基于双尺度粉末的流变成形7075合金组织与性能调控研究

近年来，随着汽车轻量化进程的加快，7xxx 系高强铝合金（Al-Zn-Mg-Cu 系）在汽车结构件上的应用逐渐受到重视。然而由于该合金的合金化程度较高，铸件容 易出现热裂、缩孔等缺陷，严重影响合金的力学性能。因此制备高强韧 7xxx 系合 金对推动车辆轻量化具有重要意义。本文首先通过半固态流变成形工艺制备 7075 合金，解决热裂和缩孔缺陷。将双尺度复合粉末引入流变成形工艺进一步改善合 金组织以获得更优性能，通过研究添加不同比例双尺度复合粉末后流变成形 7075 合金的微观组织和力学性能，分析其影响规律，并通过理论计算探讨裂纹抑制机 理。研究热处理态流变成形 7075 合金的组织与性能。本文的主要研究成果如下：（1）通过半固态流变成形工艺制备出组织为细小等轴晶的成形件，平均晶粒 尺寸为 69.5 μm，解决常规铸造工艺的热裂问题。通过高能球磨工艺制备 TiN/Ti 双 尺度复合粉末，并将其引入流变成形 7075 合金。结果表明：随着添加比例从 0.1 wt% 增加到 0.4 wt%，其晶粒尺寸呈现先减小后增大趋势。添加比例为 0.2 wt% 时， 晶粒尺寸为 46.3 μm，与未添加双尺度粉末的 7075 合金相比减小 33.4%。添加比 例为 0.3 wt% 时抗拉强度达到最大，相较于未添加的基体合金提升 13.3%，同时试 样的延伸率达到最大，相较于未添加基体合金提升 96%。（2）通过计算凝固曲线发现双尺度复合粉末的加入对凝固路线的末端影响很小，裂纹的抑制不能归因于添加双尺度复合粉末后凝固区间的改变。双尺度粉 末的加入使合金总 Q 值（生长限制因子）提高。综合分析得出：半固态流变成形工 艺的优势和双尺度复合粉末的添加促进 7075 合金晶粒细化，从而有效抑制热裂。（3）经过热处理，7075 合金组织中的大部分第二相均匀分散，同时观察到少量难熔相Al7Cu2Fe 的存在。在添加双尺度粉末的合金中 TiN 粒子更倾向于在晶界 处聚集。而过量添加时观察到由细小粒子聚集而成的约 20 μm 的粗大Al3Ti 相。热 处理后试样的晶粒尺寸出现小幅长大。随添加比例增加，热处理态合金的晶粒尺 寸呈现先减小后增大的趋势。添加 0.3 wt% 时平均晶粒尺寸最小，为 51.6 μm。在 该比例下屈服强度、抗拉强度和延伸率分别为 430.3 MPa、487.3 MPa、8.78%。过 量添加时（0.4 wt%）细化效果下降归因于粗大Al3Ti 的聚集下沉，使部分 TiN 粒子 附近的富 Ti 元素过渡区消失，导致细化效果下降。

In recent years, with the acceleration of automobile lightweight process, the application of 7xxx series of high strength aluminum alloy (Al-Zn-Mg-Cu series) in automobile structural parts has been paid more and more attention. However, due to the high degree of alloying, the casting is prone to defects such as hot cracking and shrinkage, which seriously affect the mechanical properties of the alloy. Therefore, the preparation of high strength and toughness 7xxx series alloy is of great significance for promoting vehicle lightweight. In this paper, 7075 alloy was prepared by semi-solid rheoforming to solve the defects of hot cracking and shrinkage cavity. Dual-scale composite powder was introduced into the rheoforming process to further improve the alloy microstructure and obtain better properties. The microstructure and mechanical properties of 7075 alloy were studied by adding different proportions of dual-scale composite powder, and the influence rules were analyzed, and the crack suppression mechanism was discussed by theoretical calculation. The microstructure and properties of heat treated 7075 alloy were studied. The main research achievements of this paper are as follows: （1）The semi-solid state rheoforming process was used to prepare formed parts with fine isometric crystals with an average grain size of 69.5 μm to solve the thermal cracking problem of conventional casting process. The TiN/Ti dual-scale composite powder was prepared by high-energy ball milling process and introduced into the rheoformed 7075 alloy. The results show that the grain size tends to decrease first and then increase as the addition ratio increases from 0.1 wt% to 0.4 wt%. The grain size was 46.3 μm at the addition rate of 0.2 wt%, which was 33.4% smaller than that of the 7075 alloy without the addition of double-scale powder. The maximum tensile strength was achieved at 0.3 wt% addition, which was 13.3% higher than that of the base alloy without addition, and the maximum elongation of the specimen was achieved, which was 96% higher than that of the base alloy without addition. （2）The calculated solidification curves revealed that the addition of the dual-scale composite powder had little effect on the end of the solidification route and the crack inhibition could not be attributed to the change in the solidification interval with the addition of the dual-scale composite powder. The addition of the dual-scale powder increased the total