铜原子并未在熔融锡钎料中偏聚,使得焊缝中心处形成锡的固溶体,尽管如此,固溶体内铜原子含量仍大于镍原子含量.
对于激光钎焊而言,其加热速度远远大于回流焊,尽管大量铜原子在短时加热过程中可迅速扩散至熔融钎料中,但在极短的冷却速度下不能够完全到达镍箔处,导致残留在熔融钎料中形成铜的局部偏聚,并与周边的熔融锡形成Cu-Sn金属间化合物.因凝固前端具有相似的过冷度,所形成的化合物在后续的快冷过程中呈等轴晶状长大;而对于附近的熔融锡而言,则仍形成锡的固溶体,因此激光钎焊焊缝中心的组织是由锡的固溶体和Cu-Sn金属间化合物两部分组成.
2.2 铜-锡侧界面显微组织
由2.1小节可知,焊接接头两界面处呈现不同的形貌特征,且铜-锡侧界面厚度明显大于镍-锡侧.图4给出了铜−锡侧界面区域的显微组织及EDS线扫描结果.
从图4中可以看出,回流焊铜-锡界面层厚度约为2.8 μm,其呈现扇贝状形貌[23],如图4a所示. EDS结果可知,由铜箔侧到焊缝内元素含量呈现一定的浓度梯度,即Cu元素含量逐渐降低,Sn元素含量逐渐升高,其它元素未有明显波动. 结合EDS点扫描结果,界面层的显微组织变化规律为铜固溶体→Cu3Sn→Cu6Sn5. 分析表明,Cu原子在加热阶段瞬时内形成铜的固溶体,随着Cu原子的不断扩散,与熔融锡钎料在短时间内迅速发生化学反应,由于此时距离铜箔基板最近,Cu元素含量浓度最高,导致在界面处优先形成Cu3Sn金属间化合物;随着距铜箔基板的距离增加,在Cu3Sn金属间化合物阻碍下,Cu原子的扩散速度略微减慢,且Sn元素含量也显著增加,导致在Cu3Sn金属间化合物的上面又形成Cu6Sn5化合物[24-25]. 进一步观察可发现,Cu6Sn5化合物层元素分布均匀,其层厚度可达2 μm左右.
对于激光钎焊而言,铜-锡界面厚度较回流焊相比显著提高,可达5 μm左右,且呈现柱状形貌,并垂直于铜箔基板,且向着焊缝中心长大,如图4b所示. 此外,EDS线扫描的结果中可以明显看出,尽管元素分布与回流焊有着同样的变化规律,即Cu元素从铜箔到焊缝内逐渐降低,Sn元素逐渐升高,然而元素分布曲线却非常陡峭,即元素含量在距离铜箔的任一位置都呈现明显的不同,且具有较大的浓度梯度. 据此该界面处可推断为形成了多种Cu-Sn的金属间化合物,究其原因,激光钎焊较回流焊具有快速加热和快速冷却的特征,Cu原子在加热过程中迅速扩散,迁移至熔融锡钎料中,形成金属间化合物,快速的扩散速度使得金属间化合物的类型多样化. 由于凝固前方具有较大的成分过冷和温度梯度,使得形成的化合物沿着垂直于铜箔向焊缝中心快速长大,而两侧生长则受到显著抑制,因此较回流焊相比,激光钎焊铜-锡界面处形成柱状晶组织,使得界面层厚度大于回流焊.
2.3 镍-锡侧界面显微组织
图5给出了两种焊接方式镍-锡侧界面显微组织及元素分布. 从图中可以看出,回流焊接头中界面区宽度约为3 μm(图5a). 结合EDS线扫描可知,Ni和Cu两种元素界面层处元素分布较为稳定,且距离镍基板越远,其含量逐渐降低,而Sn元素含量则逐渐增加. 在回流焊过程中,镍在界面处发生扩散,并与熔融锡发生化学反应后形成Ni3Sn4金属间化合物. 与此同时,大量Cu原子从铜箔处经过熔融锡钎料扩散迁移至镍箔界面处,并溶解至Ni3Sn4,从而形成(Cu, Ni)6Sn5金属间化合物[19-22].
相比而言,在激光钎焊接过程中,尽管铜也快速扩散至熔融钎料中,但极快的冷却速度导致铜不能充分扩散至镍箔界面处,使其在焊缝内偏聚(见小节2.1). 已扩散至镍箔界面处的Cu原子的浓度梯度较大,但仍可溶入到Ni3Sn4金属间化合物,形成(Cu,Ni)6Sn5或(Cu, Ni)3Sn化合物的混合组织.与铜-锡界面处相似,凝固前端的成分过冷和温度过冷导致显微组织向焊缝中心处呈柱状晶长大. 但与铜-锡界面处仍旧存在不同之处,(Cu, Ni)6Sn5与(Cu, Ni)3Sn4两种化合物之间呈非连续状态,其原因在于镍、锡之间反应形成Ni3Sn4造成体积收缩所导致[24].
2.4 焊接接头力学性能
图6给出了两种焊接接头的最大剪切力及宏观断口形貌. 从图中可以看出,回流焊及激光钎焊接头的断裂位置都位于铜母材处,且最大剪切力分别为369和320 N.究其原因,与回流焊相比,激光钎焊因具有较高的热输入使得界面处形成了尺寸较大的金属间化合物,可能是导致强度略微下降的原因[26]. 但在实际生产中,镍-铜接头的最大剪切力不低于100 N则为符合强度设计要求. 对于上述两种焊接方式,尽管激光钎焊接头的最大剪切力略低于回流焊,但远高于实际生产要求,因此激光钎焊既能够满足实际产线镍-铜焊接的实际生产要求,又具备可选区连接、灵活、易操作等特点,可成为替代回流焊的可行性技术之一.
3. 结论
(1) 回流焊及激光钎焊均可实现镍-铜微箔的有效连接,焊缝均匀且成形良好,无明显缺陷;回流焊焊缝中心由锡的固溶体组成,激光钎焊焊缝由锡的固溶体和等轴晶状的Cu-Sn金属间化合物混合组织组成.
(2) 回流焊在铜-锡界面处显微组织变化规律为铜固溶体→Cu3Sn→Cu6Sn5金属间化合物,且呈扇贝状形貌,而在镍-锡界面处则为连续的(Cu, Ni)6Sn5金属间化合物. 激光钎焊在铜-锡界面处形成多种Cu-Sn的金属间化合物,且呈现连续的枝晶状形貌;而在镍-锡界面处则形成(Cu, Ni)6Sn5与(Cu, Ni)3Sn4,且组织呈现非连续性.
(3) 回流焊及激光钎焊接头的最大剪切力度分别可达369和320 N,均满足实际工程应用需求,激光钎焊有望成为替代回流焊的可行性技术.
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图 7 烧结纳米银剪切变形行为模拟结果
Figure 7. Predicted shear deformation behaviour of sintered silver nanoparticles. (a) von Mises stress distribution of the sintered layer with the porosity of 0.3; (b) von Mises equivalent stress curve with time of the sintered layer with different porosities; (c) displacement–load curve at the reference point under different porosities
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