High-speed maglev trains are a key development target in China's transportation field. Rocker arm shell casting is a key load-bearing component of 600km/h high-speed maglev train operating system. It has high requirements on mechanical properties and fatigue performance of body. Quality of castings is directly related to safety and reliability of the entire vehicle operation. In order to break through foreign supply blockade, realize independent supply guarantee of key components of China's wheel-rail-less high-speed trains of 600km and above, form independent intellectual property rights and complete technological iterative upgrades, localized development of rocker housing castings is of great significance. In addition to requiring high mechanical properties, rocker arm shell castings also have clear requirements for fatigue performance and reliability of castings. This topic takes rocker arm shell castings as research object. Taking into account alloy properties and structural characteristics and service requirements of castings, analysis and research are carried out from aspects such as pouring technology and melt processing, which can provide process design reference for key components of high-speed maglev train operating systems.
ZL105A alloy rocker arm shell is produced using low-pressure casting. In view of structural characteristics and performance requirements of ZL105A alloy and castings, optimization was carried out from four aspects: melt refining, alloying, casting process design and ProCAST simulation, and key points of casting process of rocker arm shell castings were analyzed.
Graphical results
The overall dimensions of rocker arm housing casting are approximately 1200mm * 160mm * 80mm (see Figure 1). Casting has a flat plate structure as a whole, and wall thickness is small (average wall thickness is about 4mm). There are many hot spots, and it is a Class I casting. Required mechanical properties of body: tensile strength ≥ 280MPa, yield strength ≥ 235MPa, elongation ≥ 3%. Castings need to withstand at least 10 million fatigue cycle loads, and fatigue performance of casting body is required to meet: when R=-1 (fatigue test stress ratio) and maximum fatigue stress is 90MPa, fatigue life is not less than 107 times. Use a resistance furnace for alloy smelting. Put high-purity aluminum, Al-12Si, and Al-50Cu master alloy into a cast iron crucible, raise temperature to 850℃ and keep it until completely melted, stir for 30 minutes; cool to 700℃, add pure Mg, and stir for 15 minutes. Using different mass fractions (0.6%, 0.78%) of refining agent (C2Cl6) and Ar gas refining injection for 20 minutes to degas and remove slag from melt, add Al-5Ti-1B intermediate alloy wire for refinement, use low pressure +sand casting production, measured chemical composition of castings is shown in Table 1.

 

Figure 1 Three-dimensional model of rocker arm housing casting

Table 1 Chemical composition of ZL105A alloy (%)

 

(a) C2Cl6 refining (b) C2Cl6 (30% more) + argon refining
Figure 2 Analysis of fracture morphology under different refining processes

 

(a)No Be  (b)w(Be)=0.15%
Figure 3 Metallographic structure of ZL105A alloy before and after adding Be

Table 2 Hydrogen content in the ZL105A alloy aluminum liquid before and after adding Be (μg/g)
There is a hot joint area in rocker arm shell casting, so an ingate is installed below hot joint to avoid internal shrinkage caused by insufficient feeding. Since height of casting is only 160mm, the width is 80mm, and average wall thickness of facade is only 4mm, taking into account ease of modeling, unpacking and effectiveness of feeding, we use sand molding, a bottom injection + slot type pouring system, and a low-pressure casting method (see Figure 4). Figures 5 and 6 respectively show temperature field and solid phase rate during solidification process of rocker arm shell. It can be seen that temperature gradient distribution of rocker arm shell casting during solidification process after filling with aluminum liquid is reasonable, and final solidified area is concentrated on the bottom gating system. This can ensure sequential solidification of casting, obtain better structure and higher mechanical properties. Figure 7 shows possible locations of shrinkage porosity based on Niyama criterion. Simulation results show that under this process condition, hot joints are effectively fed, and casting as a whole has no shrinkage porosity. Figure 8 shows pressure distribution and transmission of molten aluminum during simulated mold filling process. It can be seen that during low-pressure filling, aluminum liquid pressure gradually decreases from bottom to top, establishing a good sequential solidification pressure gradient, which can achieve effective feeding of castings.

 

Figure 4 Schematic diagram of rocker arm shell gating system design

Table 3 Pouring process parameters

 

Figure 5 Temperature field during solidification process of rocker arm shell

 

Figure 6 Solid phase ratio during solidification process of rocker arm shell

 

Figure 7 Shrinkage rate of rocker arm shell during solidification process

 

Figure 8 Pressure distribution during rocker arm shell casting process
Based on above simulation results, it can be seen that pouring process design of rocker arm shell casting is reasonable. In addition, in order to further ensure internal quality of castings during actual production process, a conformal cold iron is installed in thick part (thickness of cold iron is 10mm, see dark area in Figure 9) to speed up chilling; at the same time, filters are installed at intersection of lateral runner and inner runner to reduce inclusions in castings.

 

Figure 9 Schematic diagram of layout of cold iron and cutting position of body

 

Figure 10 Qualified rocker arm housing casting

Table 4 Mechanical properties and fatigue life of casting body samples

 

Figure 11 Tensile fracture morphology of casting body

 

Figure 12 Body fatigue fracture structure of castings
In conclusion
Rocker arm shell casting is produced according to above process. Actual casting is shown in Figure 10. After X-ray inspection, no shrinkage defects were found inside casting, which is consistent with ProCAST simulation calculation results; no pinholes or inclusions were found in casting, and internal quality complies with requirements of GB/T 9438-2008 Class I castings, achieving a successful trial production. A three-coordinate testing platform was used to conduct dimensional inspection on rocker arm shell, and all dimensions were qualified. Tensile fracture of sample cut from body is shown in Figure 11. It can be seen that there are a large number of tearing edges in the entire section. Fracture surface presents more dimples and small cleavage planes. Dimples are more numerous and deep, showing a mixed fracture characteristic of ductile fracture and cleavage fracture. There are no obvious defects such as shrinkage and porosity on fracture surface, and the overall plasticity is good. Using hexachloroethane and argon gas rotating injection compound refining, combined with Be alloying, comprehensive mechanical properties of ZL105A alloy can be effectively improved. Using a low-pressure sand casting process, combined with a bottom-cast + gap casting system and placing cold iron in thick parts, a rocker arm shell casting with good internal quality can be produced. Tensile strength, yield strength and elongation of casting body can reach up to 318 MPa, 261 MPa and 5.6%. Fatigue performance, internal quality and size of casting body all meet design index requirements.