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1、基于数值模拟的铝合金薄壁件金属型低压铸造工艺设计张立强1,2，李落星1,2，朱必武2 1.湖南大学汽车车身先进设计与制造国家重点实验室，湖南长沙，410082 2.湖南大学材料科学与工程学院，湖南长沙，410082摘要：以A356铝合金为研究对象，基于数值模拟方法使用商业铸造软件ProCAST来设计薄壁件金属型低压铸造工艺，并针对铸件表面缺陷进行分析，提出了工艺改进方案，铸造出了截面呈L型，尺寸为长300mm，宽100mm，壁厚1.5mm的完整铸件，铸件表面质量好，尺寸完整，无缩松等铸造缺陷，为铝合金薄壁件金属型低压铸造技术研究提供了有价值的参考。关键词：铝合金；低压铸造；薄壁件；数值模拟中图
2、分类号：TG249.2 文献标识码：A The Design of Low Pressure Die Casting (LPDC) Process for Thin-walled Aluminum Component with Permanent Mold Base on Numerical Simulation Zhang Li-qiang1第一作者简介：张立强，男，1978年出生，博士研究生，湖南大学材料科学与工程学院，湖南长沙（410082），E-mail：,2, Li Luo-xing1,2, Zhu Bi-wu21. State Key Laboratory of Advanced
3、Design and Manufacturing for Vehicle Body ,Hunan University, Changsha, 410082, Hunan, China2.College of Materials Science Engineering, Hunan University, Changsha, 410082, Hunan, ChinaAbstract: The LPDC process of thin-walled aluminum component with permanent mould was designed by using the commercia
4、l software ProCAST. And a modified technology design was proposed by analyzing the surface defect of the aluminum alloy casting. By applying the modified technology design, a intact casting section:L model, length:300mm,width:100mm,thick:1.5mm was produced. The surface quality of casting was obvious
5、ly improved and no defects such as shrinkage were found. It provides the valuable reference for the research of LPDC technology for thin-walled aluminum component with permanent mold.Key words: Aluminum alloy,LPDC, Thin-wall casting, Numerical simulation引言近几十年来，采用低压铸造等特种铸造技术可以生产一些高质量的复杂精密薄壁铸件，又无需后续的
6、机加工成形，因此低压铸造已成为重要的先进近净成型技术。可铸造的合金有铝合金、锌合金、镁合金、铜合金，还有铸钢，其中铝合金具有密度低，比强度、比刚度高等一系列优点，被广泛应用于航空航天行业，同时也是作为汽车轻量化最理想的材料12。然而，目前铝合金大型结构件整体铸造一般采用砂型铸造，生产效率低、工人劳动强度大，因此要将铝合金大型结构件整体铸造技术应用于汽车行业，实现大规模批量生产必须用金属型取代砂型，但金属型铸造透气性差、无退让性，致使铸件容易产生冷隔、裂纹和气孔等铸造缺陷35。本文以A356铝合金为研究对象，基于数值模拟方法来设计薄壁件金属型低压铸造工艺，并对低压铸造过程进行改进，铸造出了截面呈
7、L型，尺寸为长300mm，宽100mm，壁厚1.5mm的完整铸件，铸件表面质量好，尺寸完整，无缩松等铸造缺陷，为铝合金薄壁件金属型低压铸造工艺设计提供了有价值的参考。2 计算机模拟2.1 几何模型的建立本文使用CAD软件UG对铝合金薄壁铸件进行实体造型（见图1），铸件截面呈L型，尺寸为长300mm，宽100mm，壁厚1.5mm。然后经Parasolid格式的转换导入商业铸造软件ProCAST中进行网格剖分、计算参数设置等处理。为了得到精确的模拟结果和减少计算量，根据数值计算原理和计算机性能要求,有限元模型采用非均匀四面体网格对几何模型进行网格剖分，即浇道和铸件采用较小的网格尺寸进行剖分,保证薄
8、壁处有2或3个单元，而铸型则采用较大的网格尺寸进行剖分，如图2、图3所示，计算所采用的有限元网格规模为：单元总数为608303，节点总数为130493。 图1 铸件的三维几何模型 Fig.1 3-D model of casting 图2 铸型的网格剖分 图3 铸件与浇道的网格剖分Fig.2 Mesh generation of mould Fig.3 Mesh generation of casting2.2 计算参数设置材料选用的是铝合金A356，其热物性参数如表1，表中合金密度、潜热与热导率都是随温度变化的，比热数值在0100范围内，铸件与模具间的热交换系数为3000 w/m2.k，模具
9、与自然空气间的换热系数为20 w/m2.k，模具与压缩空气间的换热系数为700 w/m2.k。表2是仿真的浇注工艺参数设置（其中模具温度指的是模具平均温度）。此工艺是通过试错法，不断调整浇注工艺参数对低压铸造充型过程进行模拟，通过比较最终得出，只有在较高的模具温度和浇注温度下，模拟充型才能充满。图5是浇注时的加压过程压力变化曲线，从图中可以看出，仿真和实验时的加压过程一致。 表1 合金的热物性参数Table 1 Thermal and physical parameters of alloy材料密度/kgm - 3比热/kJ(kgk) - 1潜热/kJkg- 1热导率/w(mk) - 1固相温
