For spot welding of two steel sheets (base metal) each of 3 mm thickness, welding current of 10000 A is applied for 0.2 s. The heat dissipated to the base metal is 1000J. Assuming that the heat required for melting 1mm3 volume of steel is 20 J and interfacial contact resistance between sheets is 0.0002 W, the volume (in mm3) of weld nugget is _______