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Effect of Investment Casting of ZL201A Alloy

Posted: 2014-10-24 21:34:12  Hits: 1609
Abstract: Effect of investment casting on the microstructure and mechanical properties of ZL201A alloy at as-cast, solid solution and aged state were analyzed, and compared with hose in sand mold casting and in metal mould casting. The experimental results show that the grain size of investment casting samples is coarser than that of ones in sand mold samples and in metal mold samples regardless of as-cast, solid solution or aged state. The tensile strength and yield strength of investment casting as-cast samples are slightly lower than those in sand mold samples and in metal mold samples, while the elongation is near to that of metal mold samples, however it is higher than that of sand mold samples no matter in as-cast state, solid solution sate or aging state. The hardness value of the investment casting samples in higher than that of sand mold samples and metal mold samples. After solid solution, the tensile strength, yield strength, elongation and hardness are improved by 1.4, 2.0, and 1.2 times and by 1.2 times respectively. After aging treatment, the tensile strength, yield strength and hardness is further improved, increased by 2.2, 4.4 times and by 2.1 times, while the elongation is near to that of as-casting samples with only 9%.
Keywords: ZL201A Alloy, sand mould, metal mould, investment casting.
 
Casting aluminum alloy, as the traditional metal materials because of its small density and high specific strength etc, is widely used in aerospace, automobile machinery and other industries. ZL201A has some advantages such as high strength, easily machining and treating surface, high fatigue performance etc. But ZL205A alloy contains V, Zr and other rare metals, especially it is very strict with Fe, Si impurity content restrictions, it must be made with high-quality raw materials such as high purity aluminum. The melting process is strict, production quality is unsteadiness, and the cost is very high. ZL201A alloy has no V, Zr elements etc, wide with impurity content restrictions, and reduces the demand for metal raw materials. Thus the production process is relatively simple. In short, it has very high economic value and a wide range of engineering application potential.
 
ZL210A alloy has been applied in the fields such as aerospace, defense etc. By doing solid solution treatment for ZL201A alloy sand sample with 545 ℃ and 555 ℃ to analysis different solid solution temperature mechanical properties of the sample, microstructure and fracture morphology, and discuss the different solid solution temperature on the alloy mould sample performance and the influence of the organization. Through research of the alloying element and content, such as optimizing design, grain refinement, heat treatment process etc, it makes the tensile strength of ZL201A alloy to be 400 ~ 500 mp after heat treatment T5, and elongation is 10% ~ 16%. Tensile strength is 460 to 500 mp after T6 heat treatment, and elongation is 3% ~ 7%. We use the sand mold and metal mold casting method to research the influence of properties and fracture morphology for ZL201A alloy with as-cast, solid solution and aging states.
 
But the report about investment casting impacts on microstructure and mechanical properties of ZL210A alloy is very little, so this topic makes a study about investment casting impacts on microstructure and mechanical properties of ZL210A alloy. It provides a reference about the alloy uses investment casting technology to produce other casting methods which are difficult to produce the whole complicated thin-walled precision parts.
 
1. Test method
1.1 Alloy smelting and sample preparation
ZL210A is made of high purity aluminum with a purity of 99.99%, AL-50CuAL-10MnAl-5TiAl-5SiAl-5MgAl-5ZnAl-5Z intermediate alloy and pure Cd. The alloy composition are shown in table 1. Use SG2 - S - 12 resistance crucible furnace to smelt alloy liquid. Use C2CL6 refining processing with a pouring temperature of 740 ℃.
 Table 1
 
Use three different methods including investment pattern shell, SLS precoated sand mold and metal mold to pour tensile test bar. Then machine the test bar into astandard tensile samples as Fig.1.
                       Fig.1
 
Heat treatment in the SX2 series box type resistance furnace. Solution treatment: 540±5℃×7h, water-cooling; Aging treatment 155±5℃ ×6.5h, furnace cooling.
 
Use XSP – 4XB type optical microscope to observe the microstructure and metallurgical photographs taken.
 
Figure 2 is microstructure photos of ZL210A alloy under different mold casting and pouring under 740 . From fig.2a, we can know that, investment casting organization grain is relatively bulky and grain boundary is also bulky. There are a lot of segregation eutectic phase in the grain boundary. At the same time, there are a lot of dispersed tiny particle in the matrix. These tiny particles are T - AL12CuMn2 phase particles and theta - AL2Cu phase particles. From fig.2, we can see, after solid solution treatment, grain boundary of investment casting sample is still relatively bulky and there are a large number of dispersed tiny particle in the matrix. These tiny particles are T - AL12CuMn2 phase particles. The tiny T phase particle in the alloy particle generally is located in the grain boundary, some T phase particle in the solid solution body. Alloying element solution into the substrate, to make the substrate metal dislocation density increased, the lattice distortion, the resistance of the dislocation movement increased, the material strengthen. From fig.2c, we can know the shape of ZL210A alloy is regular, grain boundary is tiny and smooth after aging treatment. Compared with the solid solution state, matrix and grain boundary distributed tinier particle. These dispersed tiny particles make the strength and hardness of the alloy to improve further.
 
