Effects of different welding modes on microstructure and mechanical properties of 316 stainless steel by wire arc additive manufacturing
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摘要:
基于冷金属过渡、冷金属过渡-脉冲以及直流脉冲三种焊接模式,开展了316不锈钢单道多层薄壁试样的电弧增材制造,并对试样进行了组织与性能的对比分析. 结果表明,三种模式下成形构件均无塌陷和宏观气孔现象,凝固组织以柱状树枝晶为主,并伴随有大量的二次枝晶、胞状晶,通过金相观察和背散射电子衍射技术发现,组织表现出强<001>//z织构,构件中段稳态区的平均枝晶间距随不同焊接模式下的热输入变化而变化:由小到大依次为CMT—CMT-P—DC-P. 通过X射线衍射和扫描电镜-能谱分析确定基体组织为γ-Fe(Cr0.19Fe0.7Ni0.11),基体间网状组织为残余δ-Fe. CMT-P模式下的构件强度最高,屈服强度达 237 MPa,抗拉强度达555 MPa,平均硬度值达209 HV0.3, DC-P模式构件的断后伸长率最高达52%.
Abstract:This study focused on the additive manufacturing of single-pass multi-layer thin-walled samples using 316 stainless steel. It explored three welding modes: cold metal transfer (CMT), cold metal transfer-pulse (CMT-P), and direct current-pulse (DC-P), and conducted a comparative analysis of the microstructure and mechanical properties of the samples. Under the three modes of formation, the resulting components exhibited no collapse or macroscopic porosity. The solidified structure consists predominantly of columnar dendritic crystals, along with a significant presence of secondary dendrites and cellular crystals. Upon conducting metallographic observation and utilizing electron back scatter diffraction (EBSD) technology, it was determined that the structure demonstrates a pronounced <001>//z texture. The average spacing of dendrites in the steady-state zone of the component exhibits variation based on the heat input across different welding modes, with the pattern being CMT < CMT-P < DC-P. The matrix structure was identified as γ-Fe(Cr0.19Fe0.7Ni0.11) using X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis, while the interstitial network structure was found to be residual δ-Fe. In CMT-P mode, the component exhibits the highest strength, characterized by a yield strength of 237 MPa, a tensile strength of 555 MPa, and an average hardness value of 209 HV0.3. In the DC-P mode, the component exhibits the highest elongation at break, reaching 52%.
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0. 序言
C/C复合材料是一种碳纤维增强碳基体的复合材料,因具有密度小、比强度高、耐高温、耐热冲击、耐腐蚀和耐摩擦性好等优异性能,在火箭发动机喷嘴喉部、飞机刹车片等航空航天领域被广泛应用. 将C/C复合材料与其他常用材料连接具有广泛的应用前景[1-4]. 然而,C/C复合材料表面润湿性较差,这势必会影响到C/C复合材料与同种材料或者异种材料的连接,导致焊后接头性能较差[5],因此,如何使钎料在C/C复合材料表面较好的润湿一直是国内外学者研究的重点[6].
Ag-Cu-Ti钎料熔点适中,具有良好的强度、韧性、导电性、导热性以及抗腐蚀性,被广泛应用于异种金属的连接[7]. Zhang 等人[8]通过化学沉积的方法探究了AgCuTi钎料在C/C复合材料上的润湿机制,结果表明,钎料中Ag可以促进C与Ti反应生成TiC.
文中通过TIG电弧直接在C/C复合材料表面加热的方式探究Ag-Cu-Ti钎料在C/C复合材料上的润湿情况,并借助SEM,EDS等分析方法,研究保温时间对Ag-Cu-Ti钎料在C/C复合材料上的润湿铺展的影响及钎料软化行为.
1. 试验方法
试验选用二维编制层叠C/C复合材料板,通过机械加工的方式截取成规格为65 mm × 40 mm × 3 mm的长方体用于后续的润湿试验. 所用钎料为Ag-Cu-Ti膏,其制备方法是根据文献资料设计的配比,如表1所示,即在Ag-Cu共晶点处加入原子分数为4.5%的活性元素Ti,制备Ag68.8Cu26.7Ti4.5合金钎料粉末,添加定量丙酮、乙二醇将其配置成膏状. 因丙酮有强烈的挥发性,导致膏状状态不能长时间保持,且铺展均匀性差. 因此,对配制方法做了改良:用制备的Ag68.8Cu26.7Ti4.5合金钎料粉末,在其中加入粘结剂使其呈膏状,粘结剂含量占总含量的8% ~ 15%. 粘结剂的牌号为Titd-YNJ01.
