Gao Hui Wu

Microstructure and Properties of a 70vol.% SiCp/Al-12Si Composite for Electronic Packaging

For the purpose of electronic packaging applications, a homogenous and void free 4032Al (Al-12wt.%Si) matrix composite with 70vol.% SiC particles was fabricated by squeeze casting technology. TEM observations indicated that SiC particles acted as heterogeneous nucleation sites for Si phases in the matrix and the SiC-Al interfaces were clean and free from any interfacial reaction products. The composite possessed lower coefficients of thermal expansion (CTEs) and excellent thermal conduction properties, and the CTEs could be further reduced by annealing treatment because of the alteration of original thermal residual stresses within the specimens. The incorporation of high volume fraction of SiC particles also induced enhanced mechanical properties for the composite, and its moduli even exceeded 200GPa. Some diodes were finally produced with nickel plated SiCp/Al baseplates and the results of thermal cycling tests between -55ı and 150ıfor these diodes were presented.

Microstructure and Mechanical Properties of High Densification Mo/Cu Composites

Microstructure and mechanical properties of the 55%, 60% and 67% Mo/Cu composites for electronic packaging application fabricated by a patent squeeze casting route have been investigated. The results show that Mo particles are homogeneously distributed in the matrix, and the Mo-Cu interfaces are clean, free from interfacial reaction products and amorphous layers. The densification of the Mo/Cu composites is higher than 99%. The as-received composites exhibit a Brinell hardness varying from HB178.1 to HB196.9 and an elastic modulus varying from 177GPa to 213 GPa. The tensile strength of the composites is higher than 480MPa. Moreover, the composites display favorable plasticity, while the elongation of the 55% Mo/Cu composite is as high as 5%. Obtaining high tensile strength and elongation in the composite is attributed to the high densification, as well as the clean and smooth Mo-Cu interfaces, both resulting from the cost-effective squeeze-casting technology.

Synthesis Nano-SiCp/MoSi2 Composites by In Situ Reaction Sintering and Low Temperature Oxidation Behavior

Nano-SiCp/MoSi2 composites with different SiC volume fraction have been synthesized by in situ reactive hot press sintering with the milled Mo, Si and C elements powders. The X-ray diffraction (XRD) detection shows the composites are composed of α-MoSi2 and β-SiC, and transmission electron microscopy (TEM) shows that the particle sizes of β-SiC formed by in situ reaction are in range of 20-100nm, and most of the them locate in grain boundary and some particles embed in MoSi2 grain. The isothermal oxidation and thermal cycling test of nano-SiCp/MoSi2 composites were carried out in air at temperatures of 500°C. The pest phenomenon was not found during oxidation and cyclic oxidation at 500°C for 500 hours and 50 cycles, respectively. All the composites have both excellent oxidation resistant and good cyclic oxidation resistant. The mechanisms of oxidation at low temperature were discussed also.

Influence of Nowonty Phase on the Fracture Toughness of SiC/MoSi2 Composites

MoSi2-SiC precursor powder has been synthesized via the mechanical alloy method with the elements Mo, Si and C powder. The SiC/MoSi2 composites with different SiC volume fraction have been prepared by reactive hot-pressing the precursor powder at 1350 °C. There is a significant increase in the fracture toughness due to addition of SiC reinforcement. However, the intermediary ternary phase exists in this system—namely, Mo5Si3C, Nowotny phase, which has negative effect on the fracture toughness, and can be eliminated through high temperature treatment at 1600 °C for 2 hrs.

Mechanical Properties of Interpenetrating Graphite/2024Al Composites Produced by Improved Squeeze Exhaust Casting

The graphite/2024Al composites have been fabricated by improved Squeeze Exhaust Casting (SQEC) method. Two kinds of graphite preforms with porosities of 13% and 17% respectively were infiltrated with 2024Al (Al-5Cu-2Mg) alloy under the pressure of 73MPa. The disadvantages of traditional Squeeze Casting (SQC) were avoided and the distribution of aluminum alloy appeared homogenous 3D network in the composites. Flexural strength and Young’s modulus were determined at room temperature. Compared to graphite preform, the composites exhibited a significant enhancement of mechanical properties. The flexural strength and Young’s modulus of X-Y direction of G186/2024Al composites increased from 38.6MPa to 99.7MPa and from 10.1GPa to 19.7GPa, respectively. The fracture mechanism of the composites was discussed on the basis of fracture surfaces.