From fig.2d, fig.2g, we can know, compared with the metallographic structure of investment casting as-cast sample, the grain of sand mold and metal mold as-cast sample is relatively tiny. This is caused by investment casting with red shell pouring. High temperature liquid metal can't quickly cool through the mold which makes the alloy liquid cool slowly in the mold. Longer cooling time makes the grain relatively bulky, and makes the alloy's mechanical properties down. Analysis fig.2e, fig.2h, we can know that,  compared with the microstructure of investment casting solid solution state, the grains of sand sample and metal mold sample have a lot of dispersed tiny particle under the solid solution state. And these small particle distributions are more evenly, grain boundary is also tinier and smoother. The grain of sand mold samples and metal mold samples are relatively tinier than the investment casting after solid solution. From fig.2f, 2i, we can know that, compared with the microstructure of investment casting ageing state, the microstructure grain boundary of sand samples and metal mold samples are tinier and fuzzier.  There are a lot of small particle dispersion distributions in the matrix, and these tiny particles are more even. Aging element through solid solution to separate out even and dispersed tiny particle to increased resistance of the dislocation movement, and making the alloy strength. 
 
 
               
               Fig.2 The microstructure of the sample under different casting method
 
Fig.3 is the effect of three kinds of casting method for strength. From fig.3a, we can know, the tensile strength of investment casting as-cast is 187 Mpa, less than metal mold casting of 220 Mpa and sand casting of 198Mpa. The sample’s tensile strength value of the sand casting, metal mold casting and investment casting are improved obviously by 1.6, 1.5, and 1.4 times that of as-cast. Solid solution treatment effect is remarkable. This is because when the ZL210A alloy is quenching, strengthening phase into the aluminum substrate, In the process of water cooling, because the speed of water cooling is fast, the solid solution strengthening phase is too late to decompose, got the supersaturated solid solution, producing solution strengthening. So it makes the strength elongation and hardness of the alloy, increase obviously. But under the solid solution state, investment casting sample’s tensile strength is lower than that of sand mold and metal mold sample. The reason is that investment casting sample grain size is bulkier than that of sand mold and metal mold, to make the tensile strength lower. After aging treatment, the sample’s tensile strength of sand mold casting, metal mold casting and investment casting is further improved. It is 1.6, 1.5, and 1.4 times that of as-cast, aging treatment effect is remarkable. This is because of the solid solution strengthening elements evenly, separate out diffusely and diffuse distribution of secondary T phase in the alloy to increase resistance of the dislocation movement, and making the alloy strength.
 
                                    (a) Tensile Strength                                                           (b) Yield Strength
                
                                                             Fig.3 The effect of casting method on strength
 
From fig.3b, we can know, investment casting samples under the as-cast condition, its yield strength of 67 Mpa is less than sand samples of 83 Mpa and metal mold samples of 94Mpa. The sample’s yield strength of metal mold and sand mold casting is respectively 1.7, 1.6, and 2.0 times that of the as-cast. Solid solution treatment effect is remarkable. After aging treatment, the yield strength of each sample is further improved. But in the whole view, investment casting yield strength of the sample is lower than the sand mould casting and the metal mold casting, this is related to bulky grain.
 
Fig.4 is the effect of different casting method for hardness value and elongation of ZL210A alloy. From fig.4, we can know, as-cast condition, the difference of the sample’s hardness HV of sand mold, metal mold and investment casting is small, each is 61, 60, 65. After solid solution treatment, the hardness of sand mold, metal mold, casting samples have different degrees of increase, were 1.2 times that of the as-cast. After aging treatment, the hardness of sand mold, metal mold, casting samples were 1.6, 1.3 and 1.2 times that of the as-cast. Aging treatment effect is remarkable, especially the investment casting sample’s intensity value increased obviously after aging treatment. Whether in as-cast state, solid solution and aging state, investment casting sample hardness values are the most excellent, its higher than that of sand mold samples and metal mold samples.
 
                                         (a) Hardness                                                              (b) Elongation
 
                                                 Fig.4 The effect of casting method on hardness and elongation
 
From fig.4, we can know, the sample’s elongation of metal mold and investment casting is 9% under the as-cast state. It is higher than that of sand mold sample by 7%. The samples’ elongation is improved after the solid solution treatment. Sand specimen type, samples’ elongation of sand mold and metal mold are further improved after aging treatment. It is 2.0 and 1.8 times of the as-cast while the elongation of investment casting is the same as the as-cast.

2 Summary.
 
2.1 Whether in as-cast state, solid solution and aging state, the grain structure of investment casting is relatively bulkier than that of the sand mold and metal mold. Grain boundary is also evident after solid solution and aging.
 
2.2 Whether in as-cast state, solid solution and aging state, the tensile strength, yield strength and elongation of investment casting are lower than those of the sand mold and metal mold. The metal mold sample has the most excellent performance, but the investment casting hardness value is higher than that of sand mold and metal mold sample.
 
2.3 Investment casting sample’s tensile strength, yield strength, hardness and elongation are 1.4, 2.0, 1.2 and 1.1 times of as-cast after solid solution treatment. Solid solution treatment effect is remarkable. Investment casting sample’s tensile strength, yield strength, hardness and elongation are 2.2, 2.4 and 2.1 times of as-cast after aging treatment, but elongation is not obviously improved, only by 9%.

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