Microstructure and Properties of Laser Melting Deposited AlSi10Mg Aluminum Alloy Repair Layer
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摘要:
采用激光熔化沉积AlSi10Mg铝合金涂层对6061铝合金进行修复,研究了激光功率（1 200~2 000 W）和扫描速度（8~16 mm·s−1）对修复层相对密度的影响,分析了致密性最好的修复层的物相组成、显微组织、残余应力、显微硬度及拉伸性能。结果表明：随着激光功率或扫描速度的增加,修复层的相对密度均基本呈先增加后减小的趋势,当激光功率为1 800 W、扫描速度为12 mm·s−1时,相对密度最高,为99.57%。修复层包含粗枝晶区域和细枝晶区域,显微组织为胞状共晶组织,由柱状枝晶和等轴枝晶组成；共晶硅相离散分布在α-Al相基体中,同时还存在少量Mg2Si沉淀相。修复层的显微硬度在96~136 HV,高于6061铝合金基体,残余拉应力为83~116 MPa；修复试样拉伸断裂位置为修复区域,抗拉强度达到了基体的98.4%,断后伸长率与基体相当,断口表面存在韧窝、解理面、孔洞等特性,断裂机理为韧脆混合断裂。
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关键词:
- 激光熔化沉积 /
- AlSi10Mg铝合金 /
- 相对密度 /
- 拉伸性能
Abstract:6061 aluminum alloy was repaired by laser melting deposition of AlSi10Mg aluminum alloy coating. The effects of laser power (1 200–2 000 W) and scanning speed (8–16 mm · s−1) on the relative density of the repair layer were investigated, and the phase composition, microstructure, residual stress, microhardness and tensile properties of the repair layer with the best densification were analyzed. The results show that with the increase of laser power or scanning speed, the relative density of the AlSi10Mg aluminum alloy repaired layer basically first increased and then decreased, and was the highest when the laser power was 1 800 W and the scanning speed was 12 mm · s−1, which was 99.57%. The repair layer contained coarse dendritic region and fine dendritic region, and the microstructure was cellular eutectic, which consisted of columnar dendrite and equiaxed dendrite. The eutectic silicon phase was distributed in the α-Al phase matrix, and there was also a small amount of Mg2Si precipitated phase. The microhardness of the repair layer was in the range of 96–136 HV, which was higher than that of the 6061 aluminum alloy matrix, and the residual tensile stress was in the range of 83–116 MPa. The tensile fracture position of the repair sample was in the repair area, the tensile strength reached 98.4% of that of the matrix, and the percentage elongation after fracture was comparable with that of the matrix. There were dimples, cleavage planes and holes on the fracture surface, and the fracture mechanism was tough-brittle mixed fracture.
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0. 引言
