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Vacuum Low-pressure Shell Casting VS Lost Foam Casting

Posted: 2014-10-13 02:16:59  Hits: 2180
Abstract: The microstructures and properties of A356 alloy produced using expendable pattern shell casting process with vacuum and low-pressure (EPSC-VL) are compared with those of lost foam casting (LFC) process by OM and SEM techniques. The results show that microstructure of A356 alloy produced by EPSC-VL is finer and denser than that of LFC, and the sizes of the primary phase and eutectic silicon are far less than those of LFC, its porosity is less than that of LFC, and its density is higher than that of LFC. The fracture mechanism of A356 alloy obtained using EPSC-VL mainly shows ductile fracture, and that of A356 alloy obtained using LFC mainly shows brittle fracture. The tensile strength, elongation as well as hardness of A356 after T6 treatment alloy fabricated using EPSC-VL are respectively 278.27 MPa, 8.10% and 93.1HB, and are respectively 20.2%, 166.4% and 17.6% higher than those of LFC. Furthermore, the surface roughness of castings made by EPSC-VL is better than that of LFC.
 
Keywords: aluminum alloy, expendable pattern casting, expendable pattern shell casting with vacuum and low-pressure, aluminium alloy casting, gravity casting, vacuum low-pressure expendable pattern shell casting, aluminium alloy investment casting, lost foam casting, vacuum lost foam low pressure casting(VLFLPC)
 
Cast aluminium alloy has excellent casting ability, good corrosion resistance, high specific strength and specific stiffness and it is close to the final shape, and other advantages. Therefore, with the rapid development of aerospace, automobile and other industries, the complicated thin-walled aluminum alloy precision castings have been more and more widely used. The expendable pattern shell casting with vacuum and low-pressure is a kind of new method that is suitable for production of complex thin-walled aluminum (magnesium) alloy precision casting. It is a new technology that combines with the Epc bubble shape precision forming technology, investment casting precision shell-making technology and low pressure casting. First of all, take the foam of the epc mode as the prototype and use investment casting shell-making technology in the bubble prototype surface crusting. After the loss of mold and roasting, pull shell into the sand box to form sand filling model, and then finally the liquid metal forms shapes under the dual force of vacuum and pressure. (Fig.1). The technology combines with advantages of the foam pattern, such as lower cost, small shrinkage, flexible size structure design and investment casting ceramic shell with high precisionseramic, etc. Being off the bubble shape before pouring, it can solve too many defects of lost foam casting such as holes, inclusion and pouring with too high temperature, etc. At the same time, the liquid metal filling gas under the double pressure of vacuum and mold filling gas is conducting the filling and solidification, so the filling and feeding ability greatly increases. As a result, it can obtain high quality castings. In the article, the organization and performance of A356 aluminum alloy are analyzed by two kinds of processes, namely aluminum alloy between expendable pattern shell casting with vacuum and low-pressure and lost foam casting.
 
1 Test
Density of foam molding technology preparation is like 0.05 g/cm3 bubble. The bubble shape material is expandable Polystyrene. Preparation ceramic shell in the surface of foam pattern, after loss of mold, and roasting, put it into the sand box, fill in the dry sand, and conduct vibration ramming. The alloy material is A356 (See chemical component in table 1). Then, put the preheating aluminum alloy into stainless steel crucible to melt. Use the Sr metamorphic, argon refining degassing. Push the sand box to low pressure casting location for pouring. During the test, the shell is not preheated. Mould filling pressure and vacuum degree are 0.04 and 0.02 MPa. The density of foam mould used for lost foam casting is 0.025 g/cm3, pouring temperature is 750 , and vacuum degree is 0.02 MPa. It is used gravity casting. The above two kinds of casting processes adopted by the specimen size and alloy processing mode are the same under the same conditions.

Table 1 Chemical composition of A356 alloy (mass fraction, %)
Si Mg Ti Sr Fe Al
7.10 0.31 0.23 0.05 0.17 Bal
 
Intercept the metallographic specimen from the joints of the tensile test specimen, use 0.5% HF (mass fraction) solution for corrosion, and use Me F - 3 type metallographic microscope for microstructure observation. Use Image Tool analysis software to measure primary phase grain average size A, then use the formula D = 2 (A/PI) 1/2 to calculate the average equivalent diameter D of the primary phase grain size. The smaller D, the smaller grain size, if not, the grain size is larger. Use JX - 2000 analysis software to measure the specimen fracture porosity. Specimen density is calculated by the Archimedes' principle.
 
