金属铸造工艺.doc

金属铸造工艺铸造是人类最早知道的金属成型方法之一。它一般是将熔融金属倒入耐火模具型腔中，并将其凝固。凝固后，所需的成品是从难冶塑的的模具中要么用打破模具要么用分开模具的方法取出的。这个凝固的成品称为铸造产品。这个过程也称为铸造过程。1.1 铸造的历史最早的铸造国家是美索不达米亚，最早铸件大约在公元前3500年左右。在世界许多地区的这个时期，铜器和其他平面物体是用石头或烘烤的粘土为模具来铸造的。这些模具基本上都是单件。但在后期，要求铸造圆形铸件时，为了方便铸件的取出，模具必须分成两部分甚至多个部分。 青铜器时代（公元前2000年）的铸造工艺更加精细。也许是最早的时期，空心铸件诞生了。这些铸件内部用的是烤粘土。蜡模铸造法这种工艺被广泛应于加工精细的首饰上。 铸造技术曾在公元前1500年左右在中国得到极大的提高。在此之前，中国还未发现铸造工艺的痕迹。它既不像失蜡法铸模工艺也不广泛使用，而是特殊的使用在多件模具铸造上来制造出高难度的工作。他们花了很多时间在完善产品上甚至到每个细节，因此每一件产品都花费了大量的时间。他们可能用30个甚至更多的精细的模具来制造产品。事实上，在中国各地考古中都曾发现过这些模具。 印度河流域也文明于他们的铜铸件，在装饰，武器，工具和铜铸件上。但是并没有技术上的改进。从各种不同的出土的铜铸件和陶俑来看，印度和中国似乎有着相同的铸造技术，如片模，开模和蜡模具。 尽管印度可能会在坩埚钢的发明上闻名，但是在印度还没有发现铁制品的证据。证据表明，铁的发现是在公元前1000左右在叙利亚和波斯。印度的铁铸造技术是在公元前300左右由亚历山大王朝时代传入的。 在奎塔布的新德里附近的著名的铁柱是印度古冶铁技术的时代标志。这个长约7.2米的铁柱是由纯可锻铸铁铸成的。这铁柱被认为是在古谱塔王朝查德古谱踏二世（公元前375-413年）时期建造的。这根铁柱露在外面的的部分锈蚀率基本为零，甚至埋在地下的部分的也是在以很缓慢的速度在锈蚀。这一定是先铸造然后再捶打到现在的模样。1.2优点和局限性铸造在制造过程中被广泛应用是因为它有很多优点。由于熔融金属可以流入模具的任何一个小的地方，因此无论是内部形状复杂的还是外部形状复杂的都可以用铸造来造成。无论是有色金属还是无色金属都可以用铸造来完成。另外，铸造所需的模具的工具非常的简单和便宜。因此试生产和小批量生产，铸造是一种理想的生产方法。只有在铸造工艺过程中才能计算出所需的材料的准确数量。因此在设计过程中减少浪费材料可以实现。铸件一般从四面八方开始均匀的冷却，因此我们希望金属没有方向属性。有些金属只能用铸造过程而不是其他的过程如锻造，因为在金属成型过程中不想其他金属成分的参入。铸造能够用在任何尺寸和重量的产品制造过程中，甚至是200吨的产品。 然而，用普通的沙铸造过程中的产品的精度和表面的光洁度在许多情况下是达不到产品的要求的。考虑到这些情况，压铸产生了，在后面的章节会介绍。此外沙模铸造是劳动密集型，因此应该在机械成型和铸造机械化上有所改进。对于某些材料，通常很难消除在铸造过程中出现的问题。1.3 应用砂型铸造的典型应用是缸体，衬套，机床床，活塞，活塞环，轧辊，车轮，轴承座，供水管道和精品，以及编钟。1.4 铸造的构成在接下来的章节中，将要看到那些具有代表性的铸造细节。在看细节之前请参考图了解一下新出现的专业名词。型箱：模具型箱能使砂型保存完整。根据型箱在模具结构中的不同位置，型箱可以有很多中不同的名字比如阻力型箱下模型箱上型箱和边面中间型箱铸造使用在三模铸造中。它是由临时使用的木头型箱和长期使用的金属型箱组成。模具：模具是产品的副本的铸造所需的一些修饰。模具的型腔是由模具的帮助而制成的。分模线：这是两个组成砂模的模具的分界线。在分模铸造中它也是两个模具的分界线。浇注底板：这是一个由普通木头制成的底板使用在模具刚刚制造的时候。模具先放在浇注底板上，沙先洒在上面，然后用力捣沙制成。面砂：用炭沙洒在成型腔的内表面以达到铸造产品表面光洁度的要求。造型砂：这是一种新鲜的发酵材料以用来制造型腔。这是由使用过的烧沙组成。砂芯：在铸造中它是用在制造空腔。浇口杯：它是将熔融金属倒入模具中的一个小漏斗。浇注口：它是将熔融金属倒入模腔中的一个通道。在很多情况下它控制着熔融金属流入模腔中的流速。横流道：它是一个在熔融金属流入模具型腔之前能使熔融金属有规律流淌的一条道路。浇口：这是使熔融金属流入型腔的一个准确的流入点。芯撑：芯撑是用在在模具型腔中的砂芯来保护砂芯自身的重量和过载金属应力。冷凝：冷凝是放在模具中的产品以达到增加铸件的冷却速度来提供均匀或预期的冷却速度的目的。冒口：这是铸造中提供熔融金属的的开口，以致当有熔融金属在某处凝固时有金属减少时就能有熔融金属流入模具型腔中。1.5 砂型制造的程序下面的文章将介绍一个典型的砂型制造的过程。 首先放一个浇注底板无论是在成型平台上还是在底板上，确保表面平整。拖模箱向上放在底板上，且在模具阻力部分的中心箱放在底板上。在模具之间应该有足够的清洁度和沙箱的墙高度有秩序的在50mm到100mm之间。干面沙洒在底板和模具之间来提供一个非洒粘层。新鲜的模沙倒入拖模箱里并且模具厚度在30到50mm之间。剩余的拖模箱内装满沙子并且统一的压缩沙子。沙子应做适当的压缩以免压缩的太紧了，以免气体外露，也不能太松否则模具就会没有足够的强度。冲压结束后，沙箱内的沙子被完全使用在了每一个角落。 现在，用一个线形的在1到2mm之间有一个坚底的孔，这通风孔是打在拖箱上且穿过沙箱底部用来在铸造过程中金属凝固时排出沙箱里的空气。这就完成了拖箱的准备。 完成拖箱后就轮到露出底板上的模具了。用一个修光工具，将模具沙边周围修光一下，并且拷贝一半模具放置在拖模上，用定位销将其定位。在定位销的帮助下将拖箱的上沙箱牢牢定位。用干沙洒在拖箱和模具上。 用来打浇注道的浇注针放在一个离模具很小的距离大约50mm。而且如果需要冒口针将被放在一个合适的地方而且新鲜的模沙与拖箱伴随着洒一些沙子并夯实。沙子要过多而且通风口处和在拖箱上一样要到处都有。 浇注口针和冒口针都要小心的推出沙箱。然后在浇注口附近开一个浇口杯。上型框与拖箱分离并且任何在上型箱和拖箱内表面上的散沙用风箱吹掉。现在上型箱和两半拖模模具通过锐尖来分开，起模时从四周轻轻的逐渐打开模腔以确保在分开模具的过程中没有打破沙墙。在模具里开横流道和浇口时必须小心不能损坏模具。任何多余的和松散的沙子用风箱吹掉。现在面沙由粘沙组成洒在模腔的每一部分，横流道将使产品有个光洁的表面。 用芯盒准备着干沙。经过适当的烘烤，被放在如图所示的位置。上型箱再放在拖模上并且用脚记注意两模箱对齐。如图所示的模具是准备好了的能浇注的。模具如前所定义的，模具是产品的副本的铸造所需的一些修饰。所作的修改如下：1 增加模具的广泛性2 提供砂芯的应用3 消除一些不能用铸造的细节，因此通过进一步的处理获得更好的细节。2.1 模具的津贴这时模具的尺寸与铸造最终所需的尺寸不同。各种各样的原因决定了这是必须要的。下面介绍的是一些细节。收缩除了铋冷却外所有的金属在冷却过程中都要收缩。这是因为当温度升高时，原子之间扩增震动产生的。因此，在固体收缩和液体收缩之间有一个区别。 液态收缩是指金属体积的减少当达到金属固相线温度时，金属由液态变成固态的过程。在12章将阐述模具中的这个变化。 固态收缩是指固体体积减小产生的，此时金属固体温度降低。收缩津贴此时可以减少这种损失。 金属的温度收缩率与金属材料有关。例如，钢铁与铝比较收缩率更大。收缩率也与冶金时金属固化过程中的形式转变有关。例如，在铸造过程中白铸铁收缩率大约是21.0mm/m。然而，退火时收缩率增长了约10.5mm/m，是因为在金属内部组织收缩了10.5mm/m。和灰铸铁和球墨铸铁一样，石墨化的程度决定了它们的准确收缩率。各种各样的金属收缩率将在表格7.1中给出。 所有的金属收缩率都是有规则的变化，除非它们在某些部分受到限制。例如，在干沙铸造中的砂芯部分将受到影响而边缘部分却没有影响。因此，在一些受到限制的部分需要更好的总体尺寸津贴。实际的金属收缩率与各种个样的因素有关尤其在特殊的铸造过程中，即，铸造合金，模具材料的使用，模具的设计，模具的复杂程度和组件的大小。样板师的经验和一些审阅都将会在最终的模具收缩上体现出来。表7.1中显示的是平均值，值越大获得的尺寸越小，反之亦然。 收缩津贴也要考虑到线性尺寸。即使在内部直径尺寸的情况下（如气缸的内部），该材料具有一个向中心的倾向，因此尺寸将增加。特殊金属仍然遵守收缩原则，例如，钢材并不是和一般的不一样而是拥有相同的收缩津贴尺寸。这些收缩率的尺寸可以应用在做模的过程中。不同的收缩规律被用在不同的铸造材料上。草案的津贴在砂模中收回模具时，模具的垂直表面都粘着沙，这有可能损坏模具型腔，如图7.2所示。为了减少这种事情的发生，把模具的垂直表面采用锥形。这就称为草案津贴。草案津贴因工作的复杂性而不同。但是一般情况下，模具的内表面的草案要比外表面的高些。表7.2是一个一般性的草案指导表。草案津贴也因手工铸模和机械铸模而不同。和机械铸模相比，手工铸模需要更多的草案铸模。机械铸模因机器的不同草案也不同（新的，僵化的，正确对其的等需要较少的草案)。 有一点需要注意的是，草案始终是作为一个额外的金属成分在原有的铸造尺寸上，如下面的例子。完成或加工余量完成并取得砂铸造精度普遍较差，因此铸造时需要有改善铸件表面和铸件尺寸的功能，这一般由后续加工实现。另外，黑金属需要对材料的表面进行清洗。因此，额外的材料是需要的，被用在表面的机械加工和清洗过程中。这取决于铸件的材料和完成的所需尺寸。这大概在2到20mm之间。表7.3中提供了一般的加工余量准则。加工余量所提供的余料最终都会在加工中去除。因此在定案之前应该仔细的考虑加工余量的费用。 加工余量的类型取决于金属的铸造，所需模具的类型，表面的精度等级和表面的细节的复杂程度。一个减少加工余量的方法是将铸造过程在拖箱中完成，这样，尺寸的变化和其他因素的缺陷将在分割时消除以达到损失最小化的目的。震动津贴在分开砂模之前，模具被四周垂直敲打着以便轻轻的增大模腔为了方便分开。由于它扩大了最终的铸造，所以考虑到这一点，原先的砂模尺寸应该被减小。这是没有标准的津贴，因为它完全取决与工作人员和他的经验。 这是一种优缺点的津贴，它只提供使用在那些平行分割尺寸时的铸造。其中一个减少这补贴的方法是增加工序以便在后面的草案中去除。失真津贴当一个金属刚刚固化时非常薄弱因此很容易有失真的倾向。较薄弱的更是如此，比如，长平的部分，V型，U型的部分或者是在复杂铸件中和厚的部分链接着的细而长的部分。铸造厂应该增加多余料量以减小失真。或者，在模具的表面用一个同样量的失真来抵消相同的失真。可以通过反复的实验得到失真的数据。从一些文献中可能会得到一些例子的失真数据。2.2 模芯印记对于那些所有需要模芯的铸件，应作出规定以支持内部的模腔的模芯。一个普遍使用的方法是提供模型印记。表7.5中给出了一个提供模芯印记的例子。模芯印记大小的提供可能是根据不同的铸造而来的，在第9章中将介绍一些相关的细节。2.3 消除细节通常很难发现沙铸表面上的非常小的漏洞的细节。在这种情况下，在最终方案采取之前通过消除这些细节和漏洞来整理铸造的过程。在表7.6中给出了相关的一个例子。这些细节能够在铸造过程中消除完全取决于精度的要求，铸造流程的选择和成型方法的选用。 Metal Casting ProcessesCasting is one of the earliest metal shaping methods known to human being. It generally means pouring molten metal into a refractory mould with a cavity of the shape to be made, and allowing it to solidify. When solidified, the desired metal object is taken out from the refractory mould either by breaking the mould or taking the mould apart. The solidified object is called casting. This process is also called founding.1.1 HISTORYThe discovery of the casting process was probably around c 3500 BC in Mesopotamia. In many parts of the the world during that period, copper axes and other flat objects were made in open moulds made of stone or baked clay. These moulds are essentially in single piece. But in later periods, when round objects were required to be made.the mould was split into two or more parts to facilitate the withdrawal of the round objects. The bronze age (c 2000 BC) brought far more refinement into casting process. For the first time perhaps, core for making hollow socket in the objects was invented. These cores were made of baked clay. Also the cire perdue or lost wax