选区激光熔化成形TiB_(2)/Al-Si-Mg大尺寸复杂构件

选区激光熔化(SLM)成形大尺寸复杂构件厚度多样、成形高度较高、方向复杂,需要研究构件微观组织均匀性和力学性能稳定性。本文以SLM成形TiB_(2)/Al-Si-Mg复合材料为研究对象,分析复合材料多级微观组织,对比不同成形厚度、高度、方向下复合材料的力学性能。结果表明:复合材料表现出熔池特征结构,细小等轴晶粒组织均匀分布且随机取向,纳米TiB_(2)颗粒在材料内部弥散分布。随成形厚度增加,复合材料伸长率保持稳定,抗拉强度受本征热处理影响略微增大;在不同成形高度下,复合材料抗拉强度和伸长率保持稳定;在不同成形方向下,复合材料抗拉强度保持稳定,伸长率受熔池结构影响在水平方向略高。基于以上结果,成功制备大飞机舱门铰链臂(588 mm×318 mm×470 mm)复杂结构件。

Mg对亚共晶Al-10Mg_(2)Si合金组织与力学性能的影响

研究了不同Mg含量对含10 mass%Mg_(2)Si的亚共晶Al-Mg-Si合金显微组织、拉伸性能和断裂行为的影响。结果表明:在合金中添加1.2~2.8 mass%的Mg能够减小初生α-Al枝晶的尺寸,适量的Mg明显地细化了合金中的Mg_(2)Si共晶相,并使其形貌由粗大的汉字状转变为短条状、颗粒状和纤维状。共晶Mg_(2)Si的细化归因于Mg原子团簇在其两侧的富集,抑制了共晶Mg_(2)Si的生长,促进了其生长方向的改变。适量的Mg的添加同时提高了合金的强度和塑性,添加2.8 mass%Mg的合金拉伸性能达到最高,抗拉强度、屈服强度和伸长率分别由未添加Mg的232.4 MPa、150.4 MPa和1.5%增加到322.0 MPa、270.4 MPa和6.1%,分别提高了38.6%、79.8%和306.7%。未添加Mg的合金的断裂机制为脆性断裂,添加2.8 mass%Mg的合金的断裂机制转变为韧性断裂。

Microstructure evolution,mechanical properties and fracture behavior of Al-xSi/AZ91D bimetallic composites prepared by a compound casting

In this paper,the effect of the Si content on microstructure evolution,mechanical properties,and fracture behavior of the Al-xSi/AZ91D bimetallic composites prepared by compound casting was investigated systematically.The obtained results showed that all the Al-xSi/AZ91D bimetallic composites had a metallurgical reaction layer(MRL),whose thickness increased with increasing Si content for the hypoeutectic Al-Si/AZ91D composites,while the hypereutectic Al-Si/AZ91D composites were opposite.The MRL included eutectic layer(E layer),intermetallic compound layer(IMC layer)and transition region layer(T layer).In the IMC layer,the hypereutectic Al-Si/AZ91D composites contained some Si solid solution and flocculent Mg_(2)Si+Al-Mg IMCs phases not presented in the hypoeutectic Al-Si/AZ91D composites.Besides,increasing Si content,the thickness proportion of the T layer increased,forming an inconsistent preferred orientation of the MRL.The shear strengths of the Al-xSi/AZ91D bimetallic composites enhanced with increasing Si content,and the Al-15Si/AZ91D composite obtained a maximum shear strength of 58.6 MPa,which was 73.4% higher than the Al-6Si/AZ91D composite.The fractures of the Al-xSi/AZ91D bimetallic composites transformed from the T layer into the E layer with the increase of the Si content.The improvement of the shear strength of the Al-xSi/AZ91D bimetallic composites was attributed to the synergistic action of the Mg_(2)Si particle reinforcement,the reduction of oxidizing inclusions and the ratio of Al-Mg IMCs as well as the orientation change of the MRL.