Q value (growth limiting factor) of the alloy. The combined analysis concludes that the advantages of the semi-solid state rheoforming process and the addition of the dual-scale composite powder promote the grain refinement of 7075 alloy, thus effectively suppressing thermal cracking. （3）After heat treatment, most of the second phase in the 7075 alloy organization is uniformly dispersed, while a small amount of refractory phase Al7Cu2Fe is observed. TiN particles tend to aggregate at the grain boundaries more in the alloy with the addition of double-scale powder. In contrast, a coarse Al3Ti phase of about 20 μm, formed by the aggregation of fine particles, was observed in the case of excess addition. The grain size of the heat-treated specimens showed a small increase in size. The grain size of the heattreated alloy showed a trend of decreasing and then increasing with the increase of the addition ratio. The average grain size was the smallest at 51.6 μm with 0.3 wt% addition, and the yield strength, tensile strength and elongation were 430.3 MPa, 487.3 MPa and 8.78%, respectively, at this ratio. The decrease of refinement effect at excessive addition (0.4 wt%) was attributed to the aggregation and sinking of coarse Al3Ti, which led to the disappearance of Ti-rich elemental transition zone near some TiN particles, resulting in the decrease of refinement effect.

[1] WITIK R A, PAYET J, MICHAUD V, et al. Assessing the life cycle costs and environmental performance of lightweight materials in automobile applications[J]. Composites Part A: Applied Science and Manufacturing, 2011, 42(11): 1694-1709.
[2] XIONG F, WANG D, MA Z, et al. Structure-material integrated multi-objective lightweight design of the front end structure of automobile body[J]. Structural and Multidisciplinary Optimization, 2018, 57: 829-847.
[3] HOU W, XU X, HAN X, et al. Multi-objective and multi-constraint design optimization for hat-shaped composite T-joints in automobiles[J]. Thin-Walled Structures, 2019, 143: 106232.
[4] MALLICK P. Advanced materials for automotive applications: An overview[J]. Advanced Materials in Automotive Engineering, 2012: 5-27.
[5] WANG J, ZHANG G, ZHENG X, et al. A self-piercing riveting method for joining of continuous carbon fiber reinforced composite and aluminum alloy sheets[J]. Composite Structures, 2021, 259: 113219.
[6] ZHANG W, XU J. Advanced lightweight materials for Automobiles: A review[J]. Materials & Design, 2022: 110994.
[7] TEKUMALLA S, GUPTA M. An insight into ignition factors and mechanisms of magnesium based materials: A review[J]. Materials & Design, 2017, 113: 84-98.
[8] FROES F, FRIEDRICH H, KIESE J, et al. Titanium in the family automobile: the cost challenge [J]. Jom, 2004, 56: 40-44.
[9] 洪腾蛟, 董福龙, 丁凤娟, 等. 铝合金在汽车轻量化领域的应用研究[J]. 热加工工艺, 2020,49(4): 1-6.
[10] SPENCER D, MEHRABIAN R, FLEMINGS M. Rheological behavior of Sn-15 pct Pb in the crystallization range[J]. Metallurgical and Materials Transactions B, 1972, 3: 1925-1932.
[11] FLEMINGS M C. Behavior of metal alloys in the semisolid state[J]. Metallurgical Transactions A, 1991, 22: 957-981.
[12] ZHU Q, MIDSON S. Semi-solid moulding: Competition to cast and machine from forging in making automotive complex components[J]. Transactions of Nonferrous Metals Society of China, 2010, 20: s1042-s1047.
[13] 卢宏兴. 铝硅合金半固态压铸成形产品缺陷的形成机理及控制研究[D]. 北京有色金属研究总院, 2017.
[14] 李干, 卢宏兴, 罗敏, 等. 铝合金半固态流变成形技术研究进展[J]. 精密成形工程, 2020, 12(3): 29-48.
[15] 梁小康. Al-Si 合金半固态浆料制备技术及应用研究[D]. 北京有色金属研究总院, 2017.
[16] 胡小刚. 319S 铝合金高固相分数半固态压铸多尺度数值模拟方法与实验研究[D]. 北京：北京科技大学, 2016.
