Evaluation of contact welding and strengthening of joints by non-contact discharge

Our probe manipulation apparatus has the ability to weld using a tungsten needle-like probe as an electrode, assisting with ease of manipulation of fine metal particles. Gold particles 60-80 μm in diameter were welded to a gold substrate using the apparatus. Contact welding is carried out by applying high voltages of 4 kV or more to the probe which is in contact with the particle at a very low contact pressure. In this paper, estimation of the welding strength is done and the joint is strengthened by arcing. After welding, the joint was fractured by tensile loading, and the fractured surface was observed by SEM. The area of the joint is within a circle of 5 μm in diameter regardless of welding time of 0.5-10 s. The fracture load is estimated to be about 2.6×10−3 N. This value is 50,000 times greater than the weight of an 80-μm gold particle, but is only 0.26 gf. Arcing by the apparatus, which we term 'non-contact discharge', appears to strengthen the joint. The probe is positioned above the welded particle and 2 kV is applied to the probe. The particle is taken off after the non-contact discharge, and the surface is observed to be fractured. Non-contact discharge was conducted in air and in N2 gas flow. Only non-contact discharge in N2 gas flow is effective for strengthening the joint. The top of the particle is sputtered by the discharge streamers. The sputtered droplets are scattered around the particle during non-contact discharge in the air but pile up near the root of the particle with N2 gas flow. The droplets form an annular ridge to which the particle becomes attached. The welding strength is, therefore, increased by non-contact discharge in N2 gas flow.