Microstructure and mechanical properties of AZ31-Mg_2Si in situ composite fabricated by repetitive upsetting

AZ31-4.6% Mg2Si （mass fraction） composite was prepared by conventional casting method. Repetitive upsetting （RU） was applied to severely deforming the as-cast composite at 400 ℃ for 1, 3, and 5 passes. Finite element analysis of the material flow indicates that deformation concentrates in the bottom region of the sample after 1 pass, and much more uniform deformation is obtained after 5 passes. During multi-pass RU process, both dendritic and Chinese script type Mg2Si phases are broken up into smaller particles owing to the shear stress forced by the matrix. With the increasing number of RU passes, finer grain size and more homogeneous distribution of Mg2Si particles are obtained along with significant enhancement in both strength and ductility. AZ31-4.6%Mg2Si composite exhibits tensile strength of 284 MPa and elongation of 9.8%after 5 RU passes at 400 ℃ compared with the initial 128 MPa and 5.4%of original AZ31-4.6%Mg2Si composite.

Microstructure and dry sliding wear behavior of cast Al-Mg_2Si in-situ metal matrix composite modified by Nd

The microstructure and dry sliding wear behav- ior of cast Al-18 wt% MgaSi in-situ metal matrix com- posite modified by Nd were investigated. Experimental results show that, after introducing a proper amount of Nd, both primary and eutectic Mg2Si in the Al-18 wt% Mg2Si composite are well modified. The morphology of primary Mg2Si is changed from irregular or dendritic to polyhedral shape, and its average particle size is signifi- cantly decreased. Moreover, the morphology of the eutectic MgzSi phase is altered from flake-like to very short fibrous or dot-like. The wear rates and friction coefficient of the composites with Nd are lower than those without Nd. Furthermore, the addition of 0.5 wt% Nd changes the wear mechanism of the composite from the combination of abrasive, adhesive, and delamination wear without Nd into a single mild abrasion wear with 0.5 wt% Nd.

Effect of ultrasonic on morphology of primary Mg_2Si in in-situ Mg_2Si/Al composite

Effects of ultrasonic on morphologies of primary Mg2Si crystals in in-situ Mg2Si/A1 composite were investigated by metallographic microscopy and field emission scanning electron microscopy. The results show that the mean grain size of primary MgESi crystals is refined from 150 to 20 μm by high intensity ultrasonic, and the morphologies of primary MgESi crystals are changed as well. Optical microscopy reveals that primary MgESi crystals without ultrasonic vibration exhibit coarse particles with cavities, in which eutectic structures grow. However, primary Mg2Si crystals with ultrasonic vibration appear fine grains without any cavity. Three-dimensional morphologies of primary Mg2Si without ultrasonic vibration display octahedron and tetrakaidecahedron with hopper-like hole in the crystals. After ultrasonic vibration, primary Mg2Si particles become solid crystals with rounded comers and edges.

Mg2Si/Al复合材料的组织和力学性能研究

采用半固态挤压法制备了原位自生的Mg2Si颗粒增强铝基复合材料,研究了它们的组织,同时研究了挤压比压对力学性能的影响。结果表明,当其它制备条件相同时,在255、340和425 MPa条件下制备的半固态挤压Mg2Si/Al复合材料中的α-Al基体和Mg2Si增强相皆出现球化,但与重力铸造复合材料相比,Mg2Si增强相的颗粒尺寸没有明显变化。拉伸测试结果表明,在255 MPa、340 MPa和425 MPa条件下获得的半固态挤压Mg2Si/Al复合材料具有优于重力铸造复合材料的力学性能,其抗拉强度和伸长率分别提高71.88%、53.13%、43.75%和378%、139%、106%。在255 MPa挤压比压下制备的复合材料表现出最优的抗拉强度及伸长率。

Al-5Ti-1B与Y复合添加对铸造Mg-5Al-5Si合金的影响

【目的】为细化Mg-Al-Si合金中的粗大Mg_(2)Si相,添加Al-5Ti-1B和稀土Y对Mg-5Al-5Si合金进行变质处理。【方法】熔炼制得不同成分的Mg-Al-Si合金,使用多种测试方法对变质前后合金成分、微观组织及力学性能进行分析,并讨论其变质机理。【结果】结果表明:在Mg-5Al-5Si合金中添加Al-5Ti-1B,初生相形貌由粗大块状转变为细枝晶状或多边形状,共晶相数量随着Al-5Ti-1B添加量的增加而减少,合金硬度显著提高,这是因为TiB_(2)为Mg_(2)Si提供了形核基底。当合金中添加1.0%Al-5Ti-1B时,再添加Y会使初生相尺寸进一步减小,形貌规整。当复合添加1.0%Al-5Ti-1B和1.0%Y时,初生Mg_(2)Si相平均尺寸降低到10.2μm,形貌为细小多边形状,合金拉伸力学性能最优,稀土Y的变质机理主要为吸附毒化。

