Laser welding properties of 1800 MPa press hardening steel and low-alloy high-strength steel CR340LA
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摘要: 采用光纖激光焊接設備對1800 MPa級熱成形鋼與CR340LA低合金高強鋼進行對接激光拼焊，研究了不同激光焊接功率和焊接速度下焊接接頭的組織演變規律及熱沖壓成形性能，并對焊接接頭的力學性能和硬度進行了分析。結果表明，3種焊接工藝下激光拼焊原板綜合力學性能相差較小，由焊接接頭造成的伸長率和抗拉強度的損失均在母材的28.3%和9.1%以內。激光焊接后焊縫區均為粗大、高硬度的馬氏體結構；兩側熱影響區組織主要為鐵素體和馬氏體，接頭未出現明顯的軟化區。激光拼焊原板拉伸試樣均斷裂于CR340LA母材區，距離焊縫12 mm左右，且存在焊縫隆起現象。選取焊接功率和焊接速率分別為4000 W和0.18 m·s?1的焊接試樣在高溫下進行熱沖壓成形檢測，未出現焊縫開裂，熱成形后拼焊板具有良好性能，滿足汽車激光拼焊板使用要求，拉伸結果表明，試樣斷裂位置與未熱沖壓成形前一致，均位于CR340LA母材區，拉伸過程中，焊縫向高強度母材側偏移，在弱強度母材側產生應力集中并縮頸斷裂。Abstract: A laser tailor welding experiment of 1800 MPa press hardening steel and low-alloy high-strength steel CR340LA was carried out using an optical fiber laser. The microstructure evolution and hot stamping formability of tailor-welded blanks were investigated under different laser welding powers and welding speeds, and the mechanical properties and distribution of the microhardness of the welding joints were analyzed and studied. Results show that the comprehensive mechanical properties of the laser tailor-welded blanks have little difference under three welding processes. The loss of elongation and tensile strength caused by welding joints is within 28.3% and 9.1% of the base metal. After laser welding, the fusion zone of the tailor-welded blanks is a martensite structure, which is bulky and has high hardness. The microstructure in the heat-affected zone on both sides is mainly ferrite and martensite, and there is no obvious softening zone in the joint under the welding processes. The tensile specimens of the tailor-welded blanks are all broken in the CR340LA base metal zone, approximately 12 mm away from the weld center, and a weld heave phenomenon occurs, which may be due to the uneven distribution of material properties after welding. Hot stamping of the tailor-welded blanks with a welding power and welding speed of 4000 W and 0.18 m·s?1, respectively, was carried out at high temperature, and no weld cracks were found during the experiment. Thus, these tailor-welded blanks have good performance and meet the requirements of automobile laser tailor-welded blanks. The tensile test results show that the fracture location of the specimens is the same as that before hot stamping, both of which are located in the CR340LA base metal area. During the stretching process, the fusion zone shifts to the side of the high-strength base metal, which results in a stress concentration and necking fracture on the side of the weak-strength base metal.
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Key words:
- laser welding /
- hot stomping /
- weld offset /
- mechanical properties /
- microstructural transformation
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圖 1 母材的基本組織形貌。（a）AC1800HS；（b）CR340LA
Figure 1. Microstructure of the base material: (a) AC1800HS; (b) CR340LA
圖 3 熱沖壓成形模具（a）及U型拼焊板構件（b）
Figure 3. Hot stamping forming die (a) and U-shaped tailor welded blank component (b)
圖 4 U型件拉伸試樣取樣圖
Figure 4. Sample diagram of the tensile specimens for the U-shaped parts
圖 6 各工藝下焊接接頭宏觀形貌。（a,d）K1接頭；（b,e）K2接頭；（c,f）K3接頭
Figure 6. Macromorphology of the welded joints under various processes: (a,d) K1 joint; (b,e) K2 joint; (c,f) K3 joint
圖 7 組織檢驗結果。（a）K1焊縫區；（b）K2焊縫區；（c）K3焊縫區；（d）K1 AC1800HS側熱影響區；（e）K2 AC1800HS側熱影響區；（f）K3 AC1800HS側熱影響區；（g）K1 CR340LA側熱影響區；（h）K2 CR340LA側熱影響區；（i）K3 CR340LA側熱影響區
Figure 7. Microstructure examination results: (a) FZ of K1; (b) FZ of K2; (c) FZ of K3; (d) HAZ of the AC1800HS side of K1; (e) HAZ of the AC1800HS side of K2; (f) HAZ of the AC1800HS side of K3; (g) HAZ of the CR340LA side of K1; (h) HAZ of the CR340LA side of K2; (i) HAZ of the CR340LA side of K3
圖 9 激光拼焊原板應力應變曲線（a）及拉伸試樣斷裂位置（b）
Figure 9. Stress–strain curve (a) and fracture location (b) of the tensile specimens of the laser tailor-welded blanks
圖 10 熱成形后U型件應力應變曲線（a）及拉伸試樣斷裂位置（b）
Figure 10. Stress–strain curve (a) and fracture location (b) of the tensile specimens of the U-shaped components
圖 11 熱沖壓成形后母材及焊縫微觀組織。（a）CR340LA；（b）焊縫；（c）AC1800HS
Figure 11. Microstructure of the base metal and fusion zone after hot stamping: (a) CR340LA; (b) fusion zone; (c) AC1800HS
表 1 激光焊接母材化學成分（質量分數）
Table 1. Chemical composition of the base material
% Base metal C Si Mn B Al Nb+Ti+Mo+Cr+V AC1800HS 0.34 0.40 1.36 0.003 0.04 0.51 CR340LA 0.07 0.38 1.08 ― 0.04 0.071 
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表 2 激光焊接工藝
Table 2. Laser welding processes
No. Power/W Welding speed/(m·s?1) Heat input/(J·mm?1) K1 2000 0.04 50.0 K2 3000 0.08 37.5 K3 4000 0.11 36.4
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表 3 AC1800HS與CR340LA焊接接頭力學性能
Table 3. Mechanical properties of the welded joint between AC1800HS and CR340LA
Number Elongation/% Yield strength/MPa Tensile strength/MPa Product of tensile strength and elongation/(GPa·%) Fracture location AC1800HS 25.52±0.28 397±3 562±2 14.3 — CR340LA 26.60±0.2 421±8 548±3 14.6 — K1 19.08±0.32 350±2 513±2 9.8 CR340LA K2 20.22±0.46 362±2 506±1 10.2 CR340LA K3 21.24±0.28 361±6 498±7 10.6 CR340LA
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表 4 U型件各部位力學性能
Table 4. Mechanical properties of the various parts of the U-shaped components
Number Elongation/% Yield strength/MPa Tensile strength/MPa Fracture location AC1800HS(after hot stamping) 5.12±0.13 1232±28 1890±29 — CR340LA(after hot stamping) 23.45±0.15 417±3 597±10 — KA 19.90±0.18 299±6 499±8 CR340LA KB 15.74±0.86 356±9 550±1 CR340LA KC 21.14±0.18 335±7 481±2 CR340LA
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