摘要
挤压铸造是一种结合铸造与锻造技术特点于一体的材料精确成形技术,其特点是,液态金属在高压下结晶成形,得到的铸件缩孔减少、组织致密、力学性能优良,并可实现“近净成形”。电磁泵可以用来传输液态金属并可将其应用于铸造工艺。将电磁泵传输与充型技术运用于挤压铸造成形工艺,优点如下:电磁力作用下传输液态金属,传输较为平稳,传输过程中有效减少了氧化膜和卷气的生成;液态金属的流速和流量精确连续可调;液态金属流经磁场,铸件组织和性能得到改善等。本文介绍了一种电磁充型间接挤压铸造成形工艺,将电磁传输充型与挤压铸造技术结合起来,针对某大型复杂铝合金支架,进行了该支架电磁充型间接挤压铸造成形工艺的研究。
运用FLOW-3D软件对液态金属在T形管道中流动并传输至压室的过程进行了数值模拟。对数值模拟结果进行了液态金属传输流动行为的分析,重点讨论了传输过程中氧化膜和卷气产生的原因。研究表明:随着传输管道转角处过渡圆弧半径的增大,所产生的氧化膜与卷气逐渐减少;在数值模拟结果分析基础上,优化设计了电磁传输系统中传输管道的结构,并对其进行了数值模拟验证。验证结果表明:所优化设计的传输管道结构能有效减少氧化膜和卷气的生成。
运用JSCAST铸造模拟软件对铝合金支架进行了间接挤压铸造充型过程的数值模拟,确定了如下工艺参数:铝液的初始温度750℃,压射速度0.04m/s,模具预热温度280℃,型芯预热温度300℃。对数值模拟结果进行分析,预测了铸件的最后充型位置以及金属液提前凝固位置。然后,对支架进行了凝固过程的数值模拟,对铸件边缘至料饼方向的6个记录点凝固时的实时温度进行了比较分析,结果表明:所确定的工艺参数较为合理。铸件在凝固过程中,铸件各部位的凝固顺序为:边缘→中心→料饼依次凝固,凝固顺序十分合理。通过对充型和凝固过程的数值模拟结果分析,确定了溢流槽和排气槽的位置,设计了支架挤压铸造模具并完成了模具的外协制造。
采用ZL101铝合金对支架进行了电磁充型间接挤压铸造的试制,在试制过程中验证了电磁传输系统传输铝液和传输管道中的铝液自动回流的可靠性。试制后得到的支架铸件表面光洁,未出现欠铸、冷隔等铸造缺陷。试制结果表明:所设计的传输管道的结构、支架挤压铸造模具设计和电磁充型间接挤压铸造成形工艺参数合理。
Squeeze casting is a precision fabrication technology, which combined casting and forging processes. The casting solidification is promoted under high pressure within a reusable die, therefore, squeeze casting has many advantages such as porosity reduction, fine grain size microstructure, improvement of mechanical properties and near net shape castings. Electromagnetic pump can be used for casting process to transfer molten metal. This technology integrated in squeeze casting process has many advantages as the following: the molten metal flows smoothly with electromagnetic pressure and in a closed channel, therefore, oxidation and gas entrainment were reduced effectively; the velocity and volume of the molten metal can be controlled accurately and continuously; the casting microstructure and property are improved after molten metal passing through the magnetic field, and so on. The Aluminum alloy supporting frame referred to this paper has large scale, complex structure and requirement of high mechanical properties, and the electromagnetic transfer system was used for the indirect squeeze casting process of the frame.
The T-junction channel used for the electromagnetic transfer system was designed, then liquid metal flows from the channel to the shot sleeve was simulated with software FLOW-3D. The simulation results were investigated, and the molten metal’s flowing behavior was discussed and then the generation of surface defect and air entrainment was analyzed. The results have shown that the larger of the fillet radius, the fewer of the surface defect and entrapped air. According to the numerical results, the transfer channel structure for the electromagnetic transfer system was designed. It was shown that few oxide defect and air entrainment was generated during the molten metal flowed from the furnace to the sleeve under the design.
The molding filling process of the supporting frame was simulated with software JSCAST with initial molten aluminum temperature of 750℃, injection velocity of 0.04m/s, dies temperature of 280℃and cores Temperature of 300℃. The location of the last filling and pre-solidification were predicted. The solidification process after the filling was simulated, and the temperature evolution of 6 points in sequence of from the casting edge to the biscuit in the casting were recorded. The results showed that the casting solidification is in order of the edge to the central part and then to the biscuit. This is very helpful for the transfer of the squeeze pressure from the punch. The numerical results have shown that the selected parameters are reasonable. According to the results, vents and overflows were designed and then the dies were produced with H13 steel.
The vertical indirect squeeze casting process of the ZL101 Al alloy supporting frame with electromagnetic transfer system was trial-produced. It was found that the electromagnetic transfer system can transfer the molten alloy reliably and the alloy was automatically flowed from the transfer channel to the furnace during the casting solidification process. The castings were trial-produced successfully. It was found that surface of the castings is good and defects such as misrun or cold lap were not found. All these results indicate the feasibility of the designed electromagnetic transfer system, the designed die structure, and the parameters of the indirect squeeze casting process.
引文
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