直接熔化制备Al-Si-Al_(2)O_(3)复合材料的组织和性能

采用光学显微、XRD、SEM和EDS分析了Si含量分别为10.00wt%、7.64wt%的Al-Si-Al_(2)O_(3)复合材料的显微组织形貌以及物相组成。结果表明:Al-10.00wt%Si试样的组织细小,孔隙率较高。基体晶粒Al呈细小圆形,Si颗粒主要在基体晶界周围聚集长大,部分向基体晶粒内部生长;而Al-7.64wt%Si试样组织略微粗大,孔隙率较低。基体晶粒Al呈不规则形状,Si颗粒聚集长大现象更为明显。原位反应成功进行,Al-10.00wt%Si复合材料以Al为基体生成的Al_(2)O_(3)是增强相;另一生成物Si没有细小弥散的分布于Al基体中,而是聚集附着在基体Al晶界周围,且有部分SiO_(2)残留存在于基体晶界周围。在加热升温过程中,混合原料只出现了部分熔化现象,金属液滴从混合原料中析出,凝结为含Al量极高的合金液滴。合金液滴致密度较高,固溶体呈枝晶状,Si呈条状或针状;且有小颗粒第二相的存在,含有Al、Si、C元素。

Characteristics of two Al based functionally gradient composites reinforced by primary Si particles and Si/in situ Mg_2Si particles in centrifugal casting

Two kinds of Al based functionally gradient composite tubes reinforced by primary Si particles alone and primary Si/in situ Mg2Si particles jointly were successfully prepared by centrifugal casting,and their structural and mechanical characters were compared.It is found that the composite reinforced with primary Si particles takes a characteristic of particles distribution both in the inner and outer layers.However,composite reinforced with primary Si/Mg2Si particles jointly takes a characteristic of particles distribution only in the inner layer and shows a sudden change of particles distribution across the section of inner and outer layers.The hardness and wear resistance of Al-19Si-5Mg tube in the inner layer are greatly higher than that in the other layers of Al-19Si-5Mg tube and Al-19Si tube.Theoretical analysis reveals that the existence of Mg2Si particles is the key factor to form this sudden change of gradient distribution of two kinds of particles.Because Mg2Si particles with a lower density have a higher centripetal moving velocity than primary Si particles,in a field of centrifugal force,they would collide with primary Si particles and then impel the later to move together forward to the inner layer of the tube.

Effect of SiC particle addition on microstructure of Mg_2Si/Al composite

In the present study, by adding SiC particles into AI-Si-Mg melt, Mg2Si and SiC particles hybrid reinforced AI matrix composites were fabricated through the Mg2Si in situ synthesis in melt combined with the SiC ex situ stir casting. The as-cast microstructure containing primary Mg2Si and SiC particles that distribute homogenously in AI matrix was successfully achieved. The effects of SiC particle addition on the microstructure of Mg2Si/AI composites were investigated by using scanning electron microscopy （SEM） and XRD. The results show that, with increasing the fraction of the SiC particles from 5wt.% to 10wt.%, the morphologies of the primary Mg2Si particulates in the prepared samples remain polygonal, but the size of the primary phase decreases slightly. However, when the SiC particle addition reaches 15wt.%, the morphologies of the primary Mg2Si particulates change partially from polygonal to quadrangular with a decrease in size from 50 pm to 30 μm. The size of primary AI dendrites decreases with increasing fraction of the SiC particles from 0wt.% to 15wt.%. The morphology of the eutectic Mg2Si phase changes from a fiber-form to a short fiber-form and/or a dot-like shape with increasing fraction of the SiC particles. Furthermore, no significant change in dendrite arm spacing （DAS） was observed in the presence of SiC particles.