[17] WANNASIN J, FUCHS M, LEE J Y, et al. GISS technology: principle and applications in die casting[J]. Solid State Phenomena, 2019, 285: 470-475.
[18] QI M, KANG Y, ZHOU B, et al. A forced convection stirring process for Rheo-HPDC aluminum and magnesium alloys[J]. Journal of Materials Processing Technology, 2016, 234: 353-367.
[19] PAN Q, FINDON M, APELIAN D. The continuous rheoconversion process (CRP): A novel SSM approach[C]//8th International Conference on Semi-Solid Processing of Alloys and Composites. 2004.
[20] UGGOWITZER P J, KAUFMANN H. Evolution of Globular Microstructure in New Rheocasting and Super Rheocasting Semi-Solid Slurries[J]. Steel Research International, 2004, 75(8-9): 525-530.
[21] YURKO J A, MARTINEZ R A, FLEMINGS M C. The use of semi-solid rheocasting (SSR) for aluminum automotive castings[J]. SAE Transactions, 2003: 119-123.
[22] BROWNE D, HUSSEY M, CARR A, et al. Direct thermal method: new process for development of globular alloy microstructure[J]. International Journal of Cast Metals Research, 2003, 16(4): 418-426.
[23] GUO H M, YANG X J. Efficient refinement of spherical grains by LSPSF rheocasting process [J]. Materials Science and Technology, 2008, 24(1): 55-63.
[24] FAN Z, FANG X, JI S. Microstructure and mechanical properties of rheo-diecast (RDC) aluminium alloys[J]. Materials Science and Engineering: A, 2005, 412(1-2): 298-306.
[25] CURLE U A, MÖLLER H, GOVENDER G. R-HPDC in South Africa[J]. Solid State Phenomena, 2013, 192: 3-15.
[26] KANG Y L, XU Y, WANG Z H. Study on Microstructures and Mechanical Properties of RheoDie Casting Semi-Solid A356 Aluminum Alloy[J]. Solid State Phenomena, 2006, 116: 453-456.
[27] THANABUMRUNGKUL S, JANUDOM S, BURAPA R, et al. Industrial development of gas induced semi-solid process[J]. Transactions of Nonferrous Metals Society of China, 2010, 20: s1016-s1021.
[28] COTÉ P, LAROUCHE M E, CHEN X G. New developments with the SEED technology[J]. Solid State Phenomena, 2013, 192: 373-378.
[29] WON S J, SO H, KANG L, et al. Development of a high-strength Al-Zn-Mg-Cu-based alloy via multi-strengthening mechanisms[J]. Scripta Materialia, 2021, 205: 114216.
[30] 应向荣, 江彬彬, 叶恒强, 等. 高强度 Al-Zn-Mg-Cu 合金等通道角挤压后的微观结构研究[J]. 电子显微学报, 2022, 41: 564-570.
[31] MA Y, ADDAD A, JI G, et al. Atomic-scale investigation of the interface precipitation in a TiB2 nanoparticles reinforced Al–Zn–Mg–Cu matrix composite[J]. Acta Materialia, 2020, 185: 287- 299.
[32] MA Y, CHEN H, ZHANG M X, et al. Break through the strength-ductility trade-off dilemma in aluminum matrix composites via precipitation-assisted interface tailoring[J]. Acta Materialia, 2023, 242: 118470.
[33] LIU T S, QIU F, YANG H Y, et al. Exploring the potential of FSW-ed Al–Zn–Mg–Cu-based composite reinforced by trace in-situ nanoparticles in manufacturing workpiece with customizable size and high mechanical performances[J]. Composites Part B: Engineering, 2023, 250: 110425.
[34] 张亮, 王一笑, 袁松阳, 等. 挤压铸造 Al-5Zn-2Mg-1Cu-0.2 Sc 合金的组织与性能[J]. 特种铸造及有色合金, 2022, 41(5): 533-538.
[35] YUAN L, GUO M, QIAO D, et al. Super-high formability of Al-5.0 Zn-1.5 Mg-1.5 Cu (wt.%) alloy via gradient heterogeneous structure[J]. Materials Research Letters, 2023, 11(2): 99-107.
[36] ZHU Z, NG F L, SEET H L, et al. Superior mechanical properties of a selective-laser-melted AlZnMgCuScZr alloy enabled by a tunable hierarchical microstructure and dual-nanoprecipitation[J]. Materials Today, 2022, 52: 90-101.
[37] LI X, LI G, ZHANG M X, et al. Novel approach to additively manufacture high-strength Al alloys by laser powder bed fusion through addition of hybrid grain refiners[J]. Additive Manufacturing, 2021, 48: 102400.
