Al-Si合金半固態(tài)漿料制備技術(shù)及應用研究
發(fā)布時間：2018-07-16 11:38
【摘要】：近些年能源消耗和環(huán)境污染問題的日益突出,為輕量化材料鋁合金在汽車方面的使用提供了廣闊的空間。半固態(tài)壓鑄由于充型平穩(wěn),凝固收縮小,生產(chǎn)的鑄件致密,可熱處理強化等優(yōu)點,被認為是突破鋁合金在汽車輕量化應用方面限制的主要途徑。然而這項技術(shù)目前還沒有得到廣泛的推廣應用,主要原因在于目前的漿料制備技術(shù)難以滿足高質(zhì)量和高效率的生產(chǎn)要求。本文以常用的鋁硅鑄造合金A357和319s為研究對象,采用基于控制凝固技術(shù)的熱平衡法漿料制備技術(shù),以實現(xiàn)半固態(tài)流變壓鑄成形工程化應用為目的,開展了鋁合金半固態(tài)漿料制備、球狀晶組織演化形成機理、漿料充填行為和半固態(tài)壓鑄件組織與性能方面系統(tǒng)的研究。通過本文研究對進一步完善控制凝固技術(shù)在半固態(tài)漿料制備和推動半固態(tài)流變成形工程應用化方面具有一定的指導意義。本文主要研究的內(nèi)容和取得的研究成果如下:分析了金屬液在熱平衡法制漿過程的冷卻行為,理論推導了熱平衡法過程的傳熱模型,推導出了漿料溫度場的表達式。結(jié)果表明,在給定合金和坩堝材料的條件下,漿料溫度場的分布是澆注溫度和坩堝初始溫度的函數(shù),共同決定半固態(tài)漿料的微觀組織。以A357合金為研究對象,研究了工藝參數(shù)對漿料組織的影響規(guī)律。研究結(jié)果表明:(1)澆注溫度對漿料組織影響最大,隨著澆注溫度的降低,金屬液在液相線附近獲得較大的過冷度,隨著熔體的內(nèi)生流動在坩堝中形成大量的自由晶并保留下來,使?jié){料最終凝固組織為細小、均勻分布的球狀晶組織。采用截線法測量晶粒平均尺寸,當澆注溫度從690℃降到630℃時,邊部晶粒平均尺寸由820μm降到248μm,中部晶粒平均尺寸由706μm降到223μm,心部晶粒平均尺寸由659μm降到186μm。(2)旋轉(zhuǎn)轉(zhuǎn)速對漿料組織的影響根據(jù)澆注溫度而變化,當高于660℃澆注時,坩堝中的熔體具有較低的表觀黏度,隨著轉(zhuǎn)速的增加,熔體的對流效果加強,晶粒組織明顯細化;當?shù)陀?45℃澆注時,坩堝中熔體的表觀黏度增加,旋轉(zhuǎn)過程難以產(chǎn)生明顯的對流效果,轉(zhuǎn)速的增加對漿料組織影響不明顯。(3)隨著旋轉(zhuǎn)時間的延長,小晶粒溶解消失,大晶粒逐漸長大,以Ostwald熟化機制長大,通過LSW模型計算顆粒尺寸變化,得長大速率為22.47μm~3/s。(4)相同制漿條件下,漿料重量主要影響熔體的過冷度,漿料重量越小,澆注過程中受到的激冷效果越明顯,熔體中晶核的數(shù)目增多,晶粒越細小。通過對制漿過程的分析,建立了描述熱平衡法制漿過程中形核和長大的理論模型。在研究形核機制時,提出了一次形核和二次形核的概念�；诰ЯＳ坞x論和瞬態(tài)形核理論,對一次形核過程進行了分析,澆注過程的激冷作用,使熔體過冷形核,產(chǎn)生大量的自由晶并保存下來,這些大量晶核的存在是獲得細小、均勻球狀晶組織的主要原因;在二次形核過程中提出了邊部冷卻層和有效對流區(qū)的概念,二次形核過程中,旋轉(zhuǎn)對流促進了有效對流區(qū)內(nèi)枝晶臂的根部熔斷,實現(xiàn)二次形核過程的晶粒增殖�；谶^冷熔體中成分過冷對晶粒長大形貌的影響,研究了初生固相顆粒在高密度晶核、緩慢冷卻和旋轉(zhuǎn)對流條件下晶粒的長大機制。高密度晶核的存在減小了晶粒的長大空間和長大速率,決定了晶粒的最終尺寸,而高密度晶核溫度場和成分場的疊加以及緩慢冷卻和旋轉(zhuǎn)對流都促進了凝固界面前沿溫度場和成分場的均勻化,提高了界面的穩(wěn)定性,促進晶粒沿各方向均勻生長,最終形成球狀晶組織。在工程化應用方面,以319s合金為研究對象,對熱平衡法半固態(tài)流變成形漿料流動機制和成形件組織與力學性能進行系統(tǒng)性研究:(1)通過壓鑄過程漿料流動穩(wěn)定性的研究,建立了熱平衡法制漿工藝與漿料穩(wěn)定性的變化規(guī)律:澆注溫度一定時,漿料的穩(wěn)定性主要由固液相配比決定,表現(xiàn)為隨著漿料溫度的降低,穩(wěn)定性先增強后降低;漿料溫度一定時,漿料穩(wěn)定性主要由初生相形貌決定,表現(xiàn)為隨著澆注溫度的降低,穩(wěn)定性先增強后降低。實驗中獲得的319s合金漿料流動最穩(wěn)定時的制漿工藝為:澆注溫度630℃-640℃,漿料固相百分數(shù)35%-45%。(2)以制備高端復雜精密汽車零部件為例,對壓鑄件充型過程、組織和力學性能進行分析。研究結(jié)果表明:在優(yōu)化后的半固態(tài)漿料制備工藝下,壓鑄充填過程漿料流動平穩(wěn),鑄件內(nèi)部可獲得細小均勻分布的初生固相,鑄件致密無缺陷,經(jīng)熱處理后鑄件性能大幅提高,可滿足高端復雜精密零部件的技術(shù)要求。
[Abstract]:The problems of energy consumption and environmental pollution have become increasingly prominent in recent years. It provides a broad space for the use of lightweight material aluminum alloy in automobile. Semi solid die casting is considered to be a breakthrough in the lightweight application of automobile because of its smooth filling, small solidification shrinkage, compact production of castings and heat treatment. However, this technology has not been widely used at present. The main reason is that the current slurry preparation technology is difficult to meet the requirements of high quality and high efficiency. This paper uses the commonly used aluminum silicon cast alloy A357 and 319s as the research object, and uses the heat balance method based on controlled solidification technology to prepare the slurry. In order to realize the engineering application of semi solid rheological die casting, the preparation of semi solid slurry of aluminum alloy, the formation mechanism of spheroidal crystal structure, filling behavior of the slurry and the structure and properties of semi solid die casting are studied. Through this study, the preparation and promotion of semi-solid slurry in the control and solidification technology are further perfected. The application of semi solid rheological forming engineering has some guiding significance. The main contents and achievements of this paper are as follows: the cooling behavior of the metal liquid in the process of heat balance pulping is analyzed. The heat transfer model of the heat balance process is deduced and the expression of the temperature field of the slurry is derived. Under the condition of fixed alloy and crucible material, the distribution of the temperature field of the slurry is the function of the pouring temperature and the initial temperature of the crucible. The microstructure of the semisolid slurry is determined jointly. The effect of the process parameters on the size of the