激光快速成形金属零件力学行为研究
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摘要
激光快速成形技术是近十年来迅速发展起来的一项先进实体自由成形技术，该技术综合快速原型技术与激光熔覆技术的优点，能够实现高性能致密金属零件的直接近终成形，应用前景十分广阔。目前该技术研究尚处于实验阶段，存在许多基础问题亟待解决，本文从实验角度对激光快速成形金属零件的开裂行为、残余应力分布特性及力学性能进行了研究。
采用微观测试分析方法对激光快速成形过程熔覆层裂纹的形成机理及影响因素进行研究。研究表明：对镍基自熔合金，熔覆层开裂属于冷裂纹范畴，是熔覆合金低延性及熔覆层内应力双重作用的结果；对316L不锈钢合金，熔覆层开裂属于热裂纹范畴，是熔覆合金在凝固时晶界处的残余液相受熔覆层拉伸应力作用所导致的液膜分离的结果。两种开裂都说明激光快速成形过程受热不均匀所引起的拉伸应力是产生熔覆裂纹的外在原因。
采用小孔释放法研究激光快速成形薄板的二维残余应力分布。结果发现作用在成形件上的主要残余应力为平行于激光扫描方向的残余应力σ_y，以拉应力为主：而垂直激光扫描方向的残余应力σ_z作用相对较小。具体表现为：
对Ni20合金，在有限熔覆高度内，随着熔覆层的增加，σ_y开始由靠近基材处的压应力向拉应力转变，并且拉应力数值逐渐增大，表明随激光能量的不断输入，残余应力积累效应增大：σ_z基本表现为拉应力，数值相对较小，随着熔覆层的增加，拉应力数值逐渐减小，有改变为压应力的趋势。
对316L不锈钢合金，σ_y和σ_z在整个熔覆高度上以拉应力作用为主。开始熔覆阶段，由于残余应力的不断积累，σ_y在靠近基材一侧表现为较大的拉应力，当到达一定的熔覆高度后，随着熔覆层的进一步增加，σ_y开始有所回落，逐渐趋于稳定，保持较低的拉应力状态：σ_z数值在整个熔覆高度上相对较小，随着熔覆层的增加，拉应力数值逐渐减小，有改变为压应力的趋势。
在裂纹和残余应力分析的基础上，研究激光快速成形件的力学性能。对激光快速成形沉积态试样进行室温拉伸性能测试，结果显示在没有消除残余拉应力的情况下，Ni20和316L不锈钢激光快速成形件的强度与塑性已经接近甚至超过同类合金的传统锻压加工水平。
Laser Rapid Forming (LRF) is a new and advanced Solid Freedom Fabrication technology which has been developed rapidly in the recent ten years. This technology integrates virtues of Rapid Prototyping and Laser Cladding technology, and can attain directly high performance and dense metal part without mould, with wider application foreground. This paper studies mechanical behavior of Laser Rapid Forming metal part by experiment, including cracking behavior, residual stress distribution and mechanical property.
The forming mechanism and influencing factors of crack in laser rapid forming cladding layers are investigated through micro measurement and analysis methods. For nickel-base self-fused alloys, the cracking of cladding layers belongs to the cold crack domain, Which is the result of collective effect of low ductility of such alloys and residual stress induced in cladding layers. For 316L stainless steel alloy, the cracking of cladding layers belongs to the hot crack domain, Which is mainly caused by the separation of liquid films in the grain boundary under the effect of tensile stress in cladding layers. Two kinds of crackings show that the tensile stress induced for asymmetric temperature distribution during laser rapid forming processing is the external reason which results in cracks.
The planar residual stresses of Laser Rapid Forming sheet samples are measured by the hole-drilling method. The results show that the residual stress y parallel to laser scanning direction is the major residual stress of samples, which is positive. Comparably, the residual stress z perpendicular to laser scanning direction is smaller.
For Ni20 alloy, In the limited cladding height, y is negative near substrate and begins to turn to tensile stress as cladding layers increase, the tensile stress numerical value increases gradually, which implies that residual stress accumulates as laser energy inputs continuously. Z is positive in cladding layers, numerical value is small. The tensile stress decreases as cladding layers increase, has a trend to turn to compressive stress.
