Effects of forced cooling on mechanical properties and fracture behavior of heavy section ductile iron

To develop materials suitable for spent-nuclear-fuel containers, the effect of forced cooling on mechanical properties and fracture toughness of heavy section ductile iron was investigated. Two cubic castings with different cooling processes were prepared: casting A was prepared in a totally sand mold, and casting B was prepared in a sand mold with two chilling blocks placed on the left and right sides of the mold. Three positions in each casting with different solidification cooling rates were chosen. In-situ SEM tensile experiment was used to observe the dynamic tensile process. Fracture analysis was conducted to study the influence of vermicular and slightly irregular spheroidal graphite on the fracture behavior of heavy section ductile iron. Results show that the tensile strength, elongation, impact toughness and fracture toughness at different positions of the two castings all decrease with decreasing cooling rate. With the increase of solidification time, the fracture mechanism of conventional casting A changes from ductile fracture to brittle fracture, and that of casting B with forced cooling changes from ductile fracture to a mixture of ductile-brittle fracture.     

微观组织对A201铝合金铸件机械性能之研究

系统改变A201铝合金平板铸件的长度、厚度及冒口大小,探讨微观组织中微孔隙量及树枝状晶胞尺寸(DCS)对A201铸件抗拉强度及伸长率的影响程度,进而于铸造实务工作时,可为冒口设计及金属凝固之参考,为研究的目的。砂型的种类有三种,分别是100%石英砂的A类石英砂、50%石英砂及50%铬砂的B类、及100%铬砂的C类。实验结果显示,A201铝合金平板铸件的机械性质同时受空孔量及DCS之影响,当微孔隙量增加及DCS变大时,均会降低铸件的抗拉强度及伸长率,其中微孔隙量影响为主要的因素。     

基于多材质复合铸型的A356铝合金性能调控机制研究

建立变壁厚回字形结构多材质复合铸型,首先,通过对A356铝合金在多材质复合铸型的充型、凝固过程模拟仿真,获得多材质复合铸型铸件充型时间和温度场结果,锆英砂与石英砂、铬铁矿砂与石英砂过渡处凝固时间呈阶梯状递减;且锆英砂与石英砂过渡处及铬铁矿砂与石英砂过渡处铸件凝固时间更短,金属液凝固速度更快。其次,采用电子背散射衍射（EBSD）分析、电子显微探针（EPMA）分析对石英砂、铬铁矿砂、锆英砂复合铸型在重力铸造下A356铝合金铸件断口进行分析并进行抗拉强度测试。结果表明：在相同壁厚时,铬铁矿砂、锆英砂型铸件的晶粒尺寸细小,Al、Mg、Si等元素分布均匀、力学性能提高,断口呈现韧性断裂的特征;同时随着壁厚减小,同种材质砂型铸件晶粒细化、元素分布均匀、力学性能提高,断口呈韧性断裂特征。     

Controlled cooling of an aluminum alloy casting based on 3D printed rib reinforced shell mold

3D printing technology has been used for sand molding and core printing, but they simply substitute the traditional molding and core making method without changing the shape or size of the sand mold(core) and their dense structure. In this study, a new type of hollow mold based on 3D printing is presented. The new type of mold is a rib reinforced thickness-varying shell mold. This mold design can realize the controlled cooling of castings, i.e., different cooling rates at different areas, and improve the temperature uniformity of a casting after its solidifi cation. Therefore, the performance of castings can be improved and their residual stress and deformation can be reduced. This kind of new mold was applied to a stress frame of A356 aluminum alloy. The 3D printed rib reinforced thickness-varying shell mold was compared with the traditional dense mold, and the castings obtained by these two kinds of molds were also compared. The experimental results showed that the rib reinforced shell mold increased the cooling rate of the casting by 30%, tensile strength by 17%, yield strength by 11%, elongation by 67%, and decreased its deformation by 43%, while sand consumption was greatly reduced by 90%.     

Dynamic measurements of the load on castings and the contraction of castings during cooling in sand molds

The load on flange castings in sand molds was gradually increased beginning from the end of the solidification process until the final cooling stage. The maximum tensile load on the flange castings in furan sand molds was larger than that of the flange castings in green sand molds. With the furan sand mold, permanent deformation in the flange castings occurred beginning from the end of the solidification process until reaching a temperature of approximately 250 °C. The mechanical interaction between the casting and the sand mold should be considered for more accurate stress calculations, particularly in furan sand molds.     