slurry is studied with A357 alloy as the research object. The results show that: (1) the pouring temperature has the greatest influence on the size of the slurry. When the pouring temperature is reduced, the metal liquid obtains a large supercooling degree near the liquidus. With the melt endogenetic flow in the crucible, a large number of free crystals are formed and retained. The size of the slurry is finally solidified into fine and uniformly distributed spherulites. The average size of the grain is measured by the intercept method, and the pouring temperature is reduced from 690 to 630. The average grain size of the edge is reduced from 820 u m to 248 m, the average size of the central grain is reduced from 706 mu m to 223 m. The average grain size of the heart is reduced from 659 Mu to 186 M. (2). The influence of rotation speed on the slurry structure varies according to the pouring temperature. The melt in the crucible has a lower apparent viscosity when it is higher than 660 C. Adding, the convection effect of the melt is strengthened and the grain structure is obviously refined. When the casting is below 645 C, the apparent viscosity of the melt is increased and the rotating process is difficult to produce obvious convection effect. The effect of the rotational speed on the size of the slurry is not obvious. (3) the small grain dissolves with the prolongation of the rotation time, and the large grains grow up gradually, with Ostwald ripening. When the mechanism grew up, the size change of the particles was calculated by LSW model. Under the same pulping condition of 22.47 m~3/s. (4), the weight of the slurry mainly affected the supercooling of the melt. The smaller the weight of the slurry, the more obvious the effect of the chilling effect in the casting process, the increase of the number of the molten MICROTEK core and the finer grain. A theoretical model describing nucleation and growth in the process of heat balance pulping is established. In the study of the nucleation mechanism, the concept of the first nucleation and two nucleation is proposed. Based on the theory of grain free and transient nucleation, the process of the first nucleation is analyzed and the chilling of the casting process is used to make the melt supercooled and produce a large number of free crystals. The existence of these large nuclei is the main reason for obtaining fine and uniform spheroidal crystals. In the process of two nucleation, the concept of side cooling layer and effective convection zone is proposed. In the two nucleation process, rotating convection promotes the fusion of the root of the dendrite arm in the effective convection zone and realizes the grain proliferation in the two nucleation process. Based on the effect of component supercooling in supercooled melt on grain growth, the grain growth mechanism of primary solid particles in high density nucleation, slow cooling and rotating convection is studied. The existence of high density nucleation reduces the growth space and growth rate of grain, determines the final size of grain, and the temperature field of high density nucleation and nucleation. The superposition of the composition field, the slow cooling and the rotating convection both promote the homogenization of the temperature field and the composition field at the front of the solidification interface, improve the stability of the interface, promote the uniform growth of the grain along all directions, and eventually form the spherical crystal structure. In the engineering application, the 319s alloy is used as the research object, and the semi solid rheological forming of the heat balance method is used. The mechanism of slurry flow and the microstructure and mechanical properties of the forming parts are systematically studied. (1) through the study of the stability of the slurry flow in the process of die-casting, the change law of the stability of the slurry technology and slurry is established. When the pouring temperature is fixed, the stability of the slurry is mainly determined by the ratio of the solid to liquid phase, which shows the decrease of the slurry temperature. Stability