超声外场下铝合金铸锭中显微疏松的数值模拟和试验研究
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摘要
显微疏松是铝合金铸锭的主要缺陷之一,其不仅影响铸锭的机械性能,而且会增加铸锭的初始疲劳裂纹。铸锭中的显微疏松形成主要有两个原因：液体凝固收缩和氢扩散。本文主要以7050铝合金超声铸造为切入点,旨在通过外场来抑制铸锭中显微疏松的形成,主要研究内容有以下几方面：
1、通过数值模拟,研究了氢浓度,枝晶臂间距以及超声场对7050铸锭中显微疏松的影响规律。当7050铝熔体中氢浓度小于0.12cm3/100g时,铸锭中显微疏松主要是由熔体凝固收缩后补缩不足引起的,此时,枝晶臂间距对铸锭中显微疏松影响将会成为主导因素。
2、通过7050铝合金超声铸造试验,初步获得了超声振动对枝晶臂间距和显微疏松的影响规律。当超声功率为240W时作用效果最好,一次枝晶臂间距和二次枝晶臂间距分别减小了58.1%和73.2%,铸锭横向位置处显微疏松百分比减少了14.3%-47.9%,中心位置处显微疏松百分比降幅为29.3%-44.1%。超声振动减少显微疏松的形成是除气和晶粒细化共同作用的结果,其中超声除气对抑制显微疏松起主导作用,此外,工具杆下方的声流效应显著,对促进液体补缩抑制中心疏松的形成起了积极作用。
3、通过对工具杆端面下的声压计算,低频高功率超声波极大了增加了超声在熔体中空化深度和广度,距离工具杆端面大概150mm处声压才会降低到1.6Mpa(空化阈值)。15kHz超声振动系统在液体中作用的效果比20kHz的振动系统强烈,高功率超声处理的铸锭的组织更加致密,声流效应更加显著。当超声功率为2600W,频率为15kHz时,一次平均枝晶臂间距减小到150μΡm,而二次枝晶臂间距也降低到了60μm,分别减小了66.7%和79.3%。铸锭横截面部分区域已难以观察到二次枝晶臂。铸锭中微疏松的形成得到了进一步抑制,铸锭中横截面处显微疏松百分比减少了30.4%-56.3%,中心显微疏松减少了31.1%-53.3%。
4、通过对工具杆端面的优化,研究了三种不同端面的工具杆对7050铝合金铸锭中显微疏松形成的影响,实验结果表明,平端面工具杆的轴向声强最大,且有明显的声流现象,抑制中心显微疏松形成的效果最好,然而在横向位置处,采用球端面工具杆施振效果较好,整个横截面显微疏松分布较均匀。虽然圆锥端面工具杆的作用范围最大,但其声强效果最弱,衰减最快,抑制显微疏松形成的效果最差。
Micro-porosity in castings is a major defect since it affects the mechanical properties, in particular the initiation of fatigue cracks. It is induced by two mechanisms, solidification shrinkage and gas segregation. This paper is aim to restrain the micro-porosity formation by introducing ultrasonic field into direct-chill casting. The main study is shown as follows:
1. The effect of various casting process parameters on micro-porosity formation for aluminum 7050 alloy is studied. The process parameters include hydrogen content, dendrite arm spacing and ultrasonic vibration. When the hydrogen content is below 0.12cm3/100g in the 7050 aluminum melt, micro-porosity is mainly caused by the solidification shrinkage and dendrite arm spacing plays a leading role. Ultrasonic grain refinement and degassing effect can greatly inhibit the formation of micro-porosity.
2. The functionary mechanism of ultrasonic vibration on the formation of micro-porosity in 7050 alloy is study. Experimental results showed the effect of restraining the micro-porosity formation is best when applying vibrating power 240W. PDAS and SDAS are reduced 58.1% and 73.2% respectively. The cross-section micro-porosity percentage is reduced by 14.3%-47.9% and the central micro-porosity percentage reduced by 26.3%-45.0%. The ultrasonic grain refinement and degassing effect play a (?) (?) in restraining the formation of micro-porosity and the later plays a leading role. In addition, the large-scale acoustic streaming is formed under the radiator's face. It can also contribute to improve liquid feed and restrain the formation of the central micro-porosity.
3. By calculating the sound pressure field distribution in the aluminum alloy melt induced by the radiator. High power low frequency ultrasound can increase significantly cavitation depth and scope. The pressure reduce to 1.6Mpa (cavitation threshold) until 150mm from tool rod end face. Ultrasonic vibration of 15kHz is better than that of 20kHz and high power ultrasound apparently increased the density of casting. In addition, the large-scale acoustic streaming is more significant. When applying vibrating power 2600W, PDAS reduced to 150μm and SDAS also drop to 60μm. They reduced 66.7% and 79.3% respectively. It is difficult to observe SDA in ingot casting cross section. The formation of micro-porosity has been further inhibited. The cross-section micro-porosity percentage is reduced by 30.4%-56.3%and the central micro-porosity percentage reduced by 31.1%～53.3%.
4. Micro-porosity of 7050 aluminum melt is studied by three optimized tool rod end faces. Experimental results showed axial sound intensity of the flat tool rod end face is the largest and large-scale acoustic streaming is formed under the radiator's face. Its effect of reducing DAS and inhabiting the formation of micro-porosity is the best; However, the intensity of spherical tool rod end face is stronger in cross-section. The effect zone of the conical tool rod end face is the largest, but sound Wave radiates in a small intensity and its attenuation is the fastest. Its inhibited effect is the Worst.
