Effect of Combined Addition of Cu and Aluminum Oxide Nanoparticles on Mechanical Properties and Microstructure of Al-7Si-0.3Mg Alloy

In this study, an ultrasonic cavitation based dispersion technique was used to fabricate Al-7Si-0.3Mg alloyed with Cu and reinforced with 1 wt pct Al₂O₃ nanoparticles, in order to investigate their influence on the mechanical properties and microstructures of Al-7Si-0.3Mg alloy. The combined addition of 0.5 pct Cu with 1 pct Al₂O₃ nanoparticles increased the yield strength, tensile strength, and ductility of the as-cast Al-7Si-0.3Mg alloy, mostly due to grain refinement and modification of the eutectic Si and θ-CuAl₂ phases. Moreover, Al-7Si-0.3Mg-0.5Cu-1 pct Al₂O₃ nanocomposites after T6 heat treatment showed a significant enhancement of ductility (increased by 512 pct) and tensile strength (by 22 pct). The significant enhancement of properties is attributed to the suppression of pore formation and modification of eutectic Si phases due to the addition of Al₂O₃ nanoparticles. However, the yield strength of the T6 heat-treated nanocomposites was limited in enhancement due to a reaction between Mg and Al₂O₃ nanoparticles.     

Characterization of microstructure and precipitation behavior in Al-4Cu-xTiB2 in-situ composite

Al-4Cu-xTiB₂ (x=0, 2.5, 5, 10 wt %) in-situ composites were prepared by a mixed salt route technique. The composites were characterized by X-ray diffraction techniques, to confirm that no Al₃Ti has formed, which is the advantage of mixed salt route technique. The scanning electron microscopy (SEM) analysis was carried out to determine the size distribution of TiB₂ particles in the matrix. The results showed that there was no agglomeration of TiB₂ particles throughout the matrix. The differential scanning calorimerty (DSC) studies were performed on the alloys as well as on composites to identify and characterize the precipitation sequence G.P.(I)→G.P.(II)/θ″→ θ′→ stable θ. To understand the precipitation kinetics of these precipitates in the presence of TiB₂, the solutionized samples were heat treated at different temperatures of precipitation as indicated by the DSC Thermogram and subsequently quenched to room temperature to retain the precipitates that form at corresponding high temperatures. The TEM analysis was carried out to characterize the crystal structure and morphology of the different precipitates. The analysis suggested that the precipitation occur primarily on the dislocations in the matrix as well as in the TiB₂ particle/Al-Cu matrix interface dislocations. It is believed that these dislocations are generated to accommodate the strain due to the difference in the coefficient of thermal expansion (CTE) of the TiB₂ particles and the Al-Cu matrix. TEM results displayed that the interface contained large amount of dislocations which may possibly accelerate the precipitation sequence.     

The Structure and Kinetics of the Nanoscale Precipitation Processes in Al-1.0 wt pct Mg2Si-0.4 wt pct Mg-0.5 wt pct Ag Alloy

The influence of addition of 0.4 wt pct Mg on the precipitation sequence in the balanced Al-1.0 wt pct Mg₂Si bearing 0.5 wt pct Ag has been investigated during the continuous heating of the quenched alloy from the solid solution state. Differential scanning calorimetry (DSC) and high-resolution transmission electron microscopy techniques have been used. The DSC experiments showed that all processes occurred are thermally activated. The activation energies of the precipitation processes have been determined and hence the kinetics of these precipitates have been determined. The obtained results have shown that the existence of excess Mg inhibits the formation of the early stage clusters of solute-vacancy clusters. These clusters can be assisted by the binding energies between solute Si, Mg, and Ag atoms and the excess vacancies. On the other hand, excess Mg accelerates the precipitation of random, β′-phase and β-phase precipitates.     

