The Research On The Changing Performance Of Aluminum Casting Alloy Under Pressure

The foundry is a kind of ancient manufacturing method. It is a kind of processing technic of pouring the liquid metal alloy into the foundry mold to make it cooling and solidifing to get the product. For some complicated components, if the rolled steel meterial process maching to get the product, it will need energy for 266MJ/kg, but for the percision casting,it will need energy for 62.8MJ/kg to get near eventually cast product after a little mount of processing, and it can save the energy for nearly 3/4. Therefore, casting will never be"sunset-industry",but the important part of the future advanced manufacturing engineering.There are several kinds of casting defects as gas cavity, shrinkage cavity, shrinkage porosity, segregation, non-metallic inclusion deforming, crack etc. According to the shape size of the pore inside and its distributing characteristics, gas cavity can divided into the pinhole, pore under the epiderm, and gross shrinkage. If the pinhole exceed certain standard, the mechanical behavior will descend obviously, and it will affect the stain resistance and anode oxidize function and airtightness, even make the product discard. The pore under epiderm is from the reaction of the aluminum alloy liquid and the surface of the casting mold. It affect the mechanical behavior and the stain resistance, it has a worse influence for the appearance and the size of the product, and for the product need surface working, the influence will be more worse. Gross shrinkage can make the product discard. Because of the existence of shrinkage porosity and shrinkage cavity, the mechanical behavior will have a big descend affected by the minishing of the effective loaded area and even the stress concentration. Furthermore, because of the broken of the consecution, the airtightness, surface smoothness after processing and tarnish resistance of cast products ruduce significantly. Microsegregation and macrosegregation demage the mechanical behaviour, the machine work performance, resistance of cold cracking and heat cracking of cast products in different degree. Nan-metallic inclusion (mainly include hydride, nitride, sulfide etc.) obviously influence the mechanical performance of the metal products, it broke the consecution of the metallic matrix and it makes the non-homogeneous of metal structure; it is a fatigue crack source and lowers the metal's fatigue resistence seriously; it has the different Young's madulus and expansion coefficeent with the metallic matrix, being subjected to the heating and stress, the stress concentration will appear in the sharp cape of the admixture and will lower the metal's plasticity and ductility. But under some particular condition, the non-metallic inclution in the metals also has its benefits, for example, the carbide and the nitride with high melting-point and higt dispersion can be the non-self-moving crystallize core in the alloy's solidifing, and this can refine the crystal grain, and sometimes can also produce the enhancement of the precipitation and promote the grain orientation and thus produce some favorable function such as the machinability improving.The forge pieces have the better internal organization and comprehensive mechanical behavior than the cast parts, but the mode of procuction of forging has disadvantages in big investment in the equipment and complex craft and small material utilization. There has been the usage of foundry craft replacing cast craft in the production of connecting rod, front tow hook of liberation series truck, extremely high manganese steel hammerhead, 120 claw mechanical loader track, idler wheel in the diesel engine injection pump etc, besides the satisfication of usage function, this can rose the material utilization, small the machining workload, low the production cost.The level of organization of the aluminum casting piece includes of the tree-like crystallization of dendritic clearance, close grain organization, deteroprated organization, the structure contain the hydrogen cavity loose and tree-like interspace structure of shrinkage void and shrinkage porosity. The casting piece of the high quality must have small tree-like arborescent crystal clearance; we will get the small equiax crystal after the increase of the grain refiner, and the casting piece will have excellent combination of strength and ductility; the tension performance and ductility performance of the metamorphic structure is better than the unmetamorphosed structure. But the loose structure of the hydrogen cavity will have the bad influence to the mechanical behavior; the difference between the tree-like interspace structure of shrinkage void and shrinkage porosity and the loose structure of the hydrogen cavity is that the former is come from the shrinkage of liquid to solide.We can get the aluminum alloy by join other metal or non-metal element into the pure aluminum, the comprehensive performance of alminum alloys is excellcent because of the alloying and heat treating. The influence of the change of the metal alloy is mainly to the gas cavity of casting pieces, the influence to the shrinkage void and shrinkage porosity of casting pieces is little. The influence of alloying agent to the hydrogen obsorbing expresses in two aspects: one is the influence to the performance of oxide film, the other is to the dissolvability of hydrogen.Pressure, cooling speed and solidify way all work on the shrinkage void and shrinkage porosity of casting pieces. When solidified under pressure, the pressure