【作者】 赵忠；

【导师】 樊自田； 董选普；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2010， 博士

【摘要】 消失模铸造(Lost Foam Casting,简称LFC)具有其独特的优点,被认为是“21世纪的铸造新技术”及“铸造中的绿色工程”。近年来,铝(镁)合金消失模铸造技术得到了快速发展,显示出了强大的生命力,具有广泛的应用前景。然而,制约铝(镁)合金消失模铸造技术发展的主要瓶颈有充型能力较差,组织晶粒粗大,针孔、缩孔或疏松等缺陷较严重,导致铸件的力学性能偏低。本文从研究振动场、压力场对铝(镁)合金消失模铸造组织和性能的影响入手,以改善铝(镁)合金消失模铸件组织和性能为目的,系统地研究了机械振动场和压力场作用下铝(镁)合金消失模成形理论及工艺,开发了振动场和压力场复合作用下铝(镁)合金消失模铸造新技术,并通过合金化、热处理等方法强化铸件力学性能,形成了特种消失模铸造生产高强度铝(镁)合金铸件的新方法,针对典型零件,确定了最佳工艺参数。研究了A356和AZ91D合金消失模铸造机械振动凝固工艺。研究结果表明：对中小复杂铸件,浇注温度在730-760℃时,采用频率30-60Hz、振幅0.11-0.45mm、峰值加速度1-4g (g=9.8m/s2)的垂直振动,对其组织细化、性能改善、流动性提高等效果明显。A356和AZ91D合金消失模铸造振动凝固铸态最大抗拉强度达到182MPa和165MPa,比普通消失模铸造的分别提高了25%和22%。但峰值加速度大于4g的强振动,容易导致铸件变形、粘砂、气孔、夹杂等缺陷,降低了铸件力学性能。通过对铝(镁)合金消失模铸造振动凝固的理论分析,建立了振动峰值加速度与真空负压之关系式,表明真空度是保证振动稳定性的关键因素。建立了振动破碎枝晶的力学模型,模型表明振动峰值加速度越大,枝晶所受应力越大,则枝晶更容易被折断。建立了机械振动影响消失模铸造铝(镁)合金充型能力的数学模型,模型表明峰值加速度和振幅增加,合金充型能力增强。研究了A356和AZ91D合金消失模铸造压力凝固工艺。结果表明：对中小复杂铸件,如果浇注温度取730-760℃,压力值大于0.5MPa,加压速率为0.003-0.03MPa/s,加压开始于凝固初期(即固相率小于20%),保压时间大于10min,则铸件综合性能较好。A356和AZ91D合金消失模铸造压力凝固铸态最大抗拉强度达到185MPa和174MPa,比普通消失模铸造的分别提高了27%和29%。通过对铝(镁)合金消失模铸造压力凝固的理论分析,建立了枝晶间缩松补缩最小外加压力的数学模型,模型表明在凝固时间较短的凝固初期,所需压力值较小。建立了消失模铸造压力凝固铸件表面凹陷的数学模型,表明凹陷量与铸件的厚度成正比。铝合金消失模铸件针孔主要由析出性氢孔和缩孔合成,在压力凝固下针孔基本消失。研究了振动-压力复合凝固的铝(镁)合金消失模铸造新工艺,成形了小型复杂薄壁铸件。研究结果表明：消失模振动-压力复合凝固铸件的浇不足、针孔、缩孔及疏松等缺陷得到改善,组织晶粒得到细化。通过充型与凝固模拟软件对缺陷进行初步预测,并改进浇注系统和增设冒口加以有效防范,达到了改善铸件综合性能的目的,验证了消失模铸造振动压力复合凝固工艺的可行性。研究了振动-压力复合凝固的铝(镁)合金消失模铸件的热处理工艺。结果表明：A356铝合金消失模振动压力凝固铸件T4抗拉强度达到272MPa,断后伸长率6.5%。0.6%Mg改性的A356合金消失模振动压力凝固铸件T6抗拉强度301MPa,断后伸长率2.6%。0.5%Y改性的AZ91D镁合金消失模振动压力凝固试样T4和T6抗拉强度分别为236MPa和252MPa,断后伸长率分别为5.1%和2.1%。研究了铝(镁)合金消失模振动凝固组织半固态等温热处理。结果表明：A356铝合金在580℃保温60min,AZ91D+0.5% Y改性镁合金在570℃保温60min,α晶粒显著球化,其尺寸和圆度达到半固态组织要求。当消失模铸造A356铝合金采用550℃保温时间2-6h,以及消失模铸造AZ91D+0.5% Y改性镁合金采用430℃保温10-16h的近半固态热处理后,其抗拉强度分别达到261MPa和232MPa。研究了0.3%富Ce稀土对消失模铸造A356铝合金的影响。研究结果表明：稀土RE能够有效的变质细化共晶硅,减少铸件针孔,提高铸件力学性能。通过对0.6%Mg和0.3%Y改性A356铝合金消失模振动压力凝固组织性能的研究。结果表明：改性合金中的颗粒相Al3Y对基体起钉扎作用,阻碍了变形时位错的通过,起强化作用,使改性A356铝合金铸态和T6性能显著提高,抗拉强度分别达到171MPa和308MPa。研究了0.5%Y和0.9%Gd改性消失模铸造AZ91D镁合金。结果表明：稀土Gd、Y元素对α-Mg晶粒有细化作用,促使β-Mg17Al12相由连续网状结构转变为断续状和颗粒状结构。生成的Al2Y相和Al2Gd相对晶界具有钉扎作用,防止晶界滑移,增强了合金强度,其铸态抗拉强度比未改性时提高了29%。更多还原

【Abstract】 Lost foam casting (LFC) process has been considered as the New Foundry Technology in 21st Century and the Green Project in Foundry because of its unique advantages. In recent years, the LFC process of aluminum (magnesium) alloy has been developing rapidly, and shows its great vitality and wide application prospect. However, the main obstacles for LFC process development of aluminum (magnesium) alloy are filling ability poorer, microstructures coarser, and the pinhole, shrinkage or loose defects more serious, so the mechanical properties are lower. In order to solve above problems, the vibration and the pressure are applied on the LFC to improve the mechanical properties of aluminum (magnesium) alloy in this dissertation. The technologic theory of LFC process with mechanical vibration and gas pressure were systematical studied, and the new LFC technology of aluminum (magnesium) alloy under vibration-pressure solidification was developed, also the intensified ways of aluminum (magnesium) alloy were investigated by the means of alloying and