A titanium mother blank is a sheet of high purity titanium~ typically 36 inches wide by 40 inches long and 0.125 inches in thickness. It is suspended from a copper hanger bar having typical dimensions of 48~ X 1.75~ X 0.75~.
It is essential that the joint between titanium and copper be strong mechanically and also electrically and that these properties remain constant for a period of years. Cells containing titanium mother blanks are operated at 140-150F
and low current density is used to deposit very fine grained sheets of pure copper on both sides of the immersed portion of the titanium. The mother blanks are removed from the cells at the end of 24 hours and the copper deposits are removed in the form of complete sheets, 25 or 30 mils in thickness~ which resemble highly flexible foils. Titanium tlas evolved over recent years as the preferred material for cathodes for the production of starter sheets because it can be plated uniformly with copper and the deposit can be stripped readily and completely.
Titanium is commonly welded to itself by electrical r~sistance welding or by fusion welding. These methods can-not be used to join titanium to copper (or to most other Z~6 metals) because~ at welding temperatures~ titanium forms intermetallic compounds with copper. These brittle com-pounds preclude formation of a mechanically sound joint.
Titanium sheets have been joined to copper hanger bars using rivets or nuts~ bolts and washers. Through holes are provided in hoth the sheet and hanger bar and the fasteners are closed under controlled conditions. The use of fasteners has the advantage that construction of the cathode is relatively inexpensive. A disadvantage of fast-eners is that one or more fasteners on a cathode may loosen in time due to daily mechanical handling of the cathode plus daily thermal cycling from 14~-150F to ambient temper-ature. Loose fasteners result in imperfect electrical con-tact between the titanium sheet and copper hanger bar and thls causes uneven deposition of copper resulting in imper-fect starter sheets.
Titanium sheets have also been joined to copper hanger bars without use of fasteners. This is done by first making a titanium sheathed copper hanger bar by co-extrusion of the two metals at elevated temperature. The temperature is such (400 to 800C) that the two metals deform plastical-ly and intimate contact results but the temperature is below that at which metal-to-metal welding~ accompanied by deleter-ious formation of intermetallic compounds~ occurs. The ti-tanium sheet is then joined to the titanium sheath of the hanger bar by conventional welding, e,g. by electrical resis-tance welding~ This joinder method is the subject of U. S~
patent No. 3~857~774 which teaches that cathodes made in this 6Z~36 manner have more stable electrical characteristics than those made with rivets or nuts and bolts. A disadvantage of co-extruded hanger bars is that they are relatively ex-pensive due to the capital cost of the massive equipment necessary for their manufacture. A second disadvantage is that the co-extrusion process makes best use of both metals when the copper core is completely sheathed by titanium. Using January~ 1979 pricesl copper costs $0.75 per pound and titanium costs $6.50 per pound~
To the extent that titanium is used in any area other than that to which the titanium sheet is to be welded~
the co-extrusion process is wasteful of this relatively ex-pensive metal