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Effects of Three Pouring Ways on Thin-Walled Castings

Posted: 2014-11-25 22:22:00  Hits: 1178
In recent years, the parts of aero-engine and automobile engine become lightweight and precision, to make thin wall castings widely used for a range of applications. Many thin wall castings’ wall thickness is only 1.5~5 mm, thus they need higher casting requirements. A guide tube of high-power engine is thin-walled pipe, whose material is heat resistant stainless steel. Alloy melting temperature is higher, and the shrinkage is big. Because the stainless steel casting performance is poorer and the melt viscosity is large, it is easy to cause defects such as porosity, crack, cold shut and misrun for complex thin wall castings. In early testing, we used investment casting method, studied the molding technology and forming process, and got a good stainless steel thin wall casting with good forming. This study mainly comparatively researched different pouring methods of stainless steel thin-walled investment castings from the properties of microstructures.
 
1. Test Methods and Processes
1.1 The process of investment casting
The test object is a type of engine’s guide tube, whose material is heat-resistance stainless steel 0Cr18Ni9. The part structure is shown in Fig. 1. The symmetric structure of guide tube is thin wall parts. The main part of the thickness is 3 mm, and it has groove in the middle.
 
    
The pattern design process can be refereed to the article Making Technology of Complex Thin-Walled Part Casting. Pattern matching ratio: 5% transform polythene, 5% modified rosin, 20% stearic acid, 70% paraffin wax. Pattern process is under the condition of 5MPa wax injection pressure. The holding time 25s, the wax injection temperature is 58, the wax injection speed is 8 mL/s, and the cooling time is 10min. Pouring methods are respectively injection pouring, side injection pouring and bottom. Fig.2 is the pattern picture of the three kinds of pouring methods. The pouring temperature is 1620, the shell temperature is controlled between 850 and 900 , and the pouring time is about 3~4s.
 
     

1.2 Simulation of the filling process
In order to be easier to compare the filling processes of three kinds of pouring system, we use casting simulation software named AnyCasting to simulate the casting mould filling process of castings.
 
1.3 The microstructure observation and casting hardness testing
In order to analyze the effect of organization on pouring way of castings, take the casting’s top, middle and bottom parts as the metallographic analysis samples. Casting axial total height is 160 mm, (top, middle and bottom) three parts’ upper surface of metallographic sample respectively is 150 mm, 80 mm and 10mm from the bottom of the casting. Metallographic corrosion liquid is aqua regia, and the corrosion time is 10s. Use Nikon EPIPHOT 300 digital metallographic microscope to observe the microstructure.
 
Use HB3000 brinell hardness meter to test casting hardness of three parts (top, middle and bottom). Hardness testing samples are the metallographic specimen samples. Hardness test values used in the data analysis are the average value.
 
2. The Results and Analyses of Tests 
2.1 The comparison and analysis for filling process of three kinds of pouring ways
Fig. 4 - Fig. 6 are simulated diagram of top pouring, side pouring and bottom pouring, and the filling 8rate are 45% and 85%.
 
 


 
During top pouring (Fig. 4), liquid metal flows enter into the mould cavity through the runner from the ingate. Affected by gravity, fluid enters along the wall from the gate into the cavity. As the pouring continues, the amount of liquid into the cavity gets larger. Metal liquid as fan-shaped streamline continues to fill the mold, and finally, the filling part are on both sides of runner.
 
During side pouring (Fig. 5), the liquid metal pushes ahead through the sprue from ingate along both sides of the circumferential wall. The liquid level on one side of the runner rises along the vertical wall, and gatheres in the contralateral of runner along the circular metal liquid propulsion.
 
During bottom pouring (Fig. 6), liquid metal firstly enters from sprue, and then enters into the mold cavity from four dispersed ingates through runner. The filling liquid surface rises and fills molds smoothly, which can get the air on the surface of the metal liquid out well. There are four exhaust ducts in the two grooves, which have the function of exhaust and drainage, making gas not deposit in the groove angle. There are four exhaust ducts at inner and outside rising to fill. the final filling parts are casting’s upper circular place, where exhaust passage is needed. Because there are four dispersed and even ingates, liquid flow allotted everywhere is fairly. The direction of liquid surface is kept the same, which makes the whole filling process smoothly. It is not easy to produce gas volume, splash, etc, which will greatly reduce casting defects.
 
2.2 Microstructure analysis
Fig.7 is metallographic organization chart of top pouring casting, side pouring casting and bottom pouring casting. Because of the casting’s material is alloy 0Cr18Ni9, so the casting microstructure mainly is austenite, and distributes strip and island ferrite.
 
 
Fig.7a is the upper organization of top pouring casting, where a little strips and island ferrites distribute unevenly. There is a portion of the oxide inclusions because it forms a serious hotspots on the upper of the casting, which makes nucleation rate of ferrite fall down, and leads to swash severity when pouring. Compared with the middle and upper organization, strips and island ferrites in the organization obviously increase, with little impurities. Bottom of the casting is on frontier chilling area in the process of solidification, where strips and island ferrites are more, the spacing of strip ferrite is smaller and the distribution is homogeneous.
 
Fig. 7b is the middle organization of side pouring casting, where a little strips and island ferrites distribute unevenly.. The ferrite distributions of the bottom organization are more than middle, the ferrites in the upper part of the organization are more than middle-bottom, and the distribution is more homogeneous. The reason is that the ingate is located in middle-bottom of the casting, and metal liquid only enters into the cavity through here, which causes overheating in the casting part, and makes the ferrite nucleation rate fall down. While the ferrite nucleation rate of upper forefront chilling area is high.
 
In Fig.7c, the microstructure morphology of the upper, middle and bottom parts of bottom pouring casting are close. There are lots of strips and island ferrite in the austenitic, with relatively homogeneous distribution and smaller spacing of strip ferrites.
 
2.3 Casting hardness testing
The hardness test results of different pouring casting are shown in Fig. 8. We can see that the bottom pouring castings are in axial direction. The hardness increases from the bottom to upper slightly. The reason is that the upper is frontier chilling area, where the organization is dense, and the hardness is stronger. The axial hardness change trend of top pouring casting is contrary to the bottom pouring casting, whose upper hardness is weakest. The reason is that the upper is the last part of solidification, which is easy to have a casting defects. The axial hardness change trend of side injection pouring casting is similar to the bottom, but the change in the upper middle is very small. 10~80 mm is the ingate height position, which is overheating than the upper and in poor organization, so the hardness is low. As a whole, pouring ways significantly influence on the casting hardness. The average hardness of bottom pouring casting is higher than the side and top.
 
 
3. Summary
(1) The three kinds of pouring ways to get casting microstructures, namely top pouring, bottom pouring and side pouring, have obvious differences. The distribution and quantity of ferrite is related to the solidification order of the casting axial parts that caused by pouring way.
 
(2) Pouring ways significantly influence on the casting hardness. The average hardness of bottom pouring casting is higher than the side and top.

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