1、通过数值模拟,研究了氢浓度,枝晶臂间距以及超声场对7050铸锭中显微疏松的影响规律。当7050铝熔体中氢浓度小于0.12cm3/100g时,铸锭中显微疏松主要是由熔体凝固收缩后补缩不足引起的,此时,枝晶臂间距对铸锭中显微疏松影响将会成为主导因素。
2、通过7050铝合金超声铸造试验,初步获得了超声振动对枝晶臂间距和显微疏松的影响规律。当超声功率为240W时作用效果最好,一次枝晶臂间距和二次枝晶臂间距分别减小了58.1%和73.2%,铸锭横向位置处显微疏松百分比减少了14.3%-47.9%,中心位置处显微疏松百分比降幅为29.3%-44.1%。超声振动减少显微疏松的形成是除气和晶粒细化共同作用的结果,其中超声除气对抑制显微疏松起主导作用,此外,工具杆下方的声流效应显著,对促进液体补缩抑制中心疏松的形成起了积极作用。
3、通过对工具杆端面下的声压计算,低频高功率超声波极大了增加了超声在熔体中空化深度和广度,距离工具杆端面大概150mm处声压才会降低到1.6Mpa(空化阈值)。15kHz超声振动系统在液体中作用的效果比20kHz的振动系统强烈,高功率超声处理的铸锭的组织更加致密,声流效应更加显著。当超声功率为2600W,频率为15kHz时,一次平均枝晶臂间距减小到150μΡm,而二次枝晶臂间距也降低到了60μm,分别减小了66.7%和79.3%。铸锭横截面部分区域已难以观察到二次枝晶臂。铸锭中微疏松的形成得到了进一步抑制,铸锭中横截面处显微疏松百分比减少了30.4%-56.3%,中心显微疏松减少了31.1%-53.3%。
4、通过对工具杆端面的优化,研究了三种不同端面的工具杆对7050铝合金铸锭中显微疏松形成的影响,实验结果表明,平端面工具杆的轴向声强最大,且有明显的声流现象,抑制中心显微疏松形成的效果最好,然而在横向位置处,采用球端面工具杆施振效果较好,整个横截面显微疏松分布较均匀。虽然圆锥端面工具杆的作用范围最大,但其声强效果最弱,衰减最快,抑制显微疏松形成的效果最差。
Micro-porosity in castings is a major defect since it affects the mechanical properties, in particular the initiation of fatigue cracks. It is induced by two mechanisms, solidification shrinkage and gas segregation. This paper is aim to restrain the micro-porosity formation by introducing ultrasonic field into direct-chill casting. The main study is shown as follows:
1. The effect of various casting process parameters on micro-porosity formation for aluminum 7050 alloy is studied. The process parameters include hydrogen content, dendrite arm spacing and ultrasonic vibration. When the hydrogen content is below 0.12cm3/100g in the 7050 aluminum melt, micro-porosity is mainly caused by the solidification shrinkage and dendrite arm spacing plays a leading role. Ultrasonic grain refinement and degassing effect can greatly inhibit the formation of micro-porosity.
2. The functionary mechanism of ultrasonic vibration on the formation of micro-porosity in 7050 alloy is study. Experimental results showed the effect of restraining the micro-porosity formation is best when applying vibrating power 240W. PDAS and SDAS are reduced 58.1% and 73.2% respectively. The cross-section micro-porosity percentage is reduced by 14.3%-47.9% and the central micro-porosity percentage reduced by 26.3%-45.0%. The ultrasonic grain refinement and degassing effect play a (?) (?) in restraining the formation of micro-porosity and the later plays a leading role. In addition, the large-scale acoustic streaming is formed under the radiator's face. It can also contribute to improve liquid feed and restrain the formation of the central micro-porosity.
3. By calculating the sound pressure field distribution in the aluminum alloy melt induced by the radiator. High power low frequency ultrasound can increase significantly cavitation depth and scope. The pressure reduce to 1.6Mpa (cavitation threshold) until 150mm from tool rod end face. Ultrasonic vibration of 15kHz is better than that of 20kHz and high power ultrasound apparently increased the density of casting. In addition, the large-scale acoustic streaming is more significant. When applying vibrating power 2600W, PDAS reduced to 150μm and SDAS also drop to 60μm. They reduced 66.7% and 79.3% respectively. It is difficult to observe SDA in ingot casting cross section. The formation of micro-porosity has been further inhibited. The cross-section micro-porosity percentage is reduced by 30.4%-56.3%and the central micro-porosity percentage reduced by 31.1%～53.3%.
4. Micro-porosity of 7050 aluminum melt is studied by three optimized tool rod end faces. Experimental results showed axial sound intensity of the flat tool rod end face is the largest and large-scale acoustic streaming is formed under the radiator's face. Its effect of reducing DAS and inhabiting the formation of micro-porosity is the best; However, the intensity of spherical tool rod end face is stronger in cross-section. The effect zone of the conical tool rod end face is the largest, but sound Wave radiates in a small intensity and its attenuation is the fastest. Its inhibited effect is the Worst.
引文
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