Metal Injection Moulding of W-10%Cu material with ultra fine composite powder
Technical Paper: PIM International, Vol.5 No.4 December 2011, pages 74-78, 2198 words
 Xiamen Honglu Tungsten & Molybdenum Industry Co. Ltd, 361021, Xiamen, China
 China National R&D Center for Tungsten Technology, Xiamen Tungsten Co. Ltd, 361026, Xiamen, China
W-10Cu is one of the most widely used materials for thermal management applications. The sinterability of this material is, however, not enough for reaching high density with normal powder. An in-house developed W-10Cu ultrafine composite powder has been employed for MIM. The torque rheology has been investigated to the feedstock with this ultrafine powder. The W-10Cu parts sintered at 1400˚C has reached relative density of 99%.
The microstructure of sintered W-10Cu is homogenous and grain size for W is about 2-3 µm. Thermal conductivity and coefficient of expansion of the sintered part are 215 W/mK and 6.7X10-6 /K respectively.
A comparison of microstructure and properties between W-10Cu MIM parts and Cu infiltrated products is presented.
Heat dissipation is becoming a crucial challenge for power semiconductors due to the increase of leakage current and decrease of device size. Therefore, advanced electronic packaging materials with improved thermal, electrical and mechanical performances play an important role in proper functions of the devices.
Tungsten-copper (W-Cu) composite is an attractive thermal management material for power electronics due to the combination of high thermal conductivity of Cu and low coefficient of thermal expansion (CTE) of W. In these kinds of composite materials, Cu generally is in the range of 10-20 wt.% (19.3-35.0 vol.%) and distributes in the W matrix.
By changing Cu content in the composite, tailored CTE is obtained and meanwhile a relatively high thermal conductivity is maintained. The fabrication method is usually infiltrating molten Cu into the press-sintered W preforms. Sometimes thermal mechanical process such as rolling is employed on the infiltrated W-Cu blanks to obtain fully dense material and desired shape. However, this process is restricted to certain geometries.
Metal injection moulding (MIM) based on composite W-Cu powders makes it possible to form the thermal management devices in complex shapes. MIM for W-Cu composite has been investigated previously [1-3]. For those with Cu content higher than 20 wt.%, blended or milled W and Cu powder could be used. However, for materials with low Cu content, such as 10 wt.% (W-10Cu), the sintered density is not high enough since the Cu content is too small in volume for a sufficient W particle rearrangement during liquid phase sintering. Even using fine W (D50 = 1.54 μm) and Cu (D50 = 3.54 μm) powder, only a sintered density of 92.4% was achieved .
On the other hand, to blend W and Cu powder homogeneously becomes more difficult with decreasing Cu contents. Ultrafine or nano-scale W-Cu composite powder is an alternative to solve these problems. Various methods such as ball milling , mechanical alloying [5, 6], oxide co-reduction , mechano-chemical process [8, 9] and thermo-chemical process [10-14] have been used to fabricate ultrafine W-Cu composite powder.
Kim et al.  investigated the MIM process for W-30Cu with ultrafine composite powder made by mechanical alloying and the sintered density was high than 96%. Senillou et. al  used W-20Cu composite powder for MIM and sintered density of 97.9% was reached. German  presented MIM product properties of W-10Cu via MIM with the density of 98%. In the present study, the in-house developed W-10Cu ultrafine composite powder via thermo-chemical process was employed for MIM. The process was evaluated regarding homogeneity of the microstructure, the density of fabricated parts and properties of the MIM parts..........
Further sections of this paper include:
- Characteristics of W-10Cu composite powder
- Feedstock mixing
- Injection moulding
- Debinding and sintering
- Property measurements
Figures and Tables:Fig. 1 SEM images of W-10Cu ultrafine composite powder
Fig. 2 Cross section of W-10Cu ultra fine composite powder showing the distribution of W and Cu in the particles
Fig. 3 Torque rheology for ultrafine W-10Cu composite powder: (a) torque vs. kneading time and (b) torque vs. solid loading
Fig. 4 Injection moulded, debinded and sintered W-10Cu tensile test specimens
Fig. 5 Sintered and polished blanks in W-10Cu for preparing the specimens to measuring thermal conductivity and coefficient of thermal expansion
Fig.6 LOM images of sintered W-10Cu parts produced by: (a) MIM with ultrafine composite powder and (b) Cu infiltration with W preforms
Fig. 7 SEM images of sintered W-10Cu parts produced by: MIM with ultrafine composite powder (a, b), Cu infiltration with W performs (c, d)
Table 1 Chemical compositions of W-10Cu composite powder
Table 2 Characteristics of the W-10Cu composite powder
Table 3 Moulding parameters for W-10Cu feedstock
Table 4 Comparisons of material properties of W-10Cu with two processes
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