10、度/k液相温度/k铸件(A356)（2.42.7）1030.96429.3129492.8184829889表2 仿真的浇注工艺参数Table 2 Technology parameter of simulation in low pressure die casting铸件材料模具材料浇注温度模具温度加压参数设置仿真结果A356H13760530见图5曲线2见图6h曲线1：实验的加压过程，曲线2：仿真的加压过程图5加压过程压力变化曲线Fig.5 Variation of pressure during the process of increasing pressure2.3 模拟结果分析及
11、讨论数值模拟是在高性能微机工作站上利用专用的低压铸造分析软件ProCAST完成的，计算时间平均为54分钟，低压铸造的充填过程计算结果见图6。从图6a图6h，可以看出金属液充满整个型腔的时间为4.38秒，充型的整个过程比较平稳，充型时铸件有明显的温度梯度，温降从距离浇口较远处开始，距离浇口较远处到浇口处温度慢慢升高。(a) 0.95s (b) 1.04s (c) 1.15s (d) 1.64s (e) 1.95s (f) 2.34s (g) 3.68s (h) 4.38s图6浇注开始后不同时刻的充型形态Fig.6 The filling state of different time of LP
12、DC 图7是低压铸造模拟过程中铸件四个不同位置温度场的计算结果。从图7a中可以看出，距离浇口较远处到浇口处有明显的温度梯度，结合图7b，可知距离浇口较远处温度较低，距离浇口近处温度较高，这样可保证铸件在凝固时达到按顺序凝固，获得较好的组织及较高的力学性能。432115mm34.5mm120mm120mm （a）温度随时间变化曲线 （b）数据采集位置图7 仿真过程中铸件不同位置温度随时间变化曲线Fig.7 The variation curves of temperature with time in different position.3 低压铸造实验3.1 实验过程低压铸造机是哈尔滨工业大
13、学设计的实验用小型低压铸造机，最大的浇注压力0.16MPa，其保温炉最高温度800，最大容量6Kg铝合金液，数据采集系统是MCT-2多功能8通道数据采集仪，采集数据时间间隔0.5秒，测量时用NiCr2NiSi型热电偶线1.0mm，最高测量温度1050，涂料配比为：65%水，5%水玻璃（硅酸钠），30%氧化锌，升液管喷涂料后预热温度700， 表3是依据仿真的浇注工艺参数设置的实际浇注工艺参数。从表3可知，实验过程中的浇注工艺参数与仿真时的一致。表3 实际的浇注工艺参数Table 3 Technology parameter in low pressure die casting铸件材料模具材料浇
14、注温度模具温度加压参数设置实验结果A356H13760见图8a见图5曲线1见相片13.2 实验结果分析及讨论基于仿真结果，共做了两次低压铸造实验，两次实验的浇注工艺参数一致，如表3。相片1是两次低压铸造生产出来的的薄壁铸件，相片1a是第一次实验的铸件，可以看出，铸件没有充满，在内浇口附近有较大的缩松产生，铸件表面粗糙。相片1b是第二次实验的铸件，铸件完全充满，表面质量好，无缩松等铸造缺陷。第一次实验铸件没有充满是因为模具顶端温度较低，仅有480如图8a，金属液充型到此位置时，温度迅速降低到凝固点以下，铸件开始凝固；铸件表面出现较大的缩松，是因为加热过程中模具温度控制不好，导致浇注时模具温度分布
15、不均匀，温差较大，凝固时如相片1a所示的缩松区域来不急补缩，形成较大的缩松；铸件表面粗糙是因为模具涂料采用手工刷涂，难于使模具型腔表面涂料均匀。第二次低压铸造实验在总结和吸收了第一次实验经验的基础上进行改进，在具体操作过程中，对模具进行加热时，为了使模具各部分温度均匀，采用间歇式加热方式，加热10分钟，停止5分钟，再继续加热，如此反复，尽可能达到模具的热平衡；另外模具涂料将手工刷涂方式改成气压喷涂，并且在刷涂料前，将涂料放在烧杯中机械搅拌均匀，这样可以大大提高模具涂料的均匀度。通过这些改进措施，低压铸造的铸件如相片b，表面质量好，尺寸完整，无缩松等铸造缺陷。图8是低压铸造过程中铸件不同位置温度
16、随时间变化的曲线，数据采集位置如图7b。图8a中，位置4的温度是模具温度，可以看出，浇注时模具温度是480，比较模具其它位置的温度此处温度较低。比较图7，低压铸造铸件在凝固过程中（固相线温度以上）的温度变化趋势与仿真时基本一致，距离浇口较远处温度较低，距离浇口近处温度较高，这样可保证铸件在凝固时按顺序凝固。缩松表面粗糙内浇口热电偶 （a） 实验1 （b） 实验2相片1 低压铸造的铸件Photo1 The LPDC casting（a） 实验1 （b） 实验2图8 低压铸造过程中铸件不同位置温度随时间变化曲线Fig.8 The variation curves of temperature wi
17、th time in different position in LPDC4 结论本文基于数值模拟方法，设计了A356铝合金薄壁件金属型低压铸造工艺，并依据模拟分析结果及对低压铸造过程改进铸造出了截面呈L型，尺寸为长300mm，宽100mm，壁厚1.5mm的完整铸件，铸件表面质量好，尺寸完整，无缩松等铸造缺陷，为铝合金薄壁件金属型低压铸造技术的研究提供了有价值的参考。参考文献1 邱庆荣,孙宝德,周尧和. 铝合金铸件在汽车上的应用J 铸造, 1998,1(6):46-49.2 周永江，洪润洲. 薄壁复杂铝合金精铸件真空吸铸工艺研究J. 特种铸造及有色合金，2000,(5):18-20.3 冯志军