Tensile specimen is a standard test bar as d10 mm. The tensile specimen is T6 heat treatment ((quenching solution treatment538 , 12 h)+ (air cooling aging treatment 165 , 6 h)), tensile test in WE - 100 type 600 kN universal material testing machine. The stretching rate is 2 mm/min. Test the specimen’s HB by HB - 3000 type hardness testing machine. Use QUAN TA - 400 type scanning electron microscope to observe eutectic silicon morphology in the organization. Analyze the fracture morphology and fracture surface composition of the tensile specimen.
 
2 Result and analysis
2.1 Microstructure
Fig.2 is morphologies of eutectic silicon of A356-T6 alloy under different processes. From Fig.2(a) showing, the primary phase of epc casting organization is thick dendrite. The grain size reaches to 327.1 μm, and there are many hole defects in the organization, and the size is larger. By contrast, from Fig.2(b) showing, the organization of expendable pattern shell casting with vacuum and low-pressure is relatively small, the grain size is 147.2μm, moreover, the structure is more compact, and the hole defects is less. Compared with the organization after T6 treatment, we can see T state organization in accordance with as-cast organization, eutectic silicon is morphology spheroidizing. After heat treatment, the distribution is more homogeneous.The epc casting organization still exist gross dendrites and larger holes (See Fig.2c), and the grain size is 310.5 μm. The organization of the vacuum low-pressure EPC shell mold casting is small and compact after T6 heat treatment (See Fig.2d), and the grain size is only144.4μm. The grain size has little change before and after heat treatment. In Fig.3, we can find the morphologies of eutectic silicon in the organization of the vacuum low-pressure epsc are less than lost foam casting, and the Silicon particle shape is round, and the size and distribution is compact.

Table 2 is comparison of density and porosity of A356 alloy under different processes. From table 2, we can see the vacuum low-pressure epsc’s pore rate is only 0.16%, less than 1.97% of epc porosity; and the density is 2.684 g/cm3, above the density 2.660 g/cm3 of the epc. As a result, the density of vacuum low pressure mold shell mould castings is superior to the density of epc It is in accordance with the law of Figure 2.
 
2.2 Appearance of fracture
Fig.4 is the low magnitude fracture morphologies of A356 alloy under different processes. From Fig.4, we can see epc tensile fracture has more holes, and the hole size is larger and deeper. It distributes in the whole section (See Fig.4a); but the vacuum low-pressure epsc tensile fracture has less holes (See Fig.4b), which is in accordance with the above law. Fig.5 is high magnitude fracture morphologies of A356 alloy under different processes. From Fig.5, we can know the tensile fracture of A356 aluminum alloy with vacuum low-pressure epc appear obvious dimple, and the dimple is deeper. The distribution is homogeneous, and the fracture mode is ductile fracture (See Fig.5d). By contrast, the tensile fracture’s dimple of lost foam casting A356 aluminum alloy is unconspicuous. The dimple is less and shallower, only is distributed in local area. And the fracture mode is brittle fracture (See Fig.5a), and there are defects of shrinkage cavity and inclusion (See Fig.5b and Fig.c).



2.3 Mechanical property
Table 3 is the comparisons of mechanical properties of A356 alloy under different processes. From table 3, we know the vacuum low-pressure epsc’s mechanical properties of A356 aluminum alloy have more obvious advantages than that of lost foam casting. The tensile strength, elongation and brinell hardness are 278.27 MPa8.10 % and 93.1HB after heat treatment, compared with the lost foam casting increased by 20.2%, 166.4% and 20.2% respectively. In addition, the vacuum low-pressure epsc with high density foam and high precision ceramic shell mold, therefore, the casting surface quality is better than that of casting surface quality.

3 Discussion
3.1 Influence of the foam pattern
In the pouring process of epc, foam pattern decomposes a lot of gas. If these gas cannot escape through the coating layer very well, it will stay in the casting and form defects the hole defects, and the shape decomposition is not complete. It can also bring the inclusion defects if residues are in the castings (See Fig.5c), which will observably reduce the casting performance. The vacuum low-pressure epsc takes off the foam pattern before pouring, so it can avoid above defects because of the foam pattern decomposition. Otherwise, in the pouring process, vacuumizing can eliminate gas inside of the cavity and reduce the hole defects. As a result, defects such as holes and inclusions of the new process for casting are greatly reduced, and the mechanical properties also have greatly improved.
 
