Patent GB-1332523-A (GB)
Title: Brazing method and appratus
IPC: B23K1/00
Filed: 1971-01-07
Abstract: 1332523 Brazing; making honeycomb panels; stamping AERONCA Inc 7 Jan 1971 [7 Jan 1970] 881/71 Headings B3A B3Q and B3R [Also in Division H5] Two face sheets 11, 12 and a honeycomb core 13 are brazed together to form a sandwich panel for e.g., supersonic speed aircraft and space vehicles, by successively (i) initially tack welding or tack brazing the sheets and core together to form an assembly 10, (ii) disposing the assembly between upper and lower ceramic dies 32, 33, (iii) raising the assembly to brazing temperature by resistance heating elements interposed between the assembly and the dies, (iv) compressing the assembly between the dies and, finally, (v) cooling the brazed assembly by gas blown through openings in the working faces of the dies. The brazing may be effected in hard vacuum or in an inert (e.g. argon) atmosphere at sub-atmospheric pressure. The face sheets and core may be of stainless steel or heat-resisting nickel alloy, the core being formed as by spot welding together corrugated strips. Brazing alloy, such as 23% Mn, 4% Cu, 2% Si, 71% Ni, is applied to the core edges as by dipping in or spraying with a brazed powder or the alloy may be in the form of interposed thin sheets. The process may be applied to the brazing of planar or, as shown, curved panels, The process may also be used to finish curved shape, by hot creeping, tacked assemblies of planar or lesser curvature configuration. Apparatus.-The upper and lower dies 32, 33 are respectively fast and vertically slidable in half-housings 15, 16 fastenable together by screw C-clamps 26 and flange sealed by an O-ring 24. Argon entry and vacuum lines 73, 75 are connected to the half-housing 16. Workpiece cooling gas (pre-chilled argon) is directed through upper and lower lines 67 and manifolds 42, and longitudinal passages 41, 58 and transverse openings 43, 71 in the dies 32, 33. The die working faces may be smooth or networked with grooves communicating with the openings 43. A lower inflatable (e.g. fabric or stainless steel) bag 48 is disposed between the lower die 33 and the half housing 16 and connected to a line 77. The resistance heating elements comprise strips 81, 83 connected between bus-bars 78, 80, 82 which are connected to power cables 77 and preferably supported by tension springs 79. Local heater cooling for temperature uniformity across the assembly 10 may be achieved by cooling water pipes adjacent the bus-bars (Fig. 10, not shown) or, as shown, by an electron emission cooling circuit comprising probes 84 positioned adjacent the bus-bars and so connected to high voltage D.C. that, on completion of a circuit to a probe, electrons are emitted from a bus-bar to probe, monitoring being effected by thermocouples. An upper, e.g. stainless steel, welded bag 90, sized to suit the assembly 10, is, in use, disposed between the upper die 32 and the upper face sheet 12. Water vapour in the housing 15, 16 is trapped on the wall on a cold well (Fig. 8, not shown) which contains e.g., liquid nitrogen and is connected to the housing. The housing half 15 is raiseable as by a pulley and rope 18. Operational steps: (1) An insulating blanket 100, the lower heating strips 83, an insulating sheet, e.g. of fibrous silica, 101 a copper or like heat distributing sheet 102, a stainless steel slip sheet 99, the assembly 10, a slip sheet 109, the upper bag 90 (not pressurized), a copper sheet 103, a fibrous silica insulating sheet 104, the upper heating strips 81, and an insulating blanket 105 are placed, one on top of the other, over the lower die 33. (2) The upper half housing 15 is lowered and clamped on to the lower half housing 16. (3) To flush out oxygen and water vapour, the housing is repeatedly alternately evacuated and argon filled through the lines 75, 73, this being facilitated by energizing the strips 81, 83 to raise the temperature to 500 F. approx. (4) The housing is argon filled at 5 inches mercury, such pressure being maintained by continuous use of both lines 73, 75. (5) The temperature is raised to brazing temperature, e.g. 1950 F. (6) The lower bag 48 is pressurized to raise the lower die to accommodate the assembly to the dies. (7) The upper bag 90 is pressurized (at ¢ to 3 p.s.i.) to apply a brazing force normal to the assembly. (8) The strips 81, 83 are de-energized and bags 48, 90 evacuated, and at the same time pre-chilled argon is fed through the die openings 43, 71, the dies being thus separated and the brazed assembly floating between cooling gas layers.