表 1 钎料中各组分的用量Table 1. Content of the solder粉末Ag-Cu-Ti L1/g 丙酮 L2/mL 乙二醇 L3/mL 0.23 0.1 0.09 焊接设备主要包括焊接机器人、奥太氩弧焊机、P-20A型智能温度控制仪表 (加热功率为1.6 kW,控温精度达 ± 0.8 ℃)和K型热电偶(测量0 ~ 1 300 ℃的液体蒸气和气体介质以及固体的表面温度). 此外,还需四组耐火棉、丙酮、砂纸、医用棉签等若干材料.
焊前清理是保证焊接质量不可缺少的重要措施,在焊接前,对钨极、加热平台,先用砂纸打磨其表面,然后用丙酮进一步去除氧化膜. 对于C/C复合材料,先用砂纸打磨后,再放入超声波清洗机中清洗30 min,烘干后立即施焊,避免因与空气接触过久形成新的氧化膜,且避免用手直接触碰母材待焊部位.
焊接装置示意图如图1所示,焊接前先将C/C复合材料放置于加热平台上,并将膏状Ag-Cu-Ti钎料涂在C/C复合材料表面,随后将加热平台加热至560 ℃,待温度恒定后在C/C复合材料板上引燃TIG电弧,保护气选用氩气,试验参数如表2所示. 随后通过焊接机器人将TIG电弧缓慢移动到Ag-Cu-Ti钎料正上方使其熔化铺展,待膏状钎料完全熔化铺展后熄灭电弧,且维持加热平台温度为560 ℃,利用耐火棉将工件包裹开始保温(保温时间:0,15,30,60,120 min),保温期间持续通保护气. 待保温过程结束后,将加热平台温度调至室温,使工件在加热平台上缓慢冷却至室温即可.
表 2 焊接工艺参数表Table 2. Welding parameters焊接电流 I/A 钨极距离 L/mm 冷却速度 v/(℃·min−1) 保护气流量 q/(L·min−1) 预热温度 T1/℃ 保温温度T2/℃ 100 4 3.5 20 560 560 利用MERLIN Compact–场发射扫描电子显微镜观察界面微观组织及断口形貌,并用设备自带的能谱仪进行成分测试. 将制取好的每个试样沿如图2所示通过硬度计从最高点到反应界面的最短路径进行打点,打点间隔为0.03 mm,施加载荷为0.98 N,加载时间10 s.
2. 结果与分析
2.1 宏观形貌及分析
图3为不同保温时间下膏状Ag-Cu-Ti钎料在C/C复合材料表面润湿铺展宏观形貌. 从图3可知,在保温时间为0~60 min时,膏状Ag-Cu-Ti钎料在电弧力的作用下熔化并由中心向四周流动,逐渐铺展润湿,且润湿效果较好. 但表面出现不同程度的氧化,在未保温情况下,润湿钎料被氧化较为严重,当保温时间增加至60 min时,因保护气原因,工件表面被氧化程度减弱. 但当保温时间增加至120 min时,试样表面氧化最为严重,这可能是因为即使持续通保护气,但在加热平台加热下工件仍旧被外界空气氧化. 此外,在通过线切割处理该试样时,润湿后的Ag-Cu-Ti钎料从C/C复合板上脱落,其原因可能是由于界面脆性相的生成并在界面附近大量聚集,热膨胀系数与母材相差太大,导致界面应力集中加剧,无法保证牢固结合.
图4分别为保温时间0 ~ 60 min下钎料在C/C复合材料表面润湿角变化,从图中可知,不同保温时间下润湿角均为锐角,且润湿角随保温时间先增加后减小. 当保温时间为30 min,润湿角最大,当保温时间增加至60 min时,润湿角最小,说明保温时间达到一定时刻后可促进钎料在C/C复合材料表面的润湿.