first increases and then decreases; when the slurry temperature is fixed, the stability of the slurry is mainly determined by the primary phase appearance. With the lowering of the pouring temperature, the stability first increases and then decreases. The pulping process of the most stable flow of 319s alloy slurry in the experiment is: the pouring temperature is 630 C -640 C, and the percentage of the slurry is 35%-45%. (2). The mold filling process, microstructure and mechanical properties of the high end complex precision automobile parts are analyzed. The results show that the slurry flow in the die casting filling process is stable under the optimized semi solid slurry preparation process, and the small and uniform distribution of the primary solid can be obtained inside the casting. The casting is compact and without defect, and the casting is cast after heat treatment. The performance is greatly improved, which can meet the technical requirements of high-end complex precision parts.
【學位授予單位】：北京有色金屬研究總院
【學位級別】：博士
【學位授予年份】：2017
【分類號】：TG292
本文編號：2126271
[Abstract]:The problems of energy consumption and environmental pollution have become increasingly prominent in recent years. It provides a broad space for the use of lightweight material aluminum alloy in automobile. Semi solid die casting is considered to be a breakthrough in the lightweight application of automobile because of its smooth filling, small solidification shrinkage, compact production of castings and heat treatment. However, this technology has not been widely used at present. The main reason is that the current slurry preparation technology is difficult to meet the requirements of high quality and high efficiency. This paper uses the commonly used aluminum silicon cast alloy A357 and 319s as the research object, and uses the heat balance method based on controlled solidification technology to prepare the slurry. In order to realize the engineering application of semi solid rheological die casting, the preparation of semi solid slurry of aluminum alloy, the formation mechanism of spheroidal crystal structure, filling behavior of the slurry and the structure and properties of semi solid die casting are studied. Through this study, the preparation and promotion of semi-solid slurry in the control and solidification technology are further perfected. The application of semi solid rheological forming engineering has some guiding significance. The main contents and achievements of this paper are as follows: the cooling behavior of the metal liquid in the process of heat balance pulping is analyzed. The heat transfer model of the heat balance process is deduced and the expression of the temperature field of the slurry is derived. Under the condition of fixed alloy and crucible material, the distribution of the temperature field of the slurry is the function of the pouring temperature and the initial temperature of the crucible. The microstructure of the semisolid slurry is determined jointly. The effect of the process parameters on the size of the slurry is studied with A357 alloy as the research object. The results show that: (1) the pouring temperature has the greatest influence on the size of the slurry. When the pouring temperature is reduced, the metal liquid obtains a large supercooling degree near the liquidus. With the melt endogenetic flow in the crucible, a large number of free crystals are formed and retained. The size of the slurry is finally solidified into fine and uniformly distributed spherulites. The average size of the grain is measured by the intercept method, and the pouring temperature is reduced from 690 to 630. The average grain size of the edge is reduced from 820 u m to 248 m, the average size of the central grain is reduced from 706 mu m to 223 m. The average grain size of the heart is reduced from 659 Mu to 186 M. (2). The influence of rotation speed on the slurry structure varies according to the pouring temperature. The melt in the crucible has a lower apparent viscosity when it is higher than 660 C. Adding, the convection effect of the melt is strengthened and the grain structure is obviously refined. When the