Determination of the Lifetime of a Double-Oxide Film in Al Castings

One of the most important casting defects in Al alloys is thought to be the double-oxide film defect (bifilm) which has been reported to have a deleterious effect on the reproducibility of the mechanical properties of Al castings. Previous research has suggested that the atmosphere inside such bifilms could be consumed by reaction with the surrounding melt, which might decrease the size of the defects and reduce their harmful effect on mechanical properties. In order to follow the change in the composition of the interior atmosphere of a bifilm, analog air bubbles were held inside Al alloy melts, for varying lengths of time, and subjected to stirring, followed by solidification. The bubble contents were then analyzed using a mass spectrometer to determine the changes in their compositions with time. The results suggested that initially oxygen and then nitrogen inside the bubble were consumed, and hydrogen dissolved in the melt diffused into the bubble. The consumption rates of O and N as well as the rate of H diffusion were dependent upon the type of oxide, which was dependent on the alloy composition. The reaction rates were the fastest with MgO (in an Al-5Mg alloy), slower with alumina (in commercial-purity Al alloy), and the slowest with MgAl₂O₄ spinel (in an Al-7Si-0.3Mg alloy). It was estimated that the times required for typical bifilm defects in the different alloys to lose their entire oxygen and nitrogen contents were about 345 seconds (~6 minutes), in the case of Al-5Mg; 538 seconds (~9 minutes), in the case of a commercial purity alloy; and 1509 seconds (~25 minutes), in the case of the Al-7Si-0.3Mg alloy (2L99) due to the different oxides that the different alloys would be expected to form.     

Coating of 6028 Aluminum Alloy Using Aluminum Piston Alloy and Al-Si Alloy-Based Nanocomposites Produced by the Addition of Al-Ti5-B1 to the Matrix Melt

The Al-12 pctSi alloy and aluminum-based composites reinforced with TiB₂ and Al₃Ti intermetallics exhibit good wear resistance, strength-to-weight ratio, and strength-to-cost ratio when compared to equivalent other commercial Al alloys, which make them good candidates as coating materials. In this study, structural AA 6028 alloy is used as the base material. Four different coating materials were used. The first one is Al-Si alloy that has Si content near eutectic composition. The second, third, and fourth ones are Al-6 pctSi-based reinforced with TiB₂ and Al₃Ti nano-particles produced by addition of Al-Ti5-B1 master alloy with different weight percentages (1, 2, and 3 pct). The coating treatment was carried out with the aid of GTAW process. The microstructures of the base and coated materials were investigated using optical microscope and scanning electron microscope equipped with EDX analyzer. Microhardness of the base material and the coated layer were evaluated using a microhardness tester. GTAW process results in almost sound coated layer on 6028 aluminum alloy with the used four coating materials. The coating materials of Al-12 pct Si alloy resulted in very fine dendritic Al-Si eutectic structure. The interface between the coated layer and the base metal was very clean. The coated layer was almost free from porosities or other defects. The coating materials of Al-6 pct Si-based mixed with Al-Ti5-B1 master alloy with different percentages (1, 2, and 3 pct), results in coated layer consisted of matrix of fine dendrite eutectic morphology structure inside α-Al grains. Many fine in situ TiAl₃ and TiB₂ intermetallics were precipitated almost at the grain boundary of α-Al grains. The amounts of these precipitates are increased by increasing the addition of Al-Ti5-B1 master alloy. The surface hardness of the 6028 aluminum alloy base metal was improved with the entire four used surface coating materials. The improvement reached to about 85 pct by the first type of coating material (Al-12 pctSi alloy), while it reached to 77, 83, and 89 pct by the coating materials of Al-6 pct Si-based mixed with Al-Ti5-B1 master alloy with different percentages 1, 2, and 3 pct, respectively.     

