Spark ignitable Ni–Al ball-milled powders for bonding applications

Ball milling of aluminum (Al) and nickel (Ni) particles at the NiAl composition can produce reactive powders with low spark ignition thresholds similar to magnetron sputtered multilayer foils (MFs). Such powders can replace MFs for bonding applications when fashioned into suitable geometries. For this purpose, Al and Ni particles were milled in a planetary ball mill for 9 hours at 300 rpm. Loose powder piles and 840 μm thick cold-compacted pellets with 25.4 μm Al overlayers were ignited using a low-energy spark. Additionally, the loose particles were used for bonding Al cylinders, whereas pellets with overlayers were used to bond silicon (Si) wafer pieces. Maximum temperature observed in loose powders was around 1770 K, consistent with differential scanning calorimetry results. The maximum temperature of the freestanding pellet was 1610 K, close to the calculated adiabatic temperature of 1575 K. The thermal front velocities were 0.06 and 0.22 m/s for loose powder piles and pellets, respectively. Bond interfaces of Al cylinders showed formation of NiAl3 within regions close to the interface, indicating large amounts of Ni diffusion. Likewise, bonded Si interfaces showed evidence of Si melt and dissolution into Al-rich liquid and Ni diffusion into the Al overlayers forming dendritic NiAl3.