18、, 申泽骥. 大型铝合金铸件低压铸造J 铸造, 2003,52(12):46-49.4 B.Zhang, D.M.Maijer, S.L.Cockcroft. Development of a 3-D thermal model of the low pressure die cast(LPDC) process of A356aluminum alloy wheelsJ.Materials Science and Engineering. 464 (2007) 295305.5 Penghuai Fu, Alan A. Luo, Haiyan Jiang. Low-pressure die
19、casting of magnesium alloy AM50: Response to process parameters J.journal of materials processing technology. 2008 in press.Editors note: Judson Jones is a meteorologist, journalist and photographer. He has freelanced with CNN for four years, covering severe weather from tornadoes to typhoons. Follo
20、w him on Twitter: jnjonesjr (CNN) - I will always wonder what it was like to huddle around a shortwave radio and through the crackling static from space hear the faint beeps of the worlds first satellite - Sputnik. I also missed watching Neil Armstrong step foot on the moon and the first space shutt
21、le take off for the stars. Those events were way before my time.As a kid, I was fascinated with what goes on in the sky, and when NASA pulled the plug on the shuttle program I was heartbroken. Yet the privatized space race has renewed my childhood dreams to reach for the stars.As a meteorologist, Iv
22、e still seen many important weather and space events, but right now, if you were sitting next to me, youd hear my foot tapping rapidly under my desk. Im anxious for the next one: a space capsule hanging from a crane in the New Mexico desert.Its like the set for a George Lucas movie floating to the e
23、dge of space.You and I will have the chance to watch a man take a leap into an unimaginable free fall from the edge of space - live.The (lack of) air up there Watch man jump from 96,000 feet Tuesday, I sat at work glued to the live stream of the Red Bull Stratos Mission. I watched the balloons posit
24、ioned at different altitudes in the sky to test the winds, knowing that if they would just line up in a vertical straight line we would be go for launch.I feel this mission was created for me because I am also a journalist and a photographer, but above all I live for taking a leap of faith - the fee
25、ling of pushing the envelope into uncharted territory.The guy who is going to do this, Felix Baumgartner, must have that same feeling, at a level I will never reach. However, it did not stop me from feeling his pain when a gust of swirling wind kicked up and twisted the partially filled balloon that
26、 would take him to the upper end of our atmosphere. As soon as the 40-acre balloon, with skin no thicker than a dry cleaning bag, scraped the ground I knew it was over.How claustrophobia almost grounded supersonic skydiverWith each twist, you could see the wrinkles of disappointment on the face of t