extensively used for making ornaments and fine work. Casting technology has been greatly improved by Chinese from around 1500 BC. Before that there is no evidence of any casting activity found in China. They do not appear to have been greatly familiar with the cire perdue process nor used it extensively but instead specialised in the multi-piece moulds for making highly intricate jobs. They spent a lot of time in perfecting the mould to the last detail so that hardly any finishing work was required on the casting made from the moulds. They had probably made piece moulds containing carefully fitted pieces numbering thirty or more. In fact, many such moulds have been unearthed during the archaeological in various parts of China. Indus valley civilisation is also known for their extensive use of casting of copper and bronze for ornaments,weapons,tools and utensils. But there was not much of improvement in the technology. From the various objects and figurines that were excavated from the Indus valley sites,they appear to have been familiar with all the known casting methods such as open mould,piece mould and the cire perdue process. Though India could be credited be credited with the invention of crucible stee, not of much iron founding was evident in India. There is evidence that iron founding had started around 1000 BC in Syria and Persia. It appears that iron casting technology in India has been in use from the times of the invasion of Alexander the Great, around 300 BC. The famous iron pillar presently lacated near the Qutab Minar in Delhi is an example of the metallurgical skills of ancient Indians.it is 7.2 m long and is made of pure malleable iron. This is assumed tobe of the period of Chandragupta 2(375-413 AD ) of Gupta dynasty. The rate of rusting of this pillar which stands outside is practically zero and even the buried portion is rusting at extreme slow rate. This must have been first cast and then hammered to the final shape.1.2 ADVANTAGES AND LIMITATIONSCasting process is extensively used in manufacturing because of its many advantages. Molten material flows into any small section in the mould cavity and as such any intricate shapes internal or external can be made with the casting process. It is possible to cast practically any material be it ferrous or nonferrous. Further, the necessary tools required for casting moulds are very simple and inexpensive. As a result, for trial production or production of a small lot,it is an ideal method. It is possible in casting process,to place the amount of material where exactly required. As a result, weight reduction in design can be achieved. Castings are generally cooled uniformly from all sides and therefore they are expected to have no directional properties. There are certain metals and alloys which can only be processed by the casting and not by any other process like forging because of the metallurgical considerations. Casting of any