车轴是铁路机车中一个十分重要的构件,世界各国对提高车轴的可靠性均十分重视[1-5]。我国铁道车辆的提速加载对大功率机车及高速列车车轴用钢的性能提出了更高的要求。国内常用的机车车轴材料是40钢和50钢。40钢强度稍低,但韧性好,50钢强度较高,但韧性稍差,2种材料均已无法满足高速、重载铁路机车的要求[6]。EA4T钢是一种广泛使用于国外地铁动车车轴及大功率机车车轴的钢种,也是欧洲标准EN 13261规定的高速客车车轴用钢。之前我国的合金钢车轴主要靠进口,为了快速实现国家铁路机车重载技术和提速战略,通过引进、吸收、消化已实现车轴的国产化,许多新型机车都采用了国产的EA4T钢车轴,如9 600 kW牵引机车和武汉、深圳的地铁车辆等。
EA4T车轴钢的常规热处理工艺为调质热处理（淬火＋高温回火）,根据EN 13261：2009标准要求,调质热处理EA4T钢车轴轴颈1/2半径处的显微组织应全部为马氏体/贝氏体组织（以下简称为M/B组织）。当轴颈直径较小,如180 mm时,采用常规工艺调质热处理后,其1/2半径处可以全部获得M/B组织；但当轴颈直径较大,达到280 mm时,其1/2半径处在常规工艺下获得全部的M/B组织则较为困难[7-8]。在实际生产中为了保证大尺寸车轴淬火加热时心部能够淬透,常常采用提高加热温度（高于铁素体转变为奥氏体的终了温度Ac3 30~50 ℃）的方法进行淬火[9],而回火温度及方式尚需进一步研究。为了确定大尺寸国产EA4T钢车轴的热处理工艺,作者设计了EA4T钢等效车轴的热处理工艺,研究了热处理后等效车轴不同位置的显微组织与性能,据此确定车轴的调质热处理工艺并进行车轴的现场验证。
1. 试样制备与试验方法
试验材料为东北特殊钢集团股份有限公司抚顺特殊钢股份有限公司生产的尺寸280 mm×280 mm×350 mm的热轧退火态EA4T钢车轴毛坯,化学成分如表1所示,符合EN 13261：2009标准要求。在毛坯上加工出规格ϕ280 mm的圆柱体试样,将其作为实际尺寸为ϕ256 mm的EA4T钢车轴的等效车轴,并对其进行调质热处理预试验。
表 1 EA4T钢的化学成分Table 1. Chemical composition of EA4T steel项目 质量分数/% C Si Mn Cr Mo Ni P S V Al Cu Fe 测试值 0.28 0.32 0.70 1.06 0.24 0.17 0.004 0.001 0.04 0.029 0.03 余 标准值 0.22~0.29 0.15~0.40 0.50~0.80 0.90~1.20 0.15~0.30 ≤0.30 ≤0.020 ≤0.015 ≤0.06 ≤0.30 余 根据前期试验结果,EA4T钢的Ac3为840 ℃,在常规条件下其淬火温度应为870~890 ℃。但由于EN 13261：2009标准要求,EA4T钢在淬火时应具有较高的淬透性,而提高奥氏体加热温度可以获得更高的淬透性,故将等效车轴的淬火温度范围设置为890~920 ℃。等效车轴的淬火保温时间可以采用经验公式来估算,公式[10]如下：
(1) 式中：t为保温时间,min；α为加热系数,取值范围为0.9~1.1 min·mm−1；K为加热时的修正系数,取1.2；D为工件的有效厚度,取280 mm。
由式（1）计算得到,等效车轴的淬火保温时间为5~6 h。根据上述分析并结合前期研究[8],确定淬火工艺为900 ℃×5 h。为了确定大尺寸车轴坯的回火工艺,将EA4T钢等效车轴进行900 ℃×5 h水淬处理后,分别进行595,610,650 ℃保温6 h水冷回火处理[8]。
按照EN 13261：2009进行车轴的显微组织及力学性能研究。在不同调质工艺处理后的等效车轴表层、1/2半径处和心部截取金相试样,经打磨、抛光,用体积分数4%硝酸乙醇溶液腐蚀20 s后,用清水冲洗试样,并用乙醇擦拭,再用吹风机吹干,采用NEOPHOT-21型光学显微镜观察显微组织。按照GB/T 228—2002,在车轴表层、1/2半径处和心部位置截取拉伸试样,拉伸试样的尺寸为直径10 mm标准试样尺寸的10倍,在AG-250KNISMO型电子拉压试验机上进行室温拉伸试验,拉伸速度为10 mm·min−1,相同条件下测3次取平均值。按照GB/T 229—2007,在等效车轴表层、1/2半径处和心部分别沿轴向（即横向）和径向（即纵向）截取标准夏比U型冲击试样,在JXB-300型摆锤式冲击试验机上进行室温冲击试验,冲击速度为5 m·s−1,相同条件下测3次取平均值。根据等效车轴测试结果,确定符合标准要求的调质热处理工艺后进行规格ϕ256 mm EA4T钢车轴的现场验证,拉伸试样的尺寸为直径10 mm标准试样尺寸的5倍,冲击试样为5 mm缺口深度的U型冲击试样,测试设备及参数同前。采用Zeiss Supra 55型场发射扫描电镜（SEM）观察拉伸断口形貌。
2. 试验结果与讨论
2.1 调质热处理后等效车轴的组织和性能
由表2可以看出,与EN 13261：2009标准要求的力学性能（屈服强度不低于420 MPa,抗拉强度为650~800 MPa,断后伸长率不低于18%,纵向和横向冲击吸收能量分别不低于50,25 J）相比,等效车轴经900 ℃水淬后,除了595 ℃水冷回火后不同位置的抗拉强度,以及610 ℃水冷回火后表层的抗拉强度偏高之外,其余条件下的强度、断后伸长率、断面收缩率以及纵向与横向冲击韧性均符合要求,同时还存在较大的富裕量。