[38] 李岩, 闫洪, 陈小会. Al3Ti/7075 铝基复合材料固溶处理工艺研究[J]. 热加工工艺, 2011, 40 (14): 139-142.
[39] XU D, ROMETSCH P, BIRBILIS N. Improved solution treatment for an as-rolled Al–Zn–Mg–Cu alloy. Part II. Microstructure and mechanical properties[J]. Materials science and Engineering: A, 2012, 534: 244-252.
[40] MAZIBUKO N E, CURLE U A. Effect of solution heat treatment time on a rheocast Al-Zn-Mg-Cu alloy[J]. Materials Science Forum, 2011, 690: 343-346.
[41] PENG G, CHEN K, CHEN S, et al. Evolution of the second phase particles during the heating-up process of solution treatment of Al–Zn–Mg–Cu alloy[J]. Materials Science and Engineering: A, 2015, 641: 237-241.
[42] ZOU X L, HONG Y, CHEN X H. Evolution of second phases and mechanical properties of 7075 Al alloy processed by solution heat treatment[J]. Transactions of Nonferrous Metals Society of China, 2017, 27(10): 2146-2155.
[43] CHEN S, CHEN K, PENG G, et al. Effect of heat treatment on strength, exfoliation corrosion and electrochemical behavior of 7085 aluminum alloy[J]. Materials & Design, 2012, 35: 93-98.
[44] SUN W, ZHU Y, MARCEAU R, et al. Precipitation strengthening of aluminum alloys by roomtemperature cyclic plasticity[J]. Science, 2019, 363(6430): 972-975.
[45] CHATTERJEE A, QI L, MISRA A. In situ transmission electron microscopy investigation of nucleation of GP zones under natural aging in Al-Zn-Mg alloy[J]. Scripta Materialia, 2022, 207: 114319.
[46] LUO M, LI D, QU W, et al. Mold–Slug Interfacial Heat Transfer Characteristics of Different Coating Thicknesses: Effects on Slug Temperature and Microstructure in Swirled Enthalpy Equilibration Device Process[J]. Materials, 2019, 12(11): 1836.
[47] 李想, 李元东, 樊博阳, 等. 不同铸造方法对 7075 铝合金热导率及力学性能的影响[J]. 特种铸造及有色合金, 2022, 41(1): 93-98.
[48] ESKIN D, KATGERMAN L, MOONEY J. Contraction of aluminum alloys during and after solidification[J]. Metallurgical and Materials Transactions A, 2004, 35: 1325-1335.
[49] LI G, LI X, GUO C, et al. Investigation into the effect of energy density on densification, surface roughness and loss of alloying elements of 7075 aluminium alloy processed by laser powder bed fusion[J]. Optics & Laser Technology, 2022, 147: 107621.
[50] KOU S. A criterion for cracking during solidification[J]. Acta Materialia, 2015, 88: 366-374.
[51] TAN Q, FAN Z, TANG X, et al. A novel strategy to additively manufacture 7075 aluminium alloy with selective laser melting[J]. Materials Science and Engineering: A, 2021, 821: 141638.
[52] CHAI G, BÄCKERUD L, RØLLAND T, et al. Dendrite coherency during equiaxed solidification in binary aluminum alloys[J]. Metallurgical and Materials Transactions A, 1995, 26(4):965-970.
[53] EASTON M, STJOHN D. Grain refinement of aluminum alloys: Part I. the nucleant and solute paradigms—a review of the literature[J]. Metallurgical and Materials Transactions A, 1999, 30(6): 1613-1623.
[54] STJOHN D, QIAN M, EASTON M, et al. The Interdependence Theory: The relationship between grain formation and nucleant selection[J]. Acta Materialia, 2011, 59(12): 4907-4921.
[55] EASTON M, STJOHN D. An analysis of the relationship between grain size, solute content, and the potency and number density of nucleant particles[J]. Metallurgical and Materials Transactions A, 2005, 36(7): 1911-1920.
[56] LIU X, LIU Y, ZHOU Z, et al. Enhanced strength and ductility in Al-Zn-Mg-Cu alloys fabricated by laser powder bed fusion using a synergistic grain-refining strategy[J]. Journal of Materials Science & Technology, 2022, 124: 41-52.
[57] CELENTANO D J. A thermomechanical model with microstructure evolution for aluminium alloy casting processes[J]. International Journal of Plasticity, 2002, 18(10): 1291-1335.
[58] TSAI T, CHUANG T. Relationship between electrical conductivity and stress corrosion cracking susceptibility of Al 7075 and Al 7475 alloys[J]. Corrosion, 1996, 52(6): 414-416.