Q235钢扫描激光热丝焊接工艺特性与组织性能研究
Laser welding has the characteristics of a high energy density, low heat input, and high welding efficiency; however, conventional laser welding has a small focused spot and high requirements for the welding assembly gap. To solve this problem, scholars have developed laser wire filling welding technology. Based on this, some scholars have developed laser hot wire welding technology, which can effectively improve the absorptivity of the welding wire by preheating the welding wire in advance, reduce the requirements for laser power, and improve the welding speed; however, there are still problems such as high requirements for the alignment of the laser focus and the tip of the welding wire, and an uneven weld height. In this study, the process characteristics of Q235 steel by scanning laser hot wire welding are systematically studied, and the mechanism of the influence of the scanning laser on the solidification process of weld metal is clarified, which provides technical guidance for expanding the industrial application of laser welding.
The base material used in this study is the Q235 steel plate. The size is 50 mm×120 mm×2 mm, and the structure is massive ferrite at normal temperature. The flat surfacing welding method is used in the research on the weld surface and section forming. The docking method is adopted in the study of the microstructure and properties of welded joints. According to the previous research and accumulation of this research group, the fixed wire feeding method is front wire feeding, the tilt angle of the welding torch is 45°, and the laser focus is located on the surface of the plate, that is, the defocus quantity is 0 mm. In the welding process, the shielding gas is argon with purity (volume fraction) greater than 99.99%. The gas pipe angle is 60° and the gas flow rate is approximately 20 L/min. In the butt welding experiment, the fixed laser power is 1.8 kW, the welding speed is 1.0 m/min, the preheating current of laser cold wire welding is 0 A, the preheating current of laser hot wire welding is 100 A, the scanning amplitude ranges from 0.6 mm to 1.0 mm, and the scanning frequency ranges from 100 Hz to 200 Hz.
Under different scanning parameters, the distribution of the laser energy is different, which affects the temperature field distribution of the weld pool, and then affects the macro forming, microstructure, and properties of the weld. Compared with that in non-scanning laser hot wire welding, the weld forming in scanning laser hot wire welding is smoother and straighter, and the splash is less (Fig. 5). The weld structure in non-scanning laser hot wire welding is dominated by thick side lath ferrite. Because the scanning laser enhances the flow of the molten pool through the stirring effect and breaks the coarse columnar crystals, the weld structure in scanning laser hot wire welding is dominated by fine crystalline ferrite and acicular ferrite with finer grains (Fig. 7). The tensile strength (578.8 MPa) of the scanning laser hot wire welded joint is basically the same as that (574.7 MPa) of the non- scanning laser hot wire welded joint, but the elongation is increased from 8.4% to 13.1% (Table 3). The dimple size of the tensile fracture surface of the scanning laser hot wire welded joint is more uniform, and the dimple size difference between the laser hot wire welded joint and the laser cold wire welded joint is larger; moreover, there is obvious inclusion precipitation at the bottom of the dimple, indicating that the scanning laser improves the homogeneity of the weld structure (Fig. 10). Simultaneously, the scanning laser improves the gap tolerance during butt welding. In the butt welding experiment of the Q235 steel plate with a thickness of 2 mm, the scanning laser hot wire welding ensures good weld formation without defects when the gap is 1.3 mm (Fig. 12).
In the experiment of scanning laser hot wire welding, by optimizing the process parameters, when the scanning diameter is 0.4?1.0 mm and the scanning frequency is 50?200 Hz, the welds obtained are well formed, smooth, no defects and nearly no splash, which proves that the scanning laser has a good improvement effect on the weld formation. Simultaneously, the scanning laser improves the gap tolerance of laser hot wire welding, which is conducive to achieve stable welding when the gap is uneven and obtain a weld with good fusion with the base metal side wall and no surface collapse. At the microstructure level, the stirring effect of the scanning laser on the weld pool can promote the flow of the weld pool and refine the grain. In terms of mechanical properties, compared with that in non- scanning laser hot wire welding, when the tensile strength is basically unchanged, the fracture elongation increases to 13.1% in scanning laser hot wire welding, indicating that the addition of the scanning laser can effectively improve the toughness of the weld, which is also proved by the deeper dimples in the electron microscope image of the fracture. The hardness of the fusion zone in the laser hot wire welding is the highest, followed by that of the heat affected zone, whereas the hardness of the base metal is the lowest. The hardness of the fusion zone in the scanning laser hot wire welding is lower than that in the non- scanning laser hot wire welding, mainly because the fusion zone in the non- scanning laser hot wire welding is easy to produce segregation, and the generated inclusions increase the microhardness.
1 引言
激光焊接具有热变形小、自动化程度高、加工效率高等优点,在汽车、船舶、微电子等领域中有着广泛的应用[1-3]。传统激光自熔焊接利用激光加热熔化基材以实现连接,由于聚焦光斑小(直径通常小于0.25 mm),其对焊件装配间隙的要求非常高。此外,高能激光束也会导致合金元素的蒸发烧损,并产生咬边、气孔和裂纹等缺陷,从而影响力学性能[4]。为了解决这些问题,研究者提出了激光填丝焊接技术。它不仅降低了对焊件的装配精度要求,而且通过填充材料,优化焊缝组织成分,提高力学性能。余阳春[5]采用激光填丝焊工艺焊接2.0 mm厚的铝合金板,即使间隙增至1.0 mm,焊缝仍成形良好、无塌陷。Li等[6]通过在7075高强铝合金的激光热丝焊中添加Sc粉,降低了焊缝气孔率,提高了接头的显微硬度、抗拉强度和伸长率。方乃文等[7]对比研究了TC3实心焊丝和Ti-Al-V-Mo药芯焊丝对TC4钛合金激光填丝焊接组织性能的影响,结果显示,与实心焊丝相比,药芯焊丝添加的元素没有导致焊缝偏析,焊缝组织晶粒更加细小,位错密度更高,焊接接头的抗拉强度、断后伸长率和显微硬度均较高。
在通常情况下,激光填丝焊接使用常温焊丝,也称激光冷丝焊。焊接时部分激光能量用于熔化焊丝,为了对熔池进行能量补偿,需要降低焊接速度[8],这在一定程度上牺牲了效率优势。因此,研究者利用电流对焊丝进行提前预热,即激光热丝焊。对焊丝进行预热不仅可以增大焊丝对激光的吸收率,而且焊丝预热后熔化所需的激光能量大大减少,提高了填充效率[9]。Marumoto等[10]采用激光热丝焊接技术在5.7 m/min的高焊接速度下,获得了具有足够熔深且没有成形缺陷的JIS500钢板角焊缝。Wei等[11]针对FV520马氏体时效钢激光热丝堆焊建立了一个全面的多相模型,模型预测的焊缝形貌和实际焊接结果吻合良好,并分析了焊接过程中的马兰戈尼流动行为和温度场演变规律。Liu等[12]采用有限元方法分析了激光热丝焊接温度场对热致残余应力的影响,结果表明,当热丝电压降低时,残余应力特别是横向残余应力明显减小。郑世卿等[13]通过数学推导给出了激光热丝焊接中送入熔池的焊丝的温度表达式,并通过计算和高速摄影方法发现,当送入焊丝温度接近焊丝熔点时,焊丝的熔化过渡行为最稳定。然而,激光热丝焊接对焊丝尖端和激光焦点位置的对中精度要求较高,存在焊道弯曲和焊缝高度不均匀等问题[14]。因此,研究者又引入了激光束振荡扫描热丝焊接,降低了系统对光丝对中位置的要求,利用激光束高频扫描搅拌熔池,改变了熔池的温度梯度和凝固结晶过程,进而改善了焊缝表面成形以及微观组织结构[15]。Wu等[16]对比研究了不同类型的扫描模式对HLSA钢激光焊接成形和气孔的影响,发现圆形扫描时焊缝气孔率最低,能量分布更加均匀,最大功率密度仅为无扫描时的13%。陈根余等[17]采用扫描激光填丝焊接技术对2 mm厚的2060铝锂合金进行了实验研究,实验结果表明,焊接过程稳定,焊缝气孔得到有效抑制。Li等[18]采用振荡激光热丝焊接的方法对20 mm厚的316L不锈钢进行了垂直焊接实验,结果表明,光束振荡促进了焊缝金属的润湿行为,减少了垂直焊接过程中的气孔缺陷。但是,目前关于扫描激光对焊缝显微组织和性能的影响研究还极其有限,尚无法为工业应用提供理论支撑。
本文以工业中常用的Q235低碳钢为研究对象,开展了扫描激光热丝焊接工艺研究。通过与常规激光热丝焊进行对比,研究了扫描参数对焊缝成形、缺陷和组织性能的影响,获取了扫描激光热丝焊接的优化工艺参数窗口,明晰了扫描激光对焊缝金属凝固过程的影响机制,为激光焊接应用范围的扩展提供了指导。
2 实验材料、设备及方法
2.1 实验材料
实验所用的基材为Q235碳素结构钢,其中,平板堆焊基板的厚度为3.5 mm,间隙对接基板的尺寸为50 mm×120 mm×2 mm。填充材料选用直径为1.0 mm的50C6实芯焊丝。基材和焊丝的化学成分如
表 1. 基材和焊丝的化学成分(质量分数,%)
Table 1. Chemical compositions of base metal and welding wire (mass fraction, %)
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2.2 实验装置及方法
如
优化的工艺布置和实验方法如
表 2. 焊接实验参数
Table 2. Experimental parameters for welding
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2.3 焊接接头组织分析
采用线切割机截取不同参数下的中部焊缝横截面,经过镶样、磨样、抛光后,使用质量分数为4%的硝酸乙醇对样品进行腐蚀,腐蚀时间为6~8 s。将样品烘干后使用扫描电镜(SEM)对金相样品进行观察,并使用其自带的能谱仪对焊缝组织进行元素分析。
2.4 焊接接头力学性能测试
采用硬度计对焊缝横截面进行硬度测试,测量位置位于焊缝横截面中心线及距离中心线上下0.2 mm的位置,如
图 4. 拉伸试样示意图。(a)取样示意图;(b)拉伸试样尺寸示意图
Fig. 4. Schematics of tensile specimen. (a) Sampling diagram; (b) diagram of tensile specimen size
3 实验结果与讨论
3.1 焊缝宏观形貌
如
图 5. 不同扫描参数下的焊缝表面与截面形貌
Fig. 5. Weld surface and cross section morphologies under different scanning parameters
保持扫描振幅A=0.6 mm不变,增大扫描频率,对比
3.2 微观组织分析
图 7. 焊缝中心的微观组织。(a)加入扫描激光前;(b)加入扫描激光后
Fig. 7. Microstructures of weld centers. (a) Before adding scanning laser; (b) after adding scanning laser
图 9. 扫描激光热丝焊接热影响区的微观组织。(a)粗晶区;(b)细晶区
Fig. 9. Microstructure of heat affected zone in scanning laser hot wire welding. (a) Coarse-grained region ;(b) fine crystal region
3.3 焊接接头的力学性能及分析
3.3.1 拉伸性能
焊接接头的拉伸实验结果如
表 3. 不同焊接方法得到的焊接接头的拉伸实验结果
Table 3. Tensile test results of welded joints obtained by different welding methods
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图 10. 拉伸试样的断口形貌。(a)激光冷丝焊接接头;(b)激光热丝焊接接头;(c)扫描激光热丝焊接接头
Fig. 10. Fracture morphologies of tensile specimens. (a) Laser cold wire welded joints; (b) laser hot wire welded joints; (c) scanning laser hot wire welded joints
3.3.2 接头显微硬度
图 11. 焊接接头的硬度分布。(a)激光热丝焊接;(b)扫描激光热丝焊接
Fig. 11. Hardness distributions of welded joints. (a) Laser hot wire welding; (b) scanning laser hot wire welding
3.4 扫描激光热丝焊接的间隙容忍度
为了降低激光填丝焊接技术对工件装夹精度的要求,本文还研究了不同参数下激光填丝焊的对接间隙容忍度,实验结果如
图 12. 不同对接间隙下的焊缝截面形貌
Fig. 12. Appearances of weld sections under different butt clearances
4 结论
在扫描激光热丝焊接实验中优化了工艺参数,当扫描直径为0.4~1.0 mm、扫描频率为50~200 Hz时,所得到的焊缝均成形良好,焊缝平整无缺陷,且几乎无飞溅,证明了扫描激光对焊缝成形具有良好的改善作用。同时,扫描激光提高了激光热丝焊接的间隙容忍度,有利于在间隙不均匀时实现稳定焊接,得到与母材侧壁熔合良好、表面无塌陷的焊缝。扫描激光对焊接熔池的搅拌作用能够促进熔池流动,细化晶粒,扫描激光热丝焊接焊缝熔合区组织主要为细晶铁素体和针状铁素体。相比之下,激光热丝焊接焊缝熔合区组织主要为较粗大的板条铁素体。
相比激光热丝焊接,扫描激光热丝焊接接头在抗拉强度基本不变的情况下,断口延伸率提升至13.1%,说明加入扫描激光可有效提高焊缝的韧性,断口电镜图中更深的韧窝也证明了这一点。
激光热丝焊接焊缝熔合区的硬度最高,热影响区其次,母材硬度最低。扫描激光热丝焊接焊缝的熔合区硬度低于激光热丝焊接,这主要是因为激光热丝焊接焊缝熔合区易发生偏析,生成的夹杂物提高了显微硬度。
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Article Outline
于宸乾, 任刚, 黄映杰, 高明. Q235钢扫描激光热丝焊接工艺特性与组织性能研究[J]. 中国激光, 2024, 51(12): 1202106. Chenqian Yu, Gang Ren, Yingjie Huang, Ming Gao. Process Characteristics, Microstructure, and Properties of Q235 Steel by Scanning Laser Hot Wire Welding[J]. Chinese Journal of Lasers, 2024, 51(12): 1202106.