Process for making hollow articles by differential heat treatment

Claim 1

A PROCESS OF FABRICATING AN ALUMINUM SHEET METAL PANEL HAVING A RELATIVELY SMOOTH HARD COMPONENT LAYER AND HAVING DISTENSIONS RAISED FROM THE OPPOSITE INTEGRALLY UNIFIED SOFTER COMPONENT LAYER, BOTH SAID LAYERS BEING FORMED OF SUBSTANTIALLY THE SAME COMPOSITION, SAID PROCESS COMPRISING THE STEPS OF: (A) PROVIDING A PAIR OF ALUMINUM ALLOY INGOTS HAVING SUBSTANIALLY IDENTICAL COMPOSITION, (B) SUBJECTING ONE OF SAID INGOTS TO A HOMOGENIZATION TREATMENT TO FORM A COARSE PARTICLE SIZE DISPERSION OF INSOLUBLE CONSTITUENTS THEREIN, SAID TREATMENT CONSISTING OF A SOAK OF THE INGO AT A TEMPERATURE WITHIN THE RANGE OF 900* TO 1150*F. FOR A PERIOD OF FROM 4 TO 72 HOURS, (C) ROLLING SAID INGOTS TO SHEETS WITHIN THE TEMPERATURE RANGE OF FROM 800 TO 1000*F. IN WHICH SAID COARSE PARTICLE SIZE DISPERSION IS UNAFFECTED AND IN WHICH A FINE PARTICLE SIZE DISPERSION OF INSOLUBLE CONSTITUENTS IS FORMED IN THE SHEET FROM THE OTHER OF SAID INGOTS, (D) FORMING A PANEL BLANK BY POSITIONING SAID SHEETS ADJACENT ONE ANOTHER, ONE OF SAID SHEETS HAVING APPLIED TO A CONFRONTING SURFACE THEREOF A PATTERN OF STOP WELD MATERIAL, (E) HOT ROLLING SAID BLANK TO INTEGRALLY UNIFY SAID SHEETS TOGETHER AS A COMPOSITE PANEL IN THE AREAS NOT COVERED BY SAID STOP WELD MATERIAL, (F) COLD ROLLING SAID COMPOSITE PANEL TO DEVELOP MAXIMUM STRENGTH IN SAID SHEETS BY WORