铝合金铸造工艺性能（Aluminum alloy casting process performance）

1、铝合金铸造工艺性能（Aluminum alloy casting process performance）1. Aluminum alloy casting process performanceThe performance of aluminum alloy casting is generally understood as the synthesis of those performances that are most prominent in the process of casting, crystallization and cooling. Liquidity, shrink

2、age, air tightness, casting stress, inhalation. The properties of aluminum alloy are dependent on the composition of the alloy, but it is also related to casting factors, heating temperature of alloy, complexity of casting type, pouring gate system and shape of gate.(1) liquidityLiquidity is the abi

3、lity of the alloy liquid to fill casting. The amount of liquidity determines whether the alloy can cast complex castings. It is best to be liquid in aluminum alloy.There are many influence factors in liquidity, mainly exist in the composition, temperature, and alloy liquid metal oxides, metal compou

4、nds and other pollutants of solid phased particles, but the root of the external factors for the pouring temperature and pouring pressure (commonly known as pouring head).(2) systolicContractibility is one of the main characteristics of cast aluminum alloy. Generally speaking, the alloy is made from

5、 liquid to solidified until it is cold to room temperature, which is divided into three stages, which are liquid shrinkage, solidification contraction and solid contraction. The contraction of alloy has a decisive influence on casting quality, which affects the size of shrinkage cavity, the producti

6、on of stress, the formation of crack and the change of size. The shrinkage of the casting is usually divided into contraction and contraction of the body, and the contraction of the alloy is measured in the actual production.Aluminum alloy shrinkage, usually expressed as a percentage, is called shri

7、nkage.(1) body contractionThe contraction of the body includes the contraction of the liquid and the contraction of solidification.Casting alloy liquid from casting to solidification, can appear in the last solidification place macroscopic or microscopic shrinkage, the macroscopic shrinkage cavity c

8、aused by shrinkage visible to the naked eye, and can be divided into centralized shrinkage cavity and dispersion shrinkage cavity. The pore diameter of the concentrated shrinkage cavity is large and concentrated, and is distributed in the hot section of the top or section of the casting. The dispers

9、ion of the shrinkage cavity is dispersed and small, mostly in the casting axis and the hot section. The microscopic shrinkage cavity is not visible to the naked eye, and the microshrinkage cavity is mostly distributed in the crystal boundary or dendrite branches.Shrinkage cavity and porosity are one

10、 of the main defects of castings. Found in production, casting aluminium alloy solidification range is smaller, the formation of concentrated shrinkage, solidification range is wide, the more easy dispersion shrinkage cavity formation, therefore, in the design must conform to the principle of direct

11、ional solidification casting aluminum alloy, the casting liquid is to the body during the solidification shrinkage should be alloy liquid supplement, is loose focus on the outside of the castings and shrinkage cavity at the mouth. For easy generation dispersed porous aluminum alloy castings, much la

12、rger than the riser set number concentration of shrinkage cavity, and in a setting are prone to loose cold iron, increasing local cooling speed, make its or rapid solidification at the same time.(2) the linear shrinkageLine shrinkage will directly affect the quality of casting. The larger the line c

13、ontraction, the more the tendency to crack and stress in aluminum castings; After cooling, the size and shape of the castings also changed.For different casting aluminum alloy, the shrinkage rate of casting is different, even if the same alloy, casting is different, the shrinkage rate is different,

14、and the shrinkage rate of its length, width and height is different in the same casting. It should be based on specific circumstances.(3) the thermal cracking resistanceThe production of aluminum casting hot crack, is mainly due to the casting shrinkage stress than the binding force between the meta

15、l grain, mostly from the fracture surface observation visible cracks along the grain boundary in metal is often oxidation, metallic luster. The crack is elongated along the grain boundary, the shape is serrated, the surface is wider, the interior is narrower, and some penetrate the end face of the w

16、hole casting.Aluminum alloy casting crack tendency is also different, this is because the cast aluminum alloy solidification process begin to form a complete framework of crystallization temperature and setting temperature, the greater the difference between the alloy shrinkage rate is, the greater

17、the produce hot cracking tendency are, the greater the even the same kind of alloy due to the structure of the resistance of the casting, casting, pouring process factors such as hot cracking tendency are also different. It is often used to reduce the casting of aluminum casting, or improve casting

18、system of casting aluminum alloy. Thermal cracking of aluminum casting is usually detected by thermal cracking.(4) the air tightnessThe air tightness of aluminum alloy is the degree of non-seepage of the aluminum casting in the cavity of the cavity. The air tightness actually represents the degree o

19、f density and purity of the inner structure of the castings.The air tightness of the cast aluminum alloy is related to the properties of the alloy. The smaller the solidification range of the alloy, the smaller the loose tendency and the smaller the deposition porosity, the higher the air tightness

20、of the alloy. Air tightness of the same kind of cast aluminum alloy is good or bad, but also related to the casting process, such as the lower cast aluminum alloy pouring temperature, put cold iron in order to speed up the cooling and solidification under pressure, etc., all can increase the tightne

21、ss of aluminum castings. It can also be used to plug the leakage gap to improve the air tightness of the casting.Casting stressThe casting stress includes thermal stress, phase change stress and contraction stress. There are different reasons for stress.(1) thermal stressThe thermal stress is caused