AlSi7Mg合金半固态压铸件热处理强化机理研究

对AlSi7Mg合金(A356)半固态压铸件和液态压铸件进行了不同工艺的固溶与时效热处理，分析了其显微组织与疏松度，测定了硬度、拉伸强度及延伸率等力学性能。实验得出，铝合金半固态压铸件原始态的力学性能优于液态压铸件，并且半固态压铸件时效强化效果尤其明显，拉伸强度可达330MPa以上，延伸率10％以上。这主要是由于半固态压铸件比液压件具有更加致密，且为球状的非树枝晶组织。铝合金半固态压铸件时效强化，机理主要归于弥散析出Mg2Si强化相。     

型砂及壁厚对锶变质A356铸造合金组织和性能的影响

采用硅砂、宝珠砂和铬铁矿砂3种不同砂型作为造型材料浇注多阶梯铸件试样,对比研究不同冷却条件对Al-10Sr中间合金变质A356（Al-7Si-0.3Mg）铸造合金力学性能和显微组织的影响。结果表明：在3种造型材料中,铬铁矿砂浇注试样具有最好的组织和力学性能。随着冷却速度的增加,枝晶间距显著变小,力学性能有所改善,其中伸长率对冷却速度比抗拉强度更敏感。这种性能的提高主要是由于枝晶间距更加细密和基体中自由Sr原子数的提高造成的。根据实际的试验数据,模拟建立不同砂型条件下枝晶间距和力学性能之间的回归分析模型,该模型可用于预测Sr变质A356合金铸件的抗拉强度和伸长率。     

温度梯度和冷却速率对ZL205A合金凝固组织及性能的影响

由于在不同温度梯度下ZL205A合金冷却速率对凝固组织的影响不同,采用不同壁厚的阶梯状结构的合金,在不同凝固条件下制备厚度不同的合金试样,探究了冷却速率及温度梯度对合金凝固组织及T5处理后合金抗拉伸强度及延伸率的影响,同时探索了合金的相变规律与组织和性能的关联效应。实验结果表明：由壁厚差引起的温度梯度对合金的微观组织及形貌会产生一定的影响,降低温度梯度的合金力学性能显著提升,T5处理后合金的抗拉伸强度达到506 MPa。     

热型连铸法制备ZLAlCu10合金的组织与性能

用热型产铸法制备ＺＬＡＩＣｕ１０合金试样,将其金相组织、力学性能、磨损性能、腐蚀性能及显微硬度与石墨型铸造法制备的ＺＬＡＩＣｕ１０台金试样进行比较,结果表明,尽管热型连铸件的晶粒比较粗大,但其抗拉强度、伸长率、耐磨性能、抗腐蚀性能却分别比石墨型试样提高了２７％、１８８％、８０％、１４０％,显微硬度也大大提高。     

Cooling control for castings by adopting skeletal sand mold design

The cooling control of the melt during the casting process is of great significance. A comprehensive closed-loop cooling control of castings by adopting a skeletal sand mold design was proposed. The skeletal sand mold consisting of an adaptive shell, functional cavities and a support was designed and created based on the finite difference meshes of a casting. It was applied to a round wall test casting. Two kinds of skeletal sand molds, one with lattice support and the other with enforcing ribs for this casting were designed and printed out by the 3 D printing(3 DP) method. Aluminum alloy A356 was cast by using these two sand molds. The first mold was cooled by natural convection, the other one by water spray cooling. Two sound castings were obtained. The sand mold temperature, cooling curves, microstructures, mechanical properties, residual stress and deformation were measured, compared and discussed. Water spray cooling hastened the cooling rate by 62%, increased the content of Mg and Cu in the α-Al matrix, improved the mechanical properties, and altered the surface residual stress state.     

真空压铸工艺参数对AM50镁合金力学性能的影响规律

采用阶梯试验模具及AM50合金,进行了系统的真空压铸试验,实测了不同厚度的阶梯试样在不同工艺条件下的密度及力学性能,研究了高真空压铸工艺参数对AM50镁合金力学性能的影响规律.结果表明,随着型腔真空压力的降低,铸件密度、抗拉强度和伸长率均随之提高;铸造压力对力学性能的影响在真空压铸和常规压铸中遵循基本相同的规律,即增大铸造压力可以使铸件的致密程度、抗拉强度、屈服强度和伸长率得到提高;随着高速速度的增大,薄壁铸件的抗拉强度、屈服强度和伸长率均表现出明显的增加,这一点与常规压铸的规律相反.结合高真空和高速工艺,可以使薄壁铸件的抗拉强度和伸长率得到较为明显的提升.     

热处理对半固态A356铝合金挤压铸件组织与性能的影响

将低温浇注制备的半固态A356铝合金坯料加热至半固态浇注、挤压成形。采用正交试验法研究了固溶、时效对半固态A356铝合金挤压铸件组织与力学性能的影响。结果表明,时效时间对抗拉强度、屈服强度、伸长率影响最大,且都是随着时效时间增长先上升后下降;试验6获得较好的综合力学性能,其抗拉强度、屈服强度、伸长率分别为340 MPa、325 MPa、9.56%。     

The correlation between dendritic microstructure and mechanical properties of directionally solidified hypoeutectic Al-Ni alloys

Al-Ni hypoeutectic alloys were directionally solidified under upward transient heat flow conditions. The aim of the present study is to set up correlations between the as-cast microstructure and the resulting mechanical properties of these alloys. The dependence of primary and secondary dendrite arm spacing on the alloy solute content and on solidification thermal parameters is also analyzed. The results include transient metal/mold heat transfer coefficient, tip growth rate, cooling rate, dendrite arm spacing, ultimate tensile strength, yield tensile strength and elongation. Expressions relating dendrite spacing to solidification thermal parameters and mechanical properties to the scale of the dendritic microstructure have been determined. It was found that the ultimate tensile strength and the yield tensile strength increase with increasing alloy solute content and with decreasing primary and secondary dendrite arm spacing. In contrast, the elongation was found to be independent of both alloy composition and dendritic arrangement.     