For 316L stainless steel alloy, In the whole cladding height, y and z are positive. Because residual stress accumulates gradually, y is larger tensile stress near substrate, above one height, y decreases and goes steady as cladding layers increase more,
holding low tensile stress state. In the whole cladding height, z numerical value is small, the tensile stress decreases as cladding layers increase, has a trend to turn to compressive stress.
Based on the analysis of crack and residual stress, mechanical property of Laser Rapid Forming sample is studied. By measuring the room temperature mechanical property of LRF Ni20 and 316L sample which endure residual tensile stress, the results show the room temperature yield strength and ultimate strength have approach or exceed the traditional forging level for the same alloy.
采用微观测试分析方法对激光快速成形过程熔覆层裂纹的形成机理及影响因素进行研究。研究表明：对镍基自熔合金，熔覆层开裂属于冷裂纹范畴，是熔覆合金低延性及熔覆层内应力双重作用的结果；对316L不锈钢合金，熔覆层开裂属于热裂纹范畴，是熔覆合金在凝固时晶界处的残余液相受熔覆层拉伸应力作用所导致的液膜分离的结果。两种开裂都说明激光快速成形过程受热不均匀所引起的拉伸应力是产生熔覆裂纹的外在原因。
采用小孔释放法研究激光快速成形薄板的二维残余应力分布。结果发现作用在成形件上的主要残余应力为平行于激光扫描方向的残余应力σ_y，以拉应力为主：而垂直激光扫描方向的残余应力σ_z作用相对较小。具体表现为：
对Ni20合金，在有限熔覆高度内，随着熔覆层的增加，σ_y开始由靠近基材处的压应力向拉应力转变，并且拉应力数值逐渐增大，表明随激光能量的不断输入，残余应力积累效应增大：σ_z基本表现为拉应力，数值相对较小，随着熔覆层的增加，拉应力数值逐渐减小，有改变为压应力的趋势。
对316L不锈钢合金，σ_y和σ_z在整个熔覆高度上以拉应力作用为主。开始熔覆阶段，由于残余应力的不断积累，σ_y在靠近基材一侧表现为较大的拉应力，当到达一定的熔覆高度后，随着熔覆层的进一步增加，σ_y开始有所回落，逐渐趋于稳定，保持较低的拉应力状态：σ_z数值在整个熔覆高度上相对较小，随着熔覆层的增加，拉应力数值逐渐减小，有改变为压应力的趋势。
在裂纹和残余应力分析的基础上，研究激光快速成形件的力学性能。对激光快速成形沉积态试样进行室温拉伸性能测试，结果显示在没有消除残余拉应力的情况下，Ni20和316L不锈钢激光快速成形件的强度与塑性已经接近甚至超过同类合金的传统锻压加工水平。
Laser Rapid Forming (LRF) is a new and advanced Solid Freedom Fabrication technology which has been developed rapidly in the recent ten years. This technology integrates virtues of Rapid Prototyping and Laser Cladding technology, and can attain directly high performance and dense metal part without mould, with wider application foreground. This paper studies mechanical behavior of Laser Rapid Forming metal part by experiment, including cracking behavior, residual stress distribution and mechanical property.
The forming mechanism and influencing factors of crack in laser rapid forming cladding layers are investigated through micro measurement and analysis methods. For nickel-base self-fused alloys, the cracking of cladding layers belongs to the cold crack domain, Which is the result of collective effect of low ductility of such alloys and residual stress induced in cladding layers. For 316L stainless steel alloy, the cracking of cladding layers belongs to the hot crack domain, Which is mainly caused by the separation of liquid films in the grain boundary under the effect of tensile stress in cladding layers. Two kinds of crackings show that the tensile stress induced for asymmetric temperature distribution during laser rapid forming processing is the external reason which results in cracks.
The planar residual stresses of Laser Rapid Forming sheet samples are measured by the hole-drilling method. The results show that the residual stress y parallel to laser scanning direction is the major residual stress of samples, which is positive. Comparably, the residual stress z perpendicular to laser scanning direction is smaller.
For Ni20 alloy, In the limited cladding height, y is negative near substrate and begins to turn to tensile stress as cladding layers increase, the tensile stress numerical value increases gradually, which implies that residual stress accumulates as laser energy inputs continuously. Z is positive in cladding layers, numerical value is small. The tensile stress decreases as cladding layers increase, has a trend to turn to compressive stress.