22、 by the uneven thickness of the different geometrical shapes of the castings and the cooling inconsistencies. The stress is formed at the thin wall, resulting in residual stress in the castings.Phase varying stressThe phase change stress is due to the phase change in the cooling process of some cast

23、 aluminum alloy after solidification, resulting in the change of volume size. It is mainly that the wall thickness of aluminum casting is not uniform, and different parts of the cast are caused by phase change in different time.Contraction stressThe shrinkage of aluminum casting is caused by casting

24、 and core blocking. This stress is temporary and aluminum casting is automatically removed. However, the improper opening time often results in hot cracks, especially the metal casting aluminum alloy tends to produce hot cracks under such stress.The residual stress in the cast aluminum alloy parts r

25、educes the mechanical properties of the alloy and affects the machining accuracy of the casting. Residual stress in aluminum casting can be eliminated by annealing. The alloy has no phase change due to good thermal conductivity, as long as the casting structure is reasonable, the residual stress of

26、aluminum casting is generally small.(6) inspiratory sexAluminum alloy is easy to absorb gas and is the main characteristic of casting aluminum alloy. The composition of liquid aluminium and aluminum alloy, the combustion products of organic materials and the combustion products of organic materials

27、and the reaction of the water in the mold containing water are absorbed by the aluminum liquid.The higher the temperature of the molten metal, the more hydrogen it absorbs; At 700 ,The solubility of hydrogen in the aluminum per 100 g is 0.5 0.9, when the temperature to 850 , the solubility of hydrog

28、en increases 2 3 times. When the alkali metal impurities, the solubility of hydrogen in aluminum is increased significantly.Cast aluminum alloy in addition to the melting inspiratory, also produces inspiratory when pouring into the mold, the liquid metal into the mold with the temperature drop, the

29、solubility of a gas fell, the precipitate excess gas, some from out of the gas in casting porosity, which is usually called pinhole. The gas is sometimes combined with a retracer, and the gas in the aluminum liquid remains in the shrinkage cavity. If the pressure of bubbles is very large, the pores

30、are smooth, and there is a ring of light around the hole. If the pressure of the bubble is small, the inner surface of the hole is wrinkled, and it looks like fly foot.The higher the hydrogen content in the aluminum alloy liquid, the more pinholes produced in the casting. The needle holes in aluminu

31、m castings not only reduce the air tightness and corrosion resistance of the castings, but also reduce the mechanical properties of the alloy. In order to obtain the aluminum castings without porosity or porosity, the key is the smelting conditions. If the coating agent is added to the smelting proc

32、ess, the inhalation of the alloy is greatly reduced. The aluminum melt liquid is refined, and it can control the amount of hydrogen contained in aluminum liquid.Sand castingMold casting is made of sand, clay and other auxiliary materials. Sand - type material is collectively referred to as molding m

33、aterial. The sand type of non-ferrous metal is made from sand, clay or other binder and water.Aluminum casting process is the process of metal and cast interaction. The aluminum alloy fluid is injected into the cast mould, and the heat of the molten metal is transferred to the casting mold, and the

34、mold is subjected to the thermal, mechanical and chemical effects of liquid metal. Therefore, to obtain high quality castings, besides strictly mastering the smelting process, it is necessary to design the matching, molding and pouring process of the correct design (core) sand.Iii. Metal casting1. I

35、ntroduction and process flowMetal mold casting is also called hard mold casting or permanent mold casting, is of good melting aluminum alloy pouring into the metal mold casting method, metal mold casting aluminum alloy metal core is mostly, can also be used by adopting the method of sand core or she

36、ll core, compared with the pressure casting, aluminum alloy metal mold service life is long.2. Casting advantages(1) the advantagesThe metal cooling rate is faster and the casting tissue is dense, which can be strengthened by heat treatment, and the mechanical property is about 15% higher than the s

37、and type casting.Metal casting, casting quality is stable, the surface roughness is better than sand mold casting, the reject rate is low.Labor conditions are good, productivity is high, workers are easy to master.(2) the disadvantagesThe thermal conductivity of the metal type is large and the filli

38、ng capacity is poor.The metallic type itself has no air permeability. Measures must be taken to effectively exhaust.The metal type has no anaglysis and can produce cracks and deformation when solidified.3. Common defects and prevention of metal castings(1) the pinholeMeasures to prevent the producti

39、on of pinholes:It is strictly prohibited to use contaminated casting aluminum alloy materials, organic compounds and materials that are seriously oxidized.Control the melting process and strengthen the degassing refining.Control the thickness of the metal coating, the thick and easy to produce pinho

40、le.The die temperature should not be too high, the casting thickness of the wall shall be used as a cold measure, such as copper block or water.When using sand type, water is strictly controlled and dry core is used.(2) the porosityMeasures to prevent stomatal development:Modify the unreasonable gat

41、e system to make the liquid flow smoothly and avoid the gas.The mould and core should be pre-preheated, then coated, and must be baked before using.The design mold and core should consider sufficient exhaust measures.(3) oxide slagMeasures to prevent the residue of oxidation:Strict control of smelting process, rapid melting, reducing oxidation, eliminating slag thoroughly. The al-mg alloy must be smelted under the covering agent.The furnace and tools should be cleaned, no oxides, and should be pre