不同铸型对钛合金微观组织及力学性能的影响

为了研究陶瓷和石墨铸型对ZTC4钛合金微观组织机理的影响,采用数值模拟与实验分析相结合的方法,研究了2种铸型的试样在凝固传热过程中的冷却速度、凝固速度、凝固时间对微观组织和力学性能影响。结果表明:与陶瓷型试样相比,石墨型的散热效果好、冷却速度快、凝固速度高,且石墨型的冷却速度提高了52.04%,使得微观组织为急冷生成的细小板条马氏体α,晶粒较小、取向较多、凝固厚度大,导致力学性能中的屈服强度、抗拉强度、硬度分别提高了6.7%、2.6%和4.2%,但伸长率和断面收缩率分别降低了11.62%和34.5%。同时同种铸型的不同位置对冷却速度快慢的影响依次为:中间底部顶部。     

The influence of copper on microstructure and mechanical properties of compacted graphite iron

Abstract

The influence of copper content (0·26 to 1·31 wt-%) on microstructure formation and mechanical properties of compacted graphite iron (CGI) has been evaluated through standard metallographic analysis, colour etching techniques and tensile testing of machined test bars. The properties investigated are yield strength, tensile strength and elongation. The castings were made in an industrial environment from a combination of CGI returns, pig iron, cast iron- and steel scrap. A total of four heats were cast in specially designed sampling cups (3 different cooling rates), chill wedges as well as tensile test bars machined from sand moulded cylinders (20, 45 and 85 mm in diameter). The results clearly illustrate the combined effect of copper and cooling rate on nodularity, chilling tendency as well as pearlite content. A discussion concerning the effect of graphite morphology on the ferrite growth is also included.     

铸件模数Mc对A357合金二次枝晶臂间距及致密度的影响

研究试样直径和铸件模数Mc与铸件二次枝晶臂间(SDAS)的关系，通过测试A357合金不同凝固条件下铸件不同部位的SDAS，得出砂型铸造凝固条件下SDAS与铸件模数M之间的关系式。研究了铸件模数等对铸件致密度的影响，确定了获得高致密度铸件的合理的凝固条件。     

Strengthening technology and mechanism for semi-solid die casting of aluminum alloy

Combined with theoretical evaluation,an optimized strengthening process for the semi-solid die castings of A356 aluminum alloy was obtained by studying the mechanical properties of castings solution treated and aged under different conditions in detail,then,the semi-solid die castings and liquid die castings were heat treated with the optimized process.The results show that the mechanical properties of semi-solid die castings of aluminum alloy are superior to those of the liquid die castings,especially the strengthening degree of heat treated semi-solid die castingsis much greater than that of liquid die castings with the tensile strength more than 330 MPa and the elongation more than 10％,and this is mainly contributed to the non-dendritic and more compact microstructure of semi-solid die castings.The strengthening mechanism of heat treatment for the semi-solid die castings of A356 aluminum alloy is due to the dispersive precipitation of the second phase(Mg2 Si) and formation of GP Zone.     

淬火条件对砂铸A356合金疲劳裂纹扩展速率的影响

在相同固溶和时效条件下，选择3种不同温度的水对砂铸A356合金进行了淬火处理，并对这3种不同淬火态下的合金进行了拉伸及疲劳裂纹扩展性能研究。结果表明，随淬火介质温度的降低，淬火冷却速率提高，合金的强度增加，塑性下降；当应力比R较低时，淬火条件对合金门槛区的裂纹扩展有明显影响，降低淬火介质温度可以提高合金的疲劳裂纹扩展阻力。此外，不同淬火态合金的疲劳扩展均明显地表现出与应力比R的相关性，这种相关性可以用裂纹闭合来说明。     

铸造镁合金旋转喷吹除气的试验研究

研究了旋转喷吹技术在AZ91镁合金熔体除气净化处理上的应用。采用减压凝固法考察了旋转喷吹的除气净化工艺的净化效果，确定了最佳的除气时间。利用金相显微镜观察了除气前后合金的微观组织。对除气前后的合金，进行了金属型和低压砂型铸造试验浇注，并对其力学性能进行了测试和比较。研究结果表明，经过30min旋转喷吹除气处理后，能快速显著地降低镁合金中的含气量，AZ91合金金属型和低压砂型试样的力学性能都明显地得到提高，特别是合金的伸长率得到大幅度的提高。