Effect of Bi modification treatment on microstructure,tensile properties,and fracture behavior of cast Al-Mg_2Si metal matrix composite

Bi has a good modification effect on the hypoeutectic Al-Si alloy, and the morphology of eutectic Si changes from coarse acicular to fine fibrous. Based on the similarity between Mg2Si and Si phases in crystalline structure and crystallization process, the present study investigated the effects of different concentrations of Bi on the microstructure, tensile properties, and fracture behavior of cast Al-15wt.%Mg2Si in-situ metal matrix composite. The results show that the addition of the proper amount of Bi has a significant modification effect on both primary and eutectic Mg2Si in the Al-15wt.%Mg2Si composite. With an increase in Bi content from 0 to lwt.%, the morphology of the primary Mg2Si is changed from irregular or dendritic to polyhedral shape; and its average particle size is significantly decreased from 70 to 6 μm. Moreover, the morphology of the eutectic Mg2Si phase is altered from flake-like to very short fibrous or dot-like. When the Bi addition exceeds 4.0wt.%, the primary Mg2Si becomes coarse again. However, the eutectic Mg2Si still exhibits the modified morphology. Tensile tests reveal that the Bi addition can improve the tensile strength and ductility of the material. Compared with those of the unmodified composite, the ultimate tensile strength and percentage elongation after fracture with 1.0wt.% Bi increase 51.2% and 100%, respectively. At the same time, the Bi addition changes the fracture behavior from brittle to ductile.

Microstructure and Mechanical Behavior of in Situ Primary Si/Mg_2Si Locally Reinforced Aluminum Matrix Composites Piston by Centrifugal Casting

Al-Si pistons are frequently damaged by burning piston top surface due to elevated combustion temperature, and by rubbing the first ring groove against the engine cylinder liner. To prevent piston from these damages, some technologies were invented, such as mounting high Ni cast iron ring around the first ring groove in Al alloy piston body and thermal resistant steel on piston top surface, and fabricating Al composite pistons by squeeze casting for enhancing the whole or local piston performance. In this paper, composite pistons locally reinforced with in situ primary Si and primary Mg2Si particles are fabricated by centrifugal casting. The microstructure characteristics, hardness and wear resistance of the composite piston are investigated and the motion characteristic of the in situ particles in centrifugal field is analyzed. The results of the experiments show that primary Si and Mg2Si particles mix up with each other in melt and segregate at the regions of piston top and piston ring grooves under the effect of centrifugal force. Particulate reinforced regions have a higher hardness and better wear resistance compared with the unreinforced regions and this performance increases after heat treatment. The analysis result of particle movement shows that, primary Si and primary Mg2Si particles move at approximately the same velocity in the centrifugal field, because of the growth of primary Si and fusion after colliding between primary Si particles, which compromised the velocity difference of primary Si and primary Mg2Si particles caused by the difference of their densities. Research results have some theory significance and applicative value of project in development of new aluminum matrix composites piston products.

Effect of Sb on microstructure and mechanical properties of Mg_2Si/Al-Si composites

The effect of Sb on the microstructure and mechanical properties of Mg2Si/Al-Si composites was investigated.The results show that Sb can improve the microstructure and mechanical properties of Mg2Si/Al-Si composites.When the content of Sb is 0.4%,the morphology of primary Mg2Si changes from dendrites to fine particles,the average size of Mg2Si particles is refined from 52 to 25μm,and the ultimate tensile strength and elongation of the composites increase from 102.1 MPa and 0.26% to 138.6 MPa and 0.36%,respectively.The strengthening mechanism can be attributed to the fine-grain strengthening.However,excessive Sb is disadvantageous to the modification of the composites.

Rheological behavior of semi-solid Mg_2Si/AM60 magnesium matrix composites at steady state

The microstructure and rheological behavior of semi-solid Mg2Si/AM60 magnesium matrix composite at steady state were investigated.The results show that the primary α-Mg phases are knapped by mechanical stirring and the Chinese script type reinforced Mg2Si phases exist in liquid phase and grain boundary.The analysis of apparent viscosity indicates that the apparent viscosity of semi-solid Mg2Si/AM60 magnesium matrix composite at steady state increases with increasing the volume fraction of Mg2Si and solid fraction of primary α-Mg,but decreases with increasing the shearing rate and shearing time,and the apparent viscosity keeps stable when shearing time reaches 300 s.

Structure and Mechanical Properties of Al-based Gradient Composites Reinforced with Primary Si and Mg_2Si Particles through Centrifugal Casting

The structure and mechanical properties of a new type of Al-based discontinuous gradient composites prepared by using the ternary AI-19Si-5Mg alloys as the raw material adopting the centrifugal casting method were investigated. Structurally, the composites are divided into two zones： a reinforced zone with the high volume fraction of primary Si and Mg2Si particles and an unreinforced zone with no or a few particles. In the reinforced zone, the primary particles are evenly distributed, with the sizes of the primary Si particles 80-120 μm, and that of primary Mg2Si particles 20-50 μm. The properties test results show the reinforced zone has higher Rockwell hardness and better wear resistance than the unreinforced zone, due to the complementary reinforcement relationship between the primary Si and Mg2Si particles and their high volume fraction.