Fracture toughness of discontinuously reinforced Al-4Cu-1.5Mg/TiB2 composites

The effect of particle size, particle volume fraction, and matrix microstructure on the fracture initiation toughness of a discontinuously reinforced aluminum composite was examined. The composites were Al-4 wt pct Cu-1.5 wt pct Mg reinforced with 0 to 15 vol pct of TiB₂ having an average particle diameter of 1.3 or 0.3μm producedin situ by the XD process. The room-temperature plane-strain toughness measured using compact tension specimens ranged from 19 to 25 MPa . Toughness was adversely affected by increases in TiB₂ volume fraction. The fracture toughness of all composites was affected by changes in the matrix microstructure produced by aging. The response of the composites to artificial aging deviates from that of the matrix. Fractography revealed that these composites failed in a ductile manner, with voids initiating at the reinforcing TiB₂ particles. The experimentally measured plane-strain toughness properties of Al-4Cu-l .5Mg composites with well-dispersed, 1.3-μm TiB² reinforcements agree with the Rice and Johnson model.     

Reaction steps in the combustion synthesis of NiAl/TiB2 composites

The reaction steps in the formation of NiAl/TiB₂ composites produced by reaction synthesis have been determined using differential thermal analysis (DTA). The DTA technique reveals that the formation of NiAl/TiB₂ composites occurs in a two-step reaction. NiAl forms first at approximately 550 °C, followed by TiB₂, which forms at approximately 1050 °C. Both X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS) were performed on composites produced by reaction synthesis in a hot-press facility using the same heating rate used in the DTA experiments. It was found that the formation of NiAl and TiB₂ is preceded by the formation of intermediate compounds such as Ni₃Al, TiAl, Ti₃Al, Ti₂Ni, and TiB. The relative density measured in the composites with 0, 0.25, 0.50, and 0.75 volume fractions of TiB₂ was in excess of 96 pct of the theoretical density, since, during synthesis, NiAl is a transient liquid that acts as a binder phase for the TiB₂ particles.     

Processing,Microstructure, and Mechanical Properties of B4C ― TiB2 Particulate Sintered Composites. Part I. Pressureless Sintering and Microstructure Evolution

Pressureless sintering of boron carbide ceramics containing 0-25 vol. % TiB₂ phase, produced via an in-situ chemical reaction between B₄C, TiO₂, and elemental carbon, was studied in the isothermal and constant-heating-rate regimes. The presence of TiB₂ results in a decrease in activation energy for sintering from 717 kJ/mol at 0 vol. % TiB₂ to 266 kJ/mol at 25 vol. % TiB₂. Ceramic bodies of B₄C TiB₂ particulate composites with relative densities of up to 99% were sintered without pressure at temperatures of 2050-2100°C. Grain boundary diffusion is the primary mechanism of TiB₂ particle coarsening. TiB₂ particle size is bimodal depending on whether the particle is confined within a B₄C grain or located on the grain boundary. Densification behavior of the B₄C TiB₂ system is identical at different heating rates in the temperature range of 1800-2150°C.     

Boron removal by titanium addition in solidification refining of silicon with Si-Al melt

In order to effectively remove B from Si for its use in solar cells, a process involving B removal by solidification refining of Si using a Si-Al melt with Ti addition was investigated. For clarifying the effect of Ti addition on B removal from the Si-Al melt, TiB₂ solubilities in Si-64.6 at. pct Al melt at 1173 K and Si-60.0 at. pct Al melt at 1273 K were determined by measuring the equilibrium concentrations of B and Ti in the presence of TiB₂ precipitates. The small solubilities of TiB₂ in the Si-Al melt indicate the effective removal of B from the Si-Al melt by Ti addition. Further, solidification experiments of Si-Al alloys containing B by Ti addition were performed, and the effect of Ti addition on the solidification refining of Si with the Si-Al melt was successfully confirmed.     

Modeling of feeding behavior of solidifying Al-7Si-0.3Mg alloy plate casting

The systematic change of riser size, together with the variation of geometries of solidifying Al-7Si-0.3Mg plate castings, was tested by thermal analysis to model the interdendritic feeding behavior based on Darcy’s law. This law, however, is found to be only applicable to certain thermal conditions in the solidifying casting. The applicability of Darcy’s law depends on the regime of solidification time. A new feeding efficiency parameter integrating all individual ther-mal variables, denoted as(G · t ^2/3)V_s (whereG is the thermal gradient,t is local solidification time, and V_{s i} is solidus velocity), is found satisfactory to predict the formation of porosity. The combined geometries of a casting and its riser size exert a great influence on the thermal vari-ables of Al-7Si-0.3Mg alloy in a complicated way. Together, these thermal variables synergize to govern the feeding behavior of the casting.     