urges liquid to flow along the interdentritic and then fill into the shrinkage porosity and reduce or remove the shrinkage porosity and that the density and airtightness of casting pieces are increased. At the same time, the mechanical performance is raised because the grain crystallized under the high pressure is crush to pieces continuously. The small branch form dendritic clearance can be get by increasing the cooling speed, so do the small equiax crystal. The fine metal and autectic alloys and the alloy of narrow crystalizing temperature scope are solidifing layer-by-layer under the general foundry condition, and will leave the concentrate shrinkage cavity behind at the eventual solidifing palace, generally, the feeding performance of this kinds of alloys is all right, because the shrinkage cavity is easily to be canceled( such as setting the feeder head). The center line shrinkage cavity will be get in the plank-like and bar-like casting pieces; the dispersal shrinkage cavity or called shrinkage porosity can be eventually formed in the alloys with the wide crystalizing temperature scope. But for the alloys with madium-scale crystalizing temperature scope, the solidifing way in the initial stage of the solidifing is near to the way of the narrow one, the columnar crystals stop growing at some time, and then the solidifing way is near to the wide one and then get the equiax crystal.People have beening knowing the metal's heredity from the 20's in the 20th centuries. After study the liquid structure of the aluminum base alloy, people point out that the second stability gum-form grain keep the organization characteristic of the raw material, and it is the carrier of the metallurgy organization hereditary, and also the organized heredity gene namely. The original materials with the eutectic, initial crystal, intermetallic compound, non-metallic inclusion or various segregated material melt and can become a macroscopical non-homogeneous fused mass. When the non-homogeneous fused mass transfers the quality to the characteristic size, it will fome the microstructure with its own particular composition and different with the acroteric mediator, namely the structure gene of the metal heredity.The pore, shrinkage cavity and shrinkage porosity all present heredity. The pore can clearly inherit between the crude materials and the casting pieces and take the aluminum-silicon alloy for example; the inclusion can be the foreign core of the pore nucleating, and can promote the pore to nuculate, and increase the pore rate; the electric processing can reduce the pore heredity; this article do the research on the pore heredity, the dissolvability of hydrogen in the liquid aluminum is far larger than in the solid aluminum, when melting, the hydrogen in the original ingot keep in the liquid aluminum alloy, laying foundation for the formation of the bubble; some alloying agent have important influence to the heredity of the pore, for example, Sr can cause the pore heredity of alloys.The processing parameters of the crystallization under pressure include the magnitude of pressure, the speed of the pressurization, the time of pressure maintaining, the pouring temperature of metal, the temperature of die and anti-friction etc, these processing parameters influence the quality of the work pieces directly, given the wrong processing parameters, casting defect will appear, and the orgnization and the mechanical performance of the work pieces will grow bad.When pressuring during the crystallization, the organization of the metals and alloys vary. The pressure can cause the formation of the columnar crystals of the inner part of the casting pieces with thick wall and metals and alloys with low temperature conductivity, and it can make the fir-tree crystal broken up or shed off and thus make the grain refined; except the normal segregation, the pressure can reduce the microsegregation, gravity segregation and negarive segregation in the crystallization process; when the liqudi metal crystalling under the pressure, a part of plastic yield caused of the pressure makes regional aggregation of the lattice defect of casting pieces.When pressuring during the liquid metal crystallizing, everywhere the pressure of the casting pieces distribute unevenly, from the formula of the pressure on the casting piece, we can find that there are a significant part of the pressure on the casting piece is used to overcome the friction; futhermore, the high pressure can also make the thicker crystallizing layer transformed, thus the pressure loss appear while the plunger piston squeezing; a part of the pressure from compression drift pin to the cast ingot has to be used for the plastic yield of the crystallizing layer, so the effective extrusion force of the crystallizing liquid metal is the difference of the total pressure from the drift pin and pressure of the tubal crystallizing layer, to reduce the pressure loss of the casting pieces, it is better to raise the effective pressure of the last solidifing liquid metal.This thesis mainly studies defects of the aluminum casting pieces and its influence factor, analyse the influence of the pressure during the crystallizing of aluminum casting pieces importantly, after the pressure acts on the fused mass in the crystallization process, the defects of casting pieces ruduce and the the mechanical performance of casting pieces rise, this can provide a worthy reference for the research on the changing performance of aluminum casting pieces, and provide a background foundation for the develop of the follow-up research.