Influence of Sb modification on microstructures and mechanical properties of Mg2 Si/AM60 composites

The refining effect and mechanism of Sb on Mg2Si and the microstructure of the matrix were investigated.The results indicate that there are Mg3Sb2 particles in the composites with the addition of Sb,and Mg3Sb2 can promote the formation of fine polygonal type Mg2Si by providing nucleation site.Meanwhile,the grain size of Sb modified alloy is finer than that of the matrix. The improved microstructure results in the improvement of mechanical properties.The ultimate tensile strength is increased by 12.2%with the addition of 0.8%Sb.

Modii cation ef ect of calcium-magnesia phosphate fertilizer on microstructure and mechanical properties of Mg_2Si/Mg-4Si composite

In order to modify in-situ synthesized Mg_2Si particles in Mg_2Si/Mg-4Si composite, the modif ication effect of calcium-magnesia phosphate fertilizer on primary Mg_2Si phase in Mg_2Si/Mg-4Si composite was investigated by means of X-ray diffraction(XRD), optical microscopy(OM), scanning electron microscopy(SEM) and energy dispersive spectroscopy(EDS) analysis. The results indicate that the morphology of the primary Mg_2Si phase apparently changes from coarse dendrites to f ine dispersive polygonal particles, and the mean size is decreased from 277 μm to 17 μm. With the addition of 4.0wt.% calcium-magnesia phosphate fertilizer as a modif ier, the ultimate tensile strength and elongation of the Mg_2Si/Mg-4Si composite are increased from 78.7 MPa and 2.1% to 105.2 MPa and 2.6%, as compared to those of the base composite, which is probably attributed to the formation of the phosphorous compound and the cluster of Ca compounds that acted as the heterogeneous nucleation substrates of the primary Mg_2Si particles, resulting in a ref ined distribution of these precipitates.

The evolution of microstructure and mechanical properties during high-speed direct-chill casting in different Al–Mg_2Si in situ composites

The effect of high-speed direct-chill(DC) casting on the microstructure and mechanical properties of Al–Mg_2Si in situ composites and AA6061 alloy was investigated. The microstructural evolution of the Al–Mg_2Si composites and AA6061 alloy was examined by optical microscopy, field-emission scanning electron microscopy(FE-SEM) and transmission electron microscopy(TEM). The results revealed that an increase of the casting speed substantially refined the primary Mg_2Si particles(from 28 to 12 μm), the spacing of eutectic Mg_2Si(from 3 to 0.5 μm), and the grains of AA6061 alloy(from 102 to 22 μm). The morphology of the eutectic Mg_2Si transformed from lamellar to rod-like and fibrous with increasing casting speed. The tensile tests showed that the yield strength, tensile strength, and elongation improved at higher casting speeds because of refinement of the Mg_2Si phase and the grains in the Al–Mg_2Si composites and the AA6061 alloy. High-speed DC casting is demonstrated to be an effective method to improve the mechanical properties of Al–Mg_2Si composites and AA6061 alloy billets.

Microstructural characteristics of in situ Mg_2Si/Al-Si composite by low superheat pouring

To control the morphology and size of the primary and eutectic Mg2Si phases in in situ Mg2Si/Al-Si composite and achieve a feasible and reliable technique to produce appropriate feedstock for the thixo-casting and rheo-casting of this type of material, three AI-Si matrix composites reinforced by 5wt.%, 9wt.% and 17wt.% Mg2Si with hypoeutectic, eutectic and hypereutectic compositions were prepared by the low superheat pouring （LSP） process. The effects of the pouring temperature （superheat） on the morphology and size distribution of primary phases （primary e-AI and Mg2Si）, binary （a-AI ＋ Mg2Si） eutectic cell and eutectic Mg2Si were investigated. The experimental results show that low pouring temperature （superheat） not only refines the grain structure of the primary e-AI and binary （e-AI ＋ Mg2Si） eutectic cell in three composites and promotes the formation of more non- dendritic structural semi-solid metal （SSM） slurry of these phases; but also refines the primary and eutectic Mg2Si phases, which seems to be attributed to the creation of an ideal condition for the nucleation and the acquisition of a high survival of nuclei caused by the LSP process.