Microstructure and Properties of W-Cu Brazed Joints with Filler Metals and Brazing Atmosphere AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY25080001

Chinese Library Classification Number:TG425+.2

Citation Information:

W-Cu composites/joints have attracted extensive attention in application areas such as electrodes，high-voltage electrical contacts，and plasma-facing components in magnetic confinement fusion reactors，owing to their favorable combination of thermal and mechanical properties. However，joining W and Cu remains a significant challenge，mainly due to the substantial differences in their physical and chemical properties. These include large mismatches in thermal expansion coefficients and elastic modulus，as well as the lack of mutual solubility or intermetallic compound formation in the solid state. Such mismatches can lead to high residual stress concentrations at the interface，potentially resulting in delamination or joint failure during service. Among the various joining methods reported for W-Cu systems （such as diffusion bonding，plasma spraying，explosive welding，and brazing），brazing has emerged as a practical and cost-effective technique due to its relatively low processing temperature，design flexibility，and scalability. The choice of filler metals and brazing parameters play crucial role in determining the microstructure and properties of the resulting joints. Although extensive research has been conducted on the brazing of W and Cu over the past decades，the influence of the brazing atmosphere received quite limited attention. Conducting brazing under a protective gas atmosphere has significant practical value in industrial applications. On the other hand，the presence of protective gases can alter the surface condition of the base metals and potentially affect the wettability of the filler metals. This work systematically investigated the influence of filler metal composition and brazing atmosphere on the microstructure and mechanical properties of W-Cu brazed joints. Four commercially available filler metals were selected：BAg72Cu，BCu89AgP，Ag-27Cu-3Ti and BAg49ZnCuMnNi，all in mass fraction. Brazing was conducted in a muffle furnace under pure N₂ and N₂+H₂ （N₂/H₂=6∶1，volume fraction） atmospheres，and O₂ content was below 0.5×10^-6，respectively. The joining process was performed at 900 ℃ with a holding time of 10 min，followed by furnace cooling. The brazed joints were analyzed in terms of macroscopic appearance，interfacial microstructure，elemental diffusion behavior and mechanical properties (including microhardness and shear strength). The results demonstrated a clear correlation between brazing atmosphere，filler composition，and joint quality. Under a nitrogen atmosphere，BCu89AgP and BAg49ZnCuMnNi yielded visually acceptable joints with well-filled seams and no significant unbonded areas. However，Ag-27Cu-3Ti exhibited uneven filler flow and reacted with the graphite fixture，leading to excessive accumulation of filler on one side and localized graphite adhesion. BAg72Cu showed the weakest bonding behavior，with many joints experiencing delamination of W layer during demolding. In N₂+H₂ atmosphere，BCu89AgP and BAg72Cu both produced joints with good appearance，but Ag-27Cu-3Ti showed intensified reactions with the graphite mold，and BAg49ZnCuMnNi completely failed to wet W-Cu interface，resulting in unjoined specimens. Metallographic and scanning electron microscopy/energy-dispersive spectrometry （SEM/EDS） analyses revealed that all effective joints was absence of a distinct reaction layer or intermetallic phase at W-Cu interface. For BAg72Cu，the brazed seam was composed of α-Cu and Ag-Cu eutectic phases，with no evident elemental interdiffusion into W substrate. BCu89AgP joints consisted of Cu-based solid solution and a ternary eutectic structure containing Cu₃P，Ag，and Cu. These eutectic phases improved wetting and spreading behavior and were largely unaffected by the change in atmosphere，due to the self-fluxing behavior of BCu89AgP. Ag-27Cu-3Ti joints showed a Ti-rich fine-grained region near the edge of the seam，where Ti reacted with graphite and nitrogen to form brittle phases such as TiC and TiN. Elemental mapping showed Ti preferentially dissolved in Cu matrix but was absent in Ag-rich phase. BAg49ZnCuMnNi filler in N₂ atmosphere formed a dense and uniform joint，with clear diffusion of Ni，Zn，and Mn toward W substrate. Ni，in particular，was identified as a key wetting promoter due to its partial solubility in W and ability to form intermetallics such as Ni₄W and NiW. These reactions contributed to enhanced interfacial bonding and mechanical strength. However，in N₂+H₂ atmosphere，this filler failed to spread and wet W and Cu surfaces，confirming that the hydrogen component disrupted the wetting behavior，as previously observed in wetting tests. Mechanical testing showed BAg72Cu joints had the lowest shear strength （9 MPa），due to the lack of metallurgical bonding. BCu89AgP joints exhibited consistent strengths over 200 MPa in both atmospheres，with relatively low porosity and uniform microstructure. Ag-27Cu-3Ti joints reached similar strength levels but exhibited complex fracture morphologies，including stepped features associated with Ti-rich edge zones. The highest shear strength （251 MPa） was achieved by BAg49ZnCuMnNi joints brazed in nitrogen，attributed to enhanced elemental interaction at W interface. Hardness profiling near the interface showed increased hardness values on the Cu side，due to the solution strengthening of Cu substrate caused by interdiffusion between Cu and filler metal. In conclusion，the selection of filler metal and control of brazing atmosphere were both critical to achieving high-quality W-Cu joints. BCu89AgP and BAg49ZnCuMnNi were identified as the most promising filler metals. While BCu89AgP performed well under both atmospheres，BAg49ZnCuMnNi showed superior bonding and mechanical performance in nitrogen but was incompatible with H₂-containing environments. This study provided valuable insights into the microstructure-property relationships in W-Cu brazed joints and offered practical guidance for the fabrication of high-performance components in electronic，thermal，and nuclear applications.