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1、4.3 Precision Casting Metal casting has a historic pedigree as the manufacturing process first used by man to produce intricate metal articles and art objects.It played a major role in the industrial revolution and remains the basis of current manufacturing equipment and manufactured goods.The proce
2、ss plays an important part in aerospace component production and,as such ,remains at the leading edge of technology development.Despite the competition from plastics and ceramics,metal still remain the dominant materials in the production of capital equipment and manufactured goods.Metal casting wil
3、l continue to play a major role,as a manufacturing process of considerable versatility,for the foreseeable future金属铸造被人类作为生产工艺首次使用在复杂的金属制品和艺术品上有悠久的历史。它在 工业革命中发挥了重大作用，仍然是当今设备和产品制造的基本方法。这种工艺在航空零 部件生产中起着重要的作用，因此，仍然是科技发展的前沿技术。尽管有塑料和陶瓷的竞 争，金属仍然作为主要材料来生产重要设备和产品。在可预见的未来，金属铸造作为一种 被广泛考虑的制造工艺方法将继续发挥重要作用。 In c
4、ommon with other other manufacturing processes,metal casting has not remained static and there have been significant developments in both the metallurgy of cast alloys and in casting processes.It is the developments in processing,and particularly those devised to improve precision,which is the focus
5、 of this pass age.The objectives of these introductory passages are to establish the context wthin which precision casting processes are operated.This requires a definition and description of metal casting,a consideration of the attributes of metal casting in relation to other manufacturing processe
6、s,a summary of the structure of the foundry,and a definition of the term precision in the context of metal casting与其他制造工艺相比，金属铸造不是静止不变的，在冶炼铸造合金和铸造工艺上有了重大 发展。铸造工艺的发展，尤其是为提高铸造精度的设计，是过去铸造工艺的重点。这些介 绍性段落是为了建立精密铸造工艺实施的背景。这就需要一个定义来描述金属铸造，与其 他制造工艺相比，金属铸造有自己的特征，需要概述铸造工业的结构和定义精密金属铸造。4.3.1 Metal Casting In the
7、 metal casting process a metal,or more commonly an alloy,is heated until it is molten,whereupon it is poured into a mold or die that contains a cavity which represents the shape of the component or casting to be produced.In the case of a mold the cavity is produced by a pattern which is a replica of
8、 the component required,whereas in the case of a die the cavity replicating the shape of the component is produced by machining.Molds are invariably expendable,being destroyed when the casting is removed.Dies,on the other hand,are permanent and may be used to produce a succession of casting.Patterns
9、 may also be permanent or expendable.A permanent pattern is reusable,whereas an expendable pattern is consumed during the process of mold or casting production在金属铸造过程中，一种纯金属（或更普遍的是一种合金）加热到熔化，然后浇入能呈现 零件形状的铸模或金属模型腔中。一般铸模的型腔是通过能复制零件形状的模样来生产的， 而金属模一般是通过机加工来制造具有零件外形的型腔。铸型都是一次性的，当铸件取出 时铸型都会被破坏。另一方面，金属型是永久
10、性的，可用于连续生产铸造。模样也可以是 永久性的或一次性的。永久性模样是可以重复使用的，而一次性模样在铸造生产中消失。 4.3.2 Sand Casting In the traditional sand casing process,sand is mixed with clay and water to produce a moldable mixture.The moistened clay provides the bonding agent which binds the sand grains together when the molding sand is compacted
11、around a pattern to produce a mold.In order that the pattern may be removed from the mold,to provide the cavity into which the metal will be poured ,the mold must be made in at least two parts.These mold parts can be separated for removal of the pattern and joined again to receive the metal.在传统的砂型铸造
12、工艺中，混有粘土和水的砂来生产型砂。当用紧实模样周围的型砂来造 型时，湿粘土作为粘结剂，使沙粒结合在一起。为了使模样能从型腔中取出，腾出空间来 填充金属液，铸型至少分为两部分。分开的铸型能够取模，然后合型浇注。 4.3.3 Gating and feeding The metal enters the mold cavity through a system of channels referred to as the running or gating system.The gating system transports the liquid metal from the pouring
13、ladle to the mold cavity and it has an important influence on the quality of a casting.Metal,like most materials,expands when it is heated and contracts when it cools.Contraction has a major effect on casting quality.During the melting stage the metal is heated to a temperature above its melting poi
14、nt to ensure that it will flow readily and completely fill the mold cavity before solidification commences.When this superheated liquid metal is referred to latent heat of fusion,the atoms of the metal lose energy and become closely bound together in a regular structure.A significant decrease in vol
15、ume accompanies the change or solidification contraction.If on attempt is made to compensate for these two forms of contraction then the casting will show signs of shrinkage,either in the form of surface sinks or as internal voids.To overcome this problem feeder heads are provided and act as reservo
16、irs which feed liquid metal into the casting as it solidifies,thus compensating for the shrinkage.When the entire system is solid it will cool down to ambient temperature and contract as it does so,this is referred to as solid state contraction.As a consequence of this form of contraction the casting will clearly be smaller than the pattern from which it was produced.金属液进入型腔的通道称为浇注系统。浇注系统把金属液从浇注钢包引入到型腔中，它对 铸件的质量有重要影响。金属，像大多数材料一样，受热时膨胀，冷却时收缩。收缩对铸 件质量有重大影响
