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Technological innovation

Technological innovation

The preparation and application of tungsten copper materials

W-Cu composite possesses unique properties of superior thermal and electronic managements,high microwave absorption capacity and etc.These features make the W-Cu composite a very

attractive material which has been widely used as heavy-duty electronic contactors,circuit breakersand thermal management devices,and SO on.However,due to their relatively large difference inproperties between W and Cu,the preparation process ofthis composite requires higher requirement.

W-Cu composite prepared by the conventional methods has drawbacks such as low densification, microstructure inhomogeneity and/or limited composition variation which hinders its further applications.With the rapid development of the electronic information industry and the hi【gh·tech

fields of defense industry, there are several new directions of development and requirements on high performance W-Cu composites,such as

(1)to explore the preparation process which can be applied to industrial production;

(2)to further improve the deusification and microstructure uniformity of W-Cu composites with a higher performance;

(3)to develop new high-performance W-Cu composites to meet the requirements of the high-tech fields;

(4)to expand the applications of W-Cu composites.In this paper, W-Cth W-Cu/AIN composites and W-Cu functionally graded

materials with high performance were fabricated by combining mechanical alloying technique and pressureless sintering or hot-pressing technique.

And the technic parameters of process,optimal design and properties of those composites were investigated and analyzed.The results of those studies would provide a theoretical basis and data base for the production and application of high·performance W-Cu composites.

The mechanical alloying process of nanocrystalline W-Cu composite powder with different contents was investigated.The process characteristics,structural evolution and thermal stability of the MA、Ⅳ.Cu nanocrystalline powder were investigated in detail by analysing phase transformation,

composition distribution,grain size,lattice parameter, microstrain,and morphology changes of the composite powder during mechanical alloying and subsequent annealing.The results showed that room temperature stable supersaturated W(Cu)solid solution formed after 30h,40h and 60h milling

of W-1 5Cu,W-20Cu and W-30Cu composite powder.During mechanical alloying of W-15Cu powder,the grain size decreased with increasing milling time,and the particle size first increased then decreased with increaSing milling time.After milling for 30h,particles were polyhedral in shape with smooth surface,and average particle size Was about 49m.At the initial stage,the structure of W-15Cu powder Was the ring-like composite layer generated by the Cu particles enwrapping around the W particles。In the middle stage,the composite particles of circle—like layers became finer and the space between the layers greatly reduced.The mixed microstructure of

homogeneously dispersed W and Cu WaS formed.After milling for 30h,the powder was composed of homogeneous W(Cu)solid solution phase.Annealing of as-milled W-15Cu powder reduces the

lattice deformation and internal strain,and incresased the lattice ordering degree.Meanwhile,Cu precipitated from the supersaturated W(Cu)solid solution at the temperature of 500"C.

Liquid phase sintering,or hot pressing Was used to consolidate MA nanocrystalline W二Cu powders to bulk compacts of W-15Cu,W-20Cu,W-30Cu,and bulk composites of W-Cu/AIN with

different contents of A1N(0.25,0.5,1.0,2.0wt.%).The microstructure,physical properties such as density,thermal conductivity,electrical resistivity conductivity,mechanical properties such as hardness,bending strength and etc.of these materials were characterized and investigated.

The effects of process parameters of hi energy ball milling and sintering on the microstructure and properties of W二Cu composite。and the sintering densification mechanism of MA powders were studied.The results showed that mechanical alloying technique promoted the sintering of W二Cu composite powder, strengthened the interaction and increased the contacts between W and Cu by grain refining,enhancing the microstructure homogeneity of powder and the formation of

W(Cu)solid solution。Thus,the relative density and microstructure homogeneity of W二Cu composites were effectively enhanced.Which means mechanical alloying is one of the optimal preparation processes to obtain near full dense W二Cu composites.Considering all factors,the optimal sintering process parameters were as follows:forming pressure was 350MPg sintefing temperature and time were 1200"C and 90min.The relative densities ofW-15Cu, W-20Cu and W-30Cu composites were 98.42%,99.10%and 99.34%,respectively by using the above’mentioned parameters.Finer structure bulk W二Cu composites were successfully synthesized by the means of hot pressed vacuunl sintering at 1200℃for 90min under the pressure of 25MPa. The relative densities of W二Cu composites with three different compositions were 97.87%,98.29%

and 98.94%,respectively.Addition ofsmall amount ofnano A1N particles had less negative effect on degradation of relative density.The relative density of W二Cu composites remained at about 98% when the addition of AIN Was l wt%.Nano AIN particles were even distributed in Cu phase of the

matrix.The hardness of Cu in the matrix would increase due to the reinforcement effect brought about by grain refining and dispersion strengthening.The increasing strengthening particles distributed at grain boundaries with the increasing of AIN content,would lead to the decrease of bending strength of composites by affecting the combination between adjacent particles and densification of materials during sintcdng process.However,the thermal conductivity of composites increased with increasing AIN content.

The residual thermal stress of W-Cu functionally graded materials arising from the fabrication process Was analyzed using finite element method(FEM).Based on the calculation results of the residual thermal stress and stress state in graded layers,W—Cu functionally graded materials

with three layers and four layers structure were designed.W-20%Cu-33%Cu一50%Cu FGM=2.4)and W-20%Cu/W-29.1%Cu/W-39.2%Cu/W-50%Cu FGM(b1.4)had the minimal equivalent thermal stresses and the thermal stress werc reduced by 52%and 68%respectively comparing with the non-FGM.Based on the optimization results,two kinds of W.Cu FGMs with high density and pretty microstructure were prepared by hot-pressing sintering.The thermal eonductivities of W-Cu FGMs with three layers and four layers structure were 198 W·m‘1K~and 202 W-m‘1K1 respectively.After the thermal shock test with 800"C temperature difference,no cracks were found at the interface of two kinds of FGM samples.The two kinds of FGM survived up to 86 and 1 43 thermal cycle tests respectively.Cracks were found at the interface at both ends of FGMs,which WaS consistent with the previous calculation results of the residuaI thermal stress.These results indicated the two kinds of FGMs had excellent heat resistance and exhibited good property of reducing thermal stress.