2.2 微观组织分析
图5为不同保温时间下接头微观形貌,并对图中P1,P2,P3,P4进行EDS分析,结果如表3所示. 对比不同保温时间下微观形貌,发现随着保温时间的增加,灰色区域面积增加. 除此之外,在高温下,发生C + Ti→TiC的反应. 此化合物是产生结合的必要产物,但作为一种脆性相,数量过多也会导致反应层边缘脆裂[9]. 在不保温情况下,界面处没有明显的反应层,几乎无TiC生成. 当保温时间增加到15,30和60 min时,界面处出现明显的反应层,且均匀铺展在C/C界面上,说明界面有生成TiC的可能性. 当保温时间从15 min增加至30 min时,反应层厚度由平均0.5 μm增加至平均1 μm. 当保温时间增加至60 min时,反应层非常致密且连续分布在C/C界面上,平均厚度为1.3 μm,反应层厚度出现不同程度的增加,界面分布较为均匀,反应效果最好. 结合王毅等人[10]的试验研究结果,从P4点扫描结果分析在钎料中伴随AgTi/CuTi3/Cu4Ti3等脆性化合物存在.
表 3 图5点扫描成分结果(原子分数,%)Table 3. Point scanning of Figure 5位置 Ti Ag Cu C 可能的相 P1 20.61 — — 79.39 TiC P2 5.36 9.04 85.60 — α′-Cu固溶体 P3 — 65.51 34.49 — α-Ag固溶体 P4 49.01 8.80 42.19 — AgTi/CuTi3/Cu4Ti3 图6为不同保温时间界面的线扫描结果,线扫描方向如图5中箭头所示. 由图可知,在不保温的情况下,元素扩散层厚度为4.5 μm,这可能是焊接过程和降温过程共同导致的. 在焊接过程和降温过程相同的情况下,当保温时间为15,30 和60 min时,元素扩散层分别为5.0,5.3和5.5 μm. 相对不保温情况下元素扩散层均有不同程度增加,但保温时间对元素扩散层的变化影响较小. 此外,四组线扫描数据中元素的变化趋势共同说明了Ti元素在界面附近发生富集. 因此,在聚集区TiC生成的同时伴随有Ti2Cu的产生[11].
2.3 软化性能
图7为钎料在不同保温时间下硬度分布示意图. 可以看出,保温导致钎料出现不同程度软化,这可能是因为在保温时钎料发生晶粒再结晶,使其硬度降低[12]. 除此之外,不同保温时间下钎料硬度曲线出现不同程度波动,这是因为TiC作为一种脆性相并未得到均匀分布,元素也没有得到充分扩散,造成硬度分布不均匀.
3. 结论
(1) 采用TIG电弧直接加热的方式在C/C复合材料表面润湿Ag-Cu-Ti钎料,分析焊后保温时间对润湿的影响. 当保温时间为60 min时,润湿效果最好,TiC反应层分布最均匀、致密,厚度约为1.3 μm;并且在保温时间下钎料发生软化致使钎料硬度降低.
(2) 在向反应界面接近的过程中,有AgTi/CuTi3/Cu4Ti3等脆性化合物的生成.
(3) 不同保温时间下,Ti元素均会发生不同程度扩散,当保温时间为60 min时,元素扩散层最大为5.5 μm,相对于不保温情况下仅增加了1 μm. 保温时间对元素扩散层的变化影响较小,且Ti元素在近界面处发生聚集,在聚集区伴随有Ti2Cu的产生.
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图 10 增材构件拉伸断口
Figure 10. Tensile fracture of additive components. (a) vertical macroscopic fracture morphology of CMT-P component; (b) horizonal macroscopic fracture morphology of CMT-P component; (c) CMT-P fracture morphology; (d) CMT fracture morphology (low power); (e) DC-P fracture morphology; (f) CMT fracture morphology (high power)
表 1 ER316焊丝化学成分(质量分数,%)
Table 1 Chemical composition of ER316 welding wire
C Cr Mn Si Ni Mo Fe 0.048 19.64 1.75 0.74 12.52 2.5 余量 表 2 电弧增材制造316不锈钢预设工艺参数
Table 2 Preset process parameters of 316 stainless steel by WAAM
试样 模式 焊接电流I/A 电弧电压
U/V焊接速度 v/(m·min−1) 送丝速度vs/(m·min−1) 1号 CMT-P 83 13.1 0.6 1.9 2号 CMT 92 9.6 0.6 2.0 3号 DC-P 73 17.3 0.6 2.3 表 3 构件宏观尺寸
Table 3 Macroscopic dimensions of components
试样 长度
l/cm高度
h /cm宽度
w/cm沉积的层数
n1号 15.0 7.1 0.8 60 2号 15.5 7.3 0.8 60 3号 15.0 9.2 0.8 60 -
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