heat treatment. Using the new LFC technology, the high strength aluminum (magnesium) alloy castings could be produced, and the best technologic parameters were obtained for the typical casting parts.A356 and AZ91D alloys LFC process with vibration solidification were researched. The results show that appropriate pouring temperatures are 730-760℃for middle-small size complex castings. The vertical vibration with frequencies 30-60Hz, amplitudes 0.11-0.45mm and peak accelerations 1-4g (g=9.8m/s2) has the obviouis effect on grains refinement and the improvement of mechanical properties and filling ability. The tensile strengths of A356 and AZ91D as-cast alloys in LFC with vibration solidification are attained to 182MPa and 165MPa, and up by 25% and 22% over that of conventional LFC, respectively. But it could easily lead to the defects such as casting distortion, sticky sand, porosity and inclusions when the peak acceleration is greater than 4g, and these defects should seriously decrease the mechanical properties.During the LFC vibration solidification process, the relationship between the vacuum level and the peak acceleration is established by theoretical analysis. It shows that the vacuum level is a key factor to ensure the stability vibration. The mechanical vibration model of dendrite fragmentation is established, and the model shows that the bigger the peak acceleration is, the bigger the stress of dendrite is also, and dendrites could be broken more easily and grains refinement is better. The mathematical model of the filling ability suiting for aluminum (magnesium) alloy under LFC process with vibration solidification shows that the filling ability enhance as the peak acceleration and the amplitude increasing.The LFC process of A356 and AZ91D alloys with the pressure solidification was researched. The results show that overall performances of the castings are better than conventional LFC process. The good technical parameters include the pouring temperature 730-760℃, the pressure exceeding 0.5MPa, adding pressure rate 0.003-0.03MPa/s, the forcing pressure time in the early solidification, and holding pressure time more than 10min for middle-small size complex castings. The tensile strengths of A356 and AZ91D as-cast alloys in LFC with pressure solidification are attained to 185MPa and 174MPa, and increase 27% and 29% than that of conventional LFC, respectively.The minium pressure of feeding inter-dendritic shrinkages is established by the theoretical analysis of aluminum (magnesium) alloy pressure solidification in LFC. This formula show that the minium feeding pressure is required a small value in the early solidification when the time is less. The model of casting surface depression in aluminum (magnesium) alloy LFC with pressure solidification is expressed, and this model shows that the casting surface sunken quantity is proportional to the casting thickness. The pinholes in aluminum alloy LFC parttings located in hot spot area are a combination of H pinhole and shrinkage, which would be disappeared under pressure solidification.A special LFC process technology with vibration-pressure solidification was studied and some complex thin-walled aluminum (magnesium) alloy castings were produced. The results show that the defects such as the misruns, the pinholes, the shrinkages and looses can be decreased by vibration-pressure solidification in LFC, and the microstructure grains can be significantly refined. By simulating the filling and solidifying process, the casting defects could be preliminarily predicted and be effectively prevented by improving the gating system design. Some complex cylinder castings of A356 and AZ91D alloys have been produced by LFC with vibration-pressure solidification. As a result, the feasibility of the process was verified and the purpose of improving the mechanical properties was achieved.The heat treatment process of aluminum (magnesium) alloy parts by LFC with vibration-pressure solidification was studied. The results show that the tensile strength and elongation of A356 alloy under LFC with vibration-pressure solidification after T4 heat treatment would be got to 272MPa and 6.5% respectively. The tensile strength and elongation of A356 modified 0.6%Mg under LFC with vibration-pressure solidification after T6 heat treatment would be get to 301MPa and 2.1% respectively. The tensile strength of the AZ91D castings modified 0.5%Y in LFC with vibration-pressure solidification after the T4 and T6 would be attained to 236MPa and 252MPa respectively, and their elongations would be attained to 5.1% and 2.1% respectively.The semi-solid isothermal heat treatment (SSIT) process for aluminum (magnesium) alloy in LFC with vibration was studied. The results show that the size and the roundness of spheroidized a grains could achieve the requirements of semi-solid forming process when A356 alloy insulated 60min at 580℃and AZ91D+0.5%Y alloy hot at 570℃holding 60min. The mechanical properties of A356 and AZ91D+0.5%Y alloys in LFC could be improved significantly when the nearly semi-solid heat treatment of the alloys is operated such as A356 alloy holding 2-6h at 550℃and AZ91D+0.5%Y alloy holding 10-16h at 430℃. In this way, the tensile strengths of A356 and AZ91D+0.5%Y alloys after the nearly semi-solid heat treatment are attained to 261MPa and 232MPa respectively.The A356 alloy in LFC process was modified by 0.3%RE which is the Ce-rich rare-earth, and the results show that the eutectic silicon of A356 alloy is effectively refined, and the pinholes are reduced and the mechanical properties are improved. The A356 alloy in LFC with vibration-pressure solidification is modified by 0.6%Mg and 0.3%Y, and the results show that the pinning and strengthening effects of Al3Y particles would lead to improving the mechanical properties significantly, and the tensile strengths of the castings as-cast and T6 are get to 171MPa and 308MPa respectively. The AZ91D alloy in LFC process is modified by 0.5%Y and 0.9% Gd, and the results show that rare earth Gd and Y can refine a-Mg grains, and theβ-Mg17Al12 phases are transited from continuous network structure to intermittent and granular structure. The strength of AZ91D modified by 0.5%Y and 0.9%Gd are enhanced because of Al2Gd and Al2Y pinning grains boundary and preventing grains boundary sliding, the tensile strength has increased 29% than unmodified one.更多还原