For 316L stainless steel alloy, In the whole cladding height, y and z are positive. Because residual stress accumulates gradually, y is larger tensile stress near substrate, above one height, y decreases and goes steady as cladding layers increase more,
holding low tensile stress state. In the whole cladding height, z numerical value is small, the tensile stress decreases as cladding layers increase, has a trend to turn to compressive stress.
Based on the analysis of crack and residual stress, mechanical property of Laser Rapid Forming sample is studied. By measuring the room temperature mechanical property of LRF Ni20 and 316L sample which endure residual tensile stress, the results show the room temperature yield strength and ultimate strength have approach or exceed the traditional forging level for the same alloy.
引文
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[2] D. M. Keiche et al. Using the Laser Engineered Net Shaping (LENS) Process to Produce Complex Components from a CAD Solid Model. SPIE, 2293: 91-97
[3] Eric Schlienger et al. Near Net Shape Production of Metal Components Using LENS. Proceedings of the Third Pacific Rim International Conference on Advanced Materials and Processing; Vol Ⅰ, Honolulu, Hawaii, USA, July 12-16, 1998. 1581-1586
[4] P. C. Collins et al. Fraser. The influence of the enthalpy of mixing during the laser deposition of complex titanium alloys using elemental blends. Scripta Materialia 48. 2003: 1445-1450
[5] Gary K. Lewisa, Eric Schlienger. Practical considerations and capabilities for laser assisted direct metal deposition, Materials and Design 21. 2000: 417-423
[6] D. Srivastava et al. The effect of process parameters and heat treatment on the microstructure of direct laser fabricated TiAl alloy samples. Intermetallics 9. 2001: 1003-1013
[7] X. Wu, J. Mei. Near net shape manufacturing of components using direct laser fabrication technology. Journal of Materials Processing Technology 135. 2003: 266-270
[8] Katrin Ischwendner et al. Direct laser deposition of alloys from elemental powder blends. Scripta Materialia 48. 2003: 1123-1129
[9] M. Gaumann et al. Single-crystal laser deposition ofsuperalloys: processing-microstructure maps. Acta mater. 2001: 1051-1062
[10] Shawn M. Kelly. Characterization and thermal modeling of laser formed Ti-6Al-4V. Thesis. 2002
[11] M. Gaumann et al. Epitaxial laser metal forming: analysis of microstructure formation. Materials Science & Engineering A; 1999, Vol. A271: 232-241
[12] Suman Das et al. Processing of titanium net shapes by SLS/HIP. Materials and Design 20. 1999: 115-121
[13] Suman Das et al. SLS/HIP-A Direct Freeform Fabrication Process for High Performance Metal Components. Proceedings of the Third Pacific Rim International Conference on Advanced Materials and Processing, Vol.Ⅰ, Honolulu, Hawaii, USA, July, 1998: 1587-1594
[14] Fritz B. Prinz, Lee E. Weiss. Novel applications and implementations of shape deposition manufacturing. Proceedings of the Third Pacific Rim International Conference on Advanced Materials and Processing, Vol.Ⅰ, Honolulu, Hawaii, USA, July, 1998: 1547-1554.
[15] E. Weiss et al. Shape deposition manufacturing of heterogeneous structures. SME Journal of Manufacturing Systems. 1997, Vol. 16: 239
[16] David Abbott, Frank Arcella. Laser forming titanium components. Advanced Materials & Processes. May, 1998: 29-30
[17] F. G. Arcella et al. Titanium Alloy Structures for Airframe Application by the Laser Forming Process. AIAA, Structural Dynamics & Materials Conference. 2000: 1465-1473.
[18] B. Snow et al. Rapid solidification processing of superalloy using high power lasers. Superalloys, Proceedings Fourth International Symposium, Alaitors Publishing Div, Baton Rouge, LA, 1980: 189-203
[19] Simon Pickering, AeroMet implementing novel Ti process. Metal Powder Report, 1998: Vol 53, No 2: 24-26
[20] J. Mazumder et al. Closed loop direct metal deposition: art to part. Optics and Lasers in Engineering. 2000, 34: 397-414
[21] Xue Let al. Free-form laser consolidation for producing metallurgicaly sound and functional components. Journal of Laser Application. 2000, 12(4): 160-165
[22] Griffith ML et al. Multimaterial processing by LENS. Proceedings of the Solid Freeform Fabrication Symposium, Austin, TX: University of Texas at Austin Publishers, August, 1997
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