casting is below 645 C, the apparent viscosity of the melt is increased and the rotating process is difficult to produce obvious convection effect. The effect of the rotational speed on the size of the slurry is not obvious. (3) the small grain dissolves with the prolongation of the rotation time, and the large grains grow up gradually, with Ostwald ripening. When the mechanism grew up, the size change of the particles was calculated by LSW model. Under the same pulping condition of 22.47 m~3/s. (4), the weight of the slurry mainly affected the supercooling of the melt. The smaller the weight of the slurry, the more obvious the effect of the chilling effect in the casting process, the increase of the number of the molten MICROTEK core and the finer grain. A theoretical model describing nucleation and growth in the process of heat balance pulping is established. In the study of the nucleation mechanism, the concept of the first nucleation and two nucleation is proposed. Based on the theory of grain free and transient nucleation, the process of the first nucleation is analyzed and the chilling of the casting process is used to make the melt supercooled and produce a large number of free crystals. The existence of these large nuclei is the main reason for obtaining fine and uniform spheroidal crystals. In the process of two nucleation, the concept of side cooling layer and effective convection zone is proposed. In the two nucleation process, rotating convection promotes the fusion of the root of the dendrite arm in the effective convection zone and realizes the grain proliferation in the two nucleation process. Based on the effect of component supercooling in supercooled melt on grain growth, the grain growth mechanism of primary solid particles in high density nucleation, slow cooling and rotating convection is studied. The existence of high density nucleation reduces the growth space and growth rate of grain, determines the final size of grain, and the temperature field of high density nucleation and nucleation. The superposition of the composition field, the slow cooling and the rotating convection both promote the homogenization of the temperature field and the composition field at the front of the solidification interface, improve the stability of the interface, promote the uniform growth of the grain along all directions, and eventually form the spherical crystal structure. In the engineering application, the 319s alloy is used as the research object, and the semi solid rheological forming of the heat balance method is used. The mechanism of slurry flow and the microstructure and mechanical properties of the forming parts are systematically studied. (1) through the study of the stability of the slurry flow in the process of die-casting, the change law of the stability of the slurry technology and slurry is established. When the pouring temperature is fixed, the stability of the slurry is mainly determined by the ratio of the solid to liquid phase, which shows the decrease of the slurry temperature. Stability first increases and then decreases; when the slurry temperature is fixed, the stability of the slurry is mainly determined by the primary phase appearance. With the lowering of the pouring temperature, the stability first increases and then decreases. The pulping process of the most stable flow of 319s alloy slurry in the experiment is: the pouring temperature is 630 C -640 C, and the percentage of the slurry is 35%-45%. (2). The mold filling process, microstructure and mechanical properties of the high end complex precision automobile parts are analyzed. The results show that the slurry flow in the die casting filling process is stable under the optimized semi solid slurry preparation process, and the small and uniform distribution of the primary solid can be obtained inside the casting. The casting is compact and without defect, and the casting is cast after heat treatment. The performance is greatly improved, which can meet the technical requirements of high-end complex precision parts.
【學位授予單位】：北京有色金屬研究總院
【學位級別】：博士
【學位授予年份】：2017
【分類號】：TG292
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