27、he current record holder and capcom (capsule communications), Col. Joe Kittinger. He hung his head low in mission control as he told Baumgartner the disappointing news: Mission aborted.The supersonic descent could happen as early as Sunday.The weather plays an important role in this mission. Startin
28、g at the ground, conditions have to be very calm - winds less than 2 mph, with no precipitation or humidity and limited cloud cover. The balloon, with capsule attached, will move through the lower level of the atmosphere (the troposphere) where our day-to-day weather lives. It will climb higher than
29、 the tip of Mount Everest (5.5 miles/8.85 kilometers), drifting even higher than the cruising altitude of commercial airliners (5.6 miles/9.17 kilometers) and into the stratosphere. As he crosses the boundary layer (called the tropopause), he can expect a lot of turbulence.The balloon will slowly dr
30、ift to the edge of space at 120,000 feet (22.7 miles/36.53 kilometers). Here, Fearless Felix will unclip. He will roll back the door.Then, I would assume, he will slowly step out onto something resembling an Olympic diving platform.Below, the Earth becomes the concrete bottom of a swimming pool that
31、 he wants to land on, but not too hard. Still, hell be traveling fast, so despite the distance, it will not be like diving into the deep end of a pool. It will be like he is diving into the shallow end.Skydiver preps for the big jumpWhen he jumps, he is expected to reach the speed of sound - 690 mph
32、 (1,110 kph) - in less than 40 seconds. Like hitting the top of the water, he will begin to slow as he approaches the more dense air closer to Earth. But this will not be enough to stop him completely.If he goes too fast or spins out of control, he has a stabilization parachute that can be deployed
33、to slow him down. His team hopes its not needed. Instead, he plans to deploy his 270-square-foot (25-square-meter) main chute at an altitude of around 5,000 feet (1,524 meters).In order to deploy this chute successfully, he will have to slow to 172 mph (277 kph). He will have a reserve parachute tha
34、t will open automatically if he loses consciousness at mach speeds.Even if everything goes as planned, it wont. Baumgartner still will free fall at a speed that would cause you and me to pass out, and no parachute is guaranteed to work higher than 25,000 feet (7,620 meters).It might not be the moon,
35、 but Kittinger free fell from 102,800 feet in 1960 - at the dawn of an infamous space race that captured the hearts of many. Baumgartner will attempt to break that record, a feat that boggles the mind. This is one of those monumental moments I will always remember, because there is no way Id miss this.