size and weight, even up to 200 tonnes can be made. However the dimensional accuracy and surface finish achieved by normal sand casting process would not be adequate for final application in many cases. To take these cases into consideration, some special casting processes such as die casting have been developed, the details of which are given in later chapters. Also the sand casting process is labour intensive to some extent and therefore many improvements are aimed at it such as machine moulding and foundry mechanisation. With some materials it is often difficult to remove defects arising out of the moisture present in sand casting.1.3 APPLICATIONSTypical applications of sand casting process are cylinder blocks, liners, machine tool beds, pistons,piston rings, mill rolls,wheels, housings, water supply pipes and specials, and bells.1.4 CASTING TERMSIn the following chapters ,the details of sand casting process which represents the basic process of casting would be seen. Before going into the details of the process, the basic process of casting would be seen. Before going into the details of the process, defining a number of casting vocabulary words would be appropriate. Reference may please be made to Fig.Flask: A moulding flask is one which holds the sand mould intact. Depending upon the position of the flask in the mould structure it is referred to by various names such as drag-lower moulding flask cope-upper moulding flask and cheek-intermediate moulding flask and cheek-intermediate moulding flask used in three-piece moulding. It is made up of wood for temporary applications and more generally of metal for long-term use.Pattern: pattern is a replica of the final object to be made with some modifications. The mould cavity is made with the help of the pattern.Parting line: This is the dividing line between the two moulding flasks that makes up the sand mould. In split pattern it is also the dividing line between the two halves of the pattern. Bottom board:This is a board normally made of wood which is used at the start of the mould making. The pattern is first kept on the bottom board, sand is sprinkled on it and then the ramming is done in the drag.Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of the moulding cavity to give abetter surface finish to the castings.Moulding sand: It is the freshly prepared refractory material found in the mould. This is made up of used and burnt sand.Core: It is used for making hollow cavities in castings.Pouring basin: A small funnel shaped cavity at the top of the mould into which the molten metal is poured.Sprue: The passage through which the molten metal from the pouring basin reaches the mould cavity. In many cases it controls the flow of metal into the mould.Runner: The passageways in the parting plane through which molten metal flow is regulated before they reach the mould cavity.Gate: The actual entry point through which molten metal enters