Optimization of Solution Treatment of Cast Al-7Si-0.3Mg and Al-8Si-3Cu-0.5Mg Alloys

The influence of solidification rate on the solution-treatment response has been investigated for an Al-7Si-0.3Mg alloy and an Al-8Si-3Cu-0.5Mg alloy. The concentrations of Mg, Cu, and Si in the matrix after different solution-treatment times were measured using a wavelength dispersive spectrometer. All Mg dissolves into the matrix for the Al-Si-Mg alloy when solution treated at 803 K (530 °C) because the π-Fe phase is unstable and transforms into short β-Fe plates which release Mg. The Q-Al₅Mg₈Cu₂Si₆ phase do not dissolve completely at 768 K (495 °C) in the Al-Si-Cu-Mg alloy and the concentration in the matrix reached 0.22 to 0.25 wt pct Mg. The distance between π-Fe phases and Al₂Cu phases was found to determine the solution-treatment time needed for dissolution and homogenization for the Al-Si-Mg alloy and Al-Si-Cu-Mg alloy, respectively. From the distance between the phases, a dimensionless diffusion time was calculated which can be used to estimate the solution-treatment times needed for different coarsenesses of the microstructure. A model was developed to describe the dissolution and homogenization processes.     

Interfacial reactions in aluminosilicate short fiber-reinforced aluminum matrix composites

Aluminosilicate short fibers are one of the less expensive reinforcements used for the fabrication of metal matrix composites (MMCs). The present investigation evaluates the interfacial characteristics of Al-7Si-0.4Mg (356) alloy reinforced with 10 wt pct aluminosilicate short fibers using optical microscopy, electron microscopy, and X-ray analysis. The fibers used are standard- and zirconiagrade aluminosilicate short fibers. The interfacial analysis has shown the formation of MgAl₂O₄ and Si in both grades of fibers. In addition, ZrAl₃ formation is observed in the zirconia-grade fiber because of the interaction between the matrix and the dispersoid. The zirconia-grade fiber is more susceptible to interfacial reaction than the standard-grade fiber because of the presence of the highly reactive ZrO₂ phase and a lower amount of the Al₂O₃ phase, which provides resistance to the reaction.     

Comparison of precipitates between excess Si-type and balanced-type Al-Mg-Si alloys during continuous heating

Differential scanning calorimetry (DSC) curves were obtained from Al-1.0 mass pct Mg₂Si (balanced) and Al-1.0 mass pct Mg₂Si −0.4 mass pct Si (excess Si) alloys, and precipitates corresponding to each peak at the DSC curve were interpreted by means of high-resolution transmission electron microscopy (HRTEM) observation in order to understand the precipitation sequence of metastable phases. Five peaks were obtained on the DSC curves, from which four were exothermic (A, C, D, and E) and one endothermic (B). Upon HRTEM observation, the peaks for the excess Si alloy were explained as follows: peak A–B: Guinier-Preston (GP) zones and random-type precipitates; peak B: dissolution of the GP zones and the random-type precipitates, precipitation of the β″ phase; peak C: β″ phase and precipitation of type B; peak D: dissolution of the β″ phase; precipitation of type A and β′ phase; and peak E: dissolution of the type B, type A, and β′ precipitation of the (β+Si) phase. This result is quite different from that in the balanced alloy as follows: peak A–B: GP zones and random-type precipitates; peak B: dissolution of the GP zones and the random-type precipitates, precipitation of the parallelogram-type precipitate; peak C: parallelogram-type precipitate and precipitation of β′ phase; peak D: β′ phase, dissolution of parallelogram-type precipitate; and peak E: the β-(Mg₂Si).     