3.2 Effects of vacuum and pressure
Aluminum alloy castings are easy to produce pinhole defects because the dissolved hydrogen atoms in the liquid alloy separate out hydrogen molecule in the casting solidification process. They aggregate into hydrogen bubble, and finally form pinhole in the cast. The hydrogen bubble formation conditions is:
pH2p1=p0+p2+γH+2σ/R      (1)
In the formula: pH2 is the pressure in the hydrogen bubbles of the liquid aluminum; p1 is the sum of external pressure for bubbles; p0 is atmospheric pressure; p2 is max pressure; γ is the proportion of molten aluminum; H is the height of molten aluminum above the bubbles; σ is aluminum liquid surface tension; R is bubble radius.
Usually, γH and σ in the formula (1) is very small, and it is negligible. So, formula (1) can be simplified as
pH2p1=p0+p2    (2)
From formula (2), we can know that, when increase max pressure p2, then, p1pH2. It can effectively inhibit the formation of hydrogen bubble. In the process of the vacuum low-pressure epsc, molten metal filling and solidification is under the dual function of the vacuum and pressure,and the max pressure is larger. But the epc filling and solidification only are through its dead load under negative pressure. Therefore, the vacuum low-pressure epc pinhole defects are reduced greatly compared with lost foam casting.
Otherwise, the molten metal feeding in the process of solidification is through the molten metal flowing in the dendritic crystal. If the feeding pressure of the molten metal is insufficient in the process of solidification, and the feeding is not sufficient, the casting is easy to produce defects such as shrinkage, sinkhole. We can know it as the follow formula.

In the formula: Gsc, Rsc and Psc is critical of solid temperature gradient, cooling rate and pressure respectively, and Kc is the number of criterion shrinkage. Under the certain condition of mold, Gsc and Rsc is relatively stable, Psc is the main factors influencing the casting shrinkage defects. Molten metal solidifications under certain pressure, which can drive the molten metal in liquid-solid phase to enter the solid skeleton gap to feeding, and then prevent shrinkage and shrinkage cavity defects. With the increase of external pressure, the feeding capacity of the molten metal in the dendritic crystal increases. It makes the casting shrinkage and sinkhole defects be greatly reduced. Thus, the casting density is improved. In the process of the vacuum low-pressure expendable pattern shell casting, because the molten metal is feeding under the dual role of vacuum and pressure,and the feeding pressure is big. As a result, the casting is very dense and the mechanical properties have been improved greatly.
 
3.3 The effect on molten metal temperature
  Fig.6 is cooling curves of molten metal under different processes. From Fig.6, we can see the cooling speed of the molten metal in the vacuum low-pressure epc shell mold casting is obviously faster than LFC, and the cooling speed of the LFC is lower. It is connected with the cooling speed of the powder dry sand. So, the grain size of A356 aluminum alloy with vacuum low pressure mold shell mold casting is smaller and has better mechanical properties than that of the lost foam casting.
 
4 Summary
1) The A356 aluminium alloy organization of expendable pattern shell casting with vacuum and low-pressure is smaller and more densification than lost foam casting. The vacuum low-pressure epsc’s pore rate is only 0.16%, more less than epc porosity 1.97%;
2) The tensile fracture of A356 aluminum alloy with vacuum low-pressure epc appear obvious dimple, and the dimple is deeper. The distribution is homogeneous. However, the fracture mode is ductile fracture after T6 heat treatment. But the tensile fracture’s dimple of lost foam casting A356 aluminum alloy is unconspicuous, and the dimple is less and shallower, only being distributed in local area and the fracture mode is brittle fracture.
3) For vacuum low-pressure epsc’s mechanical properties of A356 aluminum alloy, its tensile strength, elongation and brinell hardness are 278.27 MPa, 8.10 % and 93.1HB after heat treatment, increasing by 20.2%, 166.4% and 20.2% respectively compared with the lost foam casting. The casting surface quality is better than that of die-lost casting.

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