Influence of Deformation on Precipitation Behavior of 2024Al Alloy

In the present work, 2024Al alloy was rolled before solid solution, between solid solution and aging, and after aging treatments. Precipitation sequence in 2024 Al has not been altered by the rolling treatment. However, peak aging temperature of S’ phase was decreased from 264 to 254 oC, implying that the process of S’precipitation was accelerated by the deformation. After thermo-mechanical treatments, precipitates werefiner dispersed, and finest precipitate distribution was observed after treatment III (rolling between solid solution and aging), while precipitate size was all smaller than 240nm and more than 95% precipitates was smaller than 200nm. The highest dislocation density was found in 2024Al alloy after treatment III since the dislocations generated during deformation was slightly decreased during following low temperature aging treatment. Therefore, in order to improve the mechanical properties, it is suggested that the rolling treatment should be performed between solid solution and aging treatment.

Microstructure and Compression Behavior of Cenosphere Filled Aluminum Syntactic Foams

Aluminum syntactic foams were fabricated by pressure-infiltrating liquid pure aluminum into packed preforms of cenosphere fly ash. The morphology, true density and porosity of fly ash microballoons were characterized. The microstructure of the syntactic foams demonstrated uniform distribution of the microballoons in the aluminum matrix and seldom infiltration of cenosphere fly ash. These foams were subjected to quasi-static uniaxial compression tests and behaved like high strength aluminum foams under compressive deformation, exhibiting an extended plateau region in the stress–strain curves. With the decreasing of fly ash diameter, the plateau stress and absorbed energy of the syntactic foams increased. X-ray microcomputed tomography was used to examine the foam microstructures after interrupted compression and reveal the damage evolution. The current work provides a better understanding on the structure and mechanical properties of aluminum matrix syntactic foams.

Effect of Hollow Structure on the Electromagnetic Interfering (EMI) Shielding of Aluminum-Cenospheres Composites

The electromagnetic interference shielding effectiveness (EMISE) properties are measured using coaxial cable method in the frequency range of 10 -1200MHz. EMI analysis indicates EMISE properties of these three types of composites is mainly influenced by the hollow structure of cenospheres, suggesting the common effects of reflection and multiple reflections mechanisms. On the other hand, the change of matrix from pure aluminum (1199) to aluminum alloy (2024) on EMISE properties can be negligible.

Effect of silicon addition and thermal history on the thermal expansion behavior of SiC/Al composites

Two Al-Si alloys (Al-12Si and Al-20Si) and an industrial pure Al were reinforced with 70vol.% dual-sized SiC particles. The composites experienced annealing treatment, to investigate the effect of silicon addition and thermal history on the thermal expansion behavior of high SiC content aluminum matrix composites. The results showed that silicon additions led to a beneficial reduction in the coefficients of thermal expansion (CTEs) of the composites. In the temperature range between 20°C and 400°C, a continuous increase in CTEs with temperature was observed for SiCp/pure Al composite. However, the CTEs of SiCp/Al-12Si and SiCp/Al-20Si showed the maxima at 350°Cand 250°C respectively, then diminished at higher temperatures. This was related to the change of solid solubility of silicon in aluminum at elevated temperatures. The thermal expansion behavior of SiCp/Al composites was also influenced by thermal history. After annealing treatment, the CTEs were reduced when compared with those of as-cast composites. Annealing treatment reduced the original thermal residual stresses, and then altered thermal expansion behavior of the composites.

Effects of Particle Size on Microstructure of the Matrix Alloy in Aluminum Matrix Composites

Aluminum matrix composites, reinforced by 0.15μm and 5μm Al2O3 particles with 40% volume fractions were fabricated by squeeze casting technique. The microstructure characterization near the interfaces of Al2O3p/1070Al composites was investigated by SADP and HREM techniques. Results showed that high-density dislocations were generated in the 5μm-Al2O3p/Al composite due to the thermal mismatch stress. In contrast, the matrix of the 0.15μm-Al2O3p/Al composite appeared to be nearly free dislocations and some “micro distortion areas” of 1-5nm were observed, which was attributed to the dispersion of fine sub-micron particles and uniform distribution of the stress near the interfaces.