mould cavity.Chaplet: Chaplets are used to support cores inside the mould cavity to take care of its own weight and overcome the metallostatic forces.Chill: Chills are metallic objects which are placed in the mould to increase the cooling rate of castings to provide uniform or desired cooling rate.Riser: It is a reservoir of molten metal provided in the casting so that hot metal can flow back into the mould cavity when there is a reduction in volume of metal due to solidification. 1.5 SAND MOULD MAKING PROCEDUREThe procedure for making a typical sand mould is described in the following steps. First a bottom board is placed either on the moulding platform or on the floor, making the surface even. The drag moulding flask is kept upside down on the bottom board along with the drag part of the pattern at the centre of the flask on the board. There should be enough clearance between the pattern and the centre of the flask on the board. There should be of the order of 50 to 100mm. Dry facing sand is sprinkled over the board and pattern to provide a non-sticky layer. Freshly prepared moulding sand of requisite quality is now poured into the drag and on the pattern to a thickness of 30 to 50mm. Rest of the drag flask is completely filled with the backup sand and uniformly rammed to compact the sand. The ramming of sand should be done properly so as not to compact it too hard, which makes the escape of gases difficult, nor too loose so that mould not have enough strength. After the ramming is over, the excess sand in the flask is completely scraped using a flat bar to the level of the flask edges. Now, with a vent wire which is a wire of 1 to 2 mm diameter with a pointed end, vent holes are made in the drag to the full depth of the flask as well as to the pattern to facilitate the removal of gases during casting solidification. This completes the preparation of the drag. The finished drag flask is now rolled over to the bottom board exposing the pattern. Using a slick, the edges of sand around the pattern is repaired and cope half of the pattern is placed over the drag pattern,aligning it with the help of dowel pins. The cope flask on top of the drag is located aligning again with the help of the pins. The dry parting sand is sprinkled all over the drag and on the pattern. A sprue pin for making the sprue passage is located at a small distance of about 50 mm from the pattern. Also a riser pin if requires, is kept at an appropriate place and freshly prepared moulding sand similar to that of the drag along with the backing sand is sprinkled.the sand is thoroughly rammed,excess sand scraped and vent holes are made all over in the cope as in the drag. The sprue pin and the riser pin are carefully withdrawn from the flask. Later the pouring basin is cut near the top of the sprue. The cope is separated from the drag and any loose sand on the cope and drag interface of the drag is blown off with the