表 2 等效车轴经900 ℃×5 h水淬和不同温度保温6 h水冷回火后的力学性能Table 2. Mechanical properties of equivalent axle after 900 ℃×5 h water quenching and tempering at different temperatures for 6 h and water cooling回火温度/℃ 位置 屈服强度/MPa 抗拉强度/MPa 断后伸长率/% 断面收缩率/% 冲击吸收能量/J 纵向 横向 595 表层 719.0 866.0 19.0 62.5 156.0 128.0 1/2半径处 603.0 866.0 19.5 68.0 101.0 76.0 心部 694.0 861.0 18.5 62.5 107.0 93.0 610 表层 728.0 843.0 20.5 71.5 129.0 103.0 1/2半径处 628.0 763.0 27.5 65.0 101.0 155.0 心部 543.0 696.0 22.3 65.5 144.0 128.0 650 表层 629.0 781.0 19.0 67.0 141.0 164.0 1/2半径处 628.0 762.0 19.0 72.5 210.0 155.0 心部 595.0 749.0 18.3 66.5 111.0 169.0 由图1可以看出,等效车轴经900 ℃×5 h水淬和650 ℃×6 h水冷回火后,除了心部含有少量铁素体外,表层以及1/2半径处的组织基本为回火M/B组织,符合EN 13261：2009对车轴组织的要求。心部与表层、1/2半径处组织差异的原因主要在于冷却速率的不同,心部冷却速率较慢,未发生完全马氏体转变,导致生成少量铁素体,表层和1/2半径处冷却速率较快,形成了回火M/B组织。这种回火M/B组织具有较高的硬度和耐磨性,对车轴的表层性能有积极影响[11-12]。综上,确定EA4T钢车轴的调质热处理工艺为900 ℃×5 h水淬和650 ℃×6 h水冷回火。
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图 1 等效车轴经900 ℃×5 h水淬和650 ℃×6 h水冷回火后不同位置的显微组织Figure 1. Microstructures of different areas in equivalent axle after 900 ℃×5 h water quenching and 650 ℃×6 h water cooling tempering: (a) surface layer；(b) 1/2 radius position and (c) core2.2 调质热处理工艺的现场验证
在热处理车间采用规格ϕ256 mm的EA4T钢车轴对前文确定的调质热处理工艺（900 ℃×5 h水淬和650 ℃×6 h水冷回火）进行现场验证。由表3可以看出,规格ϕ256 mm的EA4T钢车轴经900 ℃×5 h水淬和650 ℃×6 h水冷回火后,不同位置的力学性能均完全满足EN 13261：2009标准要求。由图2可以看出,不同部位的组织均为回火M/B组织,也满足EN 13261：2009标准要求。
位置 屈服强度/MPa 抗拉强度/MPa 断后伸长率/% 断面收缩率/% 冲击吸收能量/J 纵向 横向 表层 653.0 788.0 21.5 70.0 65.3 72.0 1/2半径处 569.0 717.0 21.5 65.0 80.0 71.3 心部 527.0 695.0 20.0 62.0 72.0 73.3 ![]()
图 2 ϕ256 mm车轴经900 ℃×5 h水淬和650 ℃×6 h水冷回火后不同位置的显微组织Figure 2. Microstructures of different areas inϕ256 mm axle after 900 ℃×5 h water quenching and 650 ℃×6 h water cooling tempering: (a) surface layer；(b) 1/2 radius position and (c) core由图3和图4可以看出,车轴不同位置所取的拉伸试样在断裂前均发生了大量的塑性变形,为韧性断裂。宏观断口表现出明显的缩颈,且仅存在表现韧性的纤维区和剪切唇,而不存在表现脆性的放射区。纤维区微观均呈韧窝特征。在拉伸应力的作用下,试样发生颈缩而在最小截面处形成三维应力,其值在轴线方向上最大,这些三维应力使晶界、缺陷等处形成显微孔洞；随着应力的提高,孔洞不断长大且相互连接,同时产生新的孔洞,从而使裂纹缓慢形成并扩展,最终在断口上留下韧窝状的区域。综上所述,国产EA4T钢车轴经900 ℃×5 h水淬和650 ℃×6 h水冷回火的调质热处理后,其力学性能和组织均符合EN 13261：2009标准要求。
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图 3 经900 ℃×5 h水淬和650 ℃×6 h水冷回火后ϕ256 mm车轴不同位置拉伸试样试验后的宏观形貌Figure 3. Macromorphology of tensile samples in different areas ofϕ256 mm axle after 900 ℃×5 h water quenching and 650 ℃×6 h water cooling tempering after test![]()