Microstructure and properties of In situ Al/TiB2 composite fabricated by in-melt reaction method

A novel in situ reaction process-in-melt reaction method was developed. TiB₂ particles form in situ through the reaction of TiO₂, H₃BO₃, and Na₃AlF₆ in an aluminum alloy melt. The results showed that the in situ TiB₂ particles formed were spherical in shape and had an average diameter of about 0.93 μm. Moreover, the distribution of TiB₂ particles in the matrix was uniform. The interface between the TiB₂ particles and the matrix showed good cohesion. The tensile strength and the yield strength of the composite increase with increasing TiB₂ content. When TiB₂ particle content in the matrix was 10 vol pct, the tensile strength, yield strength, and elongation of Al-4.5Cu/TiB₂ composite were 417 MPa, 317 MPa, and 3.3 pct, respectively.     

Functionally Graded Al Alloy Matrix <Emphasis Type="Italic">In-Situ</Emphasis> Composites

In the present work, functionally graded (FG) aluminum alloy matrix in-situ composites (FG-AMCs) with TiB₂ and TiC reinforcements were synthesized using the horizontal centrifugal casting process. A commercial Al-Si alloy (A356) and an Al-Cu alloy were used as matrices in the present study. The material parameters (such as matrix and reinforcement type) and process parameters (such as mold temperature, mold speed, and melt stirring) were found to influence the gradient in the FG-AMCs. Detailed microstructural analysis of the composites in different processing conditions revealed that the gradients in the reinforcement modify the microstructure and hardness of the Al alloy. The segregated in-situ formed TiB₂ and TiC particles change the morphology of Si particles during the solidification of Al-Si alloy. A maximum of 20 vol pct of reinforcement at the surface was achieved by this process in the Al-4Cu-TiB₂ system. The stirring of the melt before pouring causes the reinforcement particles to segregate at the periphery of the casting, while in the absence of such stirring, the particles are segregated at the interior of the casting.     

原位反应TiC颗粒对喷射沉积Al-20Si-5Fe合金微观组织的影响

利用熔铸-原位反应喷射沉积成形技术制备了TiC颗粒增强A1-20Si-SFe复合材料.分析了内生TiC颗粒对喷射沉积Al-20Si-5Fe合金微观组织的影响.结果表明:喷射沉积A1-20SI-5Fe合金微观组织中常出现脆性、针状的A1-SI-Fe三元金属间化合物相.在A1-20Si-5Fe合金中内生一定量的TiC颗粒,有助于减小粗晶Si颗粒的尺寸、消除针状的A1-Si——Fe三元金属间化合物相.     

Co-deformation processing and modeling of <Emphasis Type="Italic">In-Situ</Emphasis> multiphase composites

A series of in-situ, deformation-processed metal matrix composites were produced by direct powder extrusion of blended constituents. The resulting composites are comprised of a metallic Ti-6Al-4V matrix containing dispersed and co-deformed discontinuously reinforced-intermetallic matrix composite (DR-IMC) reinforcements. The DR-IMCs are comprised of discontinuous TiB₂ particulate within a titanium trialuminide or near-γ Ti-47Al matrix. Thus, an example of a resulting composite would be Ti-6Al-4V+40 vol pct (Al₃Ti+30 vol pct TiB₂) or Ti-6Al-4V+40 vol pct (Ti-47Al+40 vol pct TiB₂), with the DR-IMCs having an aligned, high aspect ratio morphology as a consequence of deformation processing. The degree to which both constituents deform during extrusion has been examined using systematic variations in the percentage of TiB₂ within the DR-IMC, and by varying the percentage of DR-IMC within the metal matrix. In the former instance, variation of the TiB₂ percentage effects variations in relative flow behavior; while in the latter, varying the percentage of DR-IMC within the metallic matrix effects changes in strain distribution among components. The results indicate that successful co-deformation processing can occur within certain ranges of relative flow stress; however, the extent of commensurate flow will be limited by the constituents’ inherent capacity to plastically deform.