help of bellows. Now the cope and the drag pattern halves are withdrawn by using the draw spikes and rapping the pattern all around to slightly enlarge the mould cavity so that the mould walls are not spoiled by the withdrawing pattern. The runners and the gates are cut in the mould carefully without spoiling the mould. Any excess or loose sand found in the runners and mould cavity is blown away using the bellows. Now the facing sand in the form of a paste is applied all over the mould cavity and the runners which would give the finished casting a good surface finish. A dry sand core is prepared using a core box. After suitable baking, it is placed in the mould cavity as shown in Fig. The cope is replaced on the drag taking care of the alignment of the two by means of the pins. The mould now,as shown in Fig is ready for pouring. Patterns As has been defined earlier,a pattern is a replica of the object to be made by the casting process, with some modifications. The main modifications are:(a) the addition of pattern allowances,(b) the provision of core prints, and (c) elimination of fine details which cannot be obtained by casting and hence are to be obtained by further processing.2.1PATTERN ALLOWANCESThe dimensions of the pattern are different from the final dimensions of the casting required. This is required because of various reasons. These are detailed as follows.ShrinkageAll the metals shrink when cooling except perhaps bismuth. This is because of the inter-atomic vibrations which are amplified by an increase in temperature. However, there is a distinction to be made between liquid shrinkage and solid shrinkage. Liquid shrinkage refers to the reduction in volume when the metal changes from liquid to solid state at the solidus temperature. To account for this risers are provided in the moulds as explained in Chapter 12. Solid shrinkage is the reduction in volume caused, when metal loses temperature in solid state. The shrinkage allowance is provided to take care of this reduction. The rate of contraction with temperature is dependent on the material. For example, steel contracts to a higher degree compared to aluminium. The contractions also depend upon the metallurgical transformation taking place during the solidification. For example, white cast iron shrinks by about 21.0 mm/m during casting. However, when annealed it grows by about 10.5 mm/m, resulting in a net shrinkage of 10.5 mm/m. Similarly in grey cast iron and spheroidal graphite iron, the amount of graphitisation controls the actual shrinkage. When graphitisation is more, the shrinkage would be less and vice versa. The various rates of contraction for the materials are given in Table 7.1. As a rule all the dimensions are going to be altered uniformly unless they are restrained in some way. For example, a dry sand core at the centre of the casting may restrain the casting from contracting but the edges are not restrained. T