图 4 经900 ℃×5 h水淬和650 ℃×6 h水冷回火后ϕ256 mm车轴不同位置拉伸试样的断口SEM形貌Figure 4. Fracture SEM morphology of tensile samples in different areas ofϕ256 mm axle after 900 ℃×5 h water quenching and 650 ℃×6 h water cooling tempering: (a) surface layer, overall morphology；(b) surface layer, micromorphology of fiber area；(c) 1/2 radius position, overall morphology；(d) 1/2 radius position, micromorphology of fiber area；(e) core, overall morphology and (f) core, micromorphology of fiber area3. 结论
（1）国产ϕ280 mm EA4T钢等效车轴在进行900 ℃×5 h水淬和595 ℃×6 h水冷回火处理后不同位置以及610 ℃×6 h水冷回火后表层的抗拉强度均偏高,650 ℃×6 h水冷回火后的强度、断后伸长率、断面收缩率以及纵向与横向冲击韧性均符合EN 13261：2009标准要求,组织也基本为回火M/B组织。确定EA4T钢车轴的调质热处理工艺为900 ℃×5 h水淬和650 ℃×6 h水冷回火。
（2）现场验证得到经900 ℃×5 h水淬和650 ℃×6 h水冷回火后,国产ϕ256 mm EA4T钢车轴表层、1/2半径处和心部处的力学性能和显微组织均符合EN 13261：2009标准要求,不同位置取样拉伸后均发生韧性断裂,断口均由纤维区和剪切唇组成,纤维区呈韧窝形貌。
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图 2 不同扫描速度下激光熔化沉积修复层的相对密度随激光功率的变化曲线
Figure 2. Curves of relative density vs laser power of laser melting deposited repair layer under different scanning speeds
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图 4 激光熔化沉积修复层的横截面显微组织
Figure 4. Cross section microstructures of laser melting deposited repair layer: (a) overall morphology; (b) partially enlarged view of molten pool boundary; (c) enlarged view of bottom area; (d) enlarged view of central area and (e) enlarged view of top area
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图 5 激光熔化沉积修复层横截面的SEM形貌
Figure 5. Cross section SEM morphology of laser melting deposited repair layer: (a) overall morphology and (b) partial enlargement morphology
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图 6 激光熔化沉积修复试样的截面硬度分布曲线
Figure 6. Section hardness distribution curves of laser melting deposited repair sample: (a) along deposition direction and (b) perpendicular to deposition direction
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图 7 激光熔化沉积修复试样的残余应力分布
Figure 7. Residual stress distribution of laser melting deposited repair sample: (a) perpendicular to deposition direction on surface and (b) along deposition direction on section
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图 8 基体和激光熔化沉积修复试样的真应力-真应变曲线
Figure 8. True stress-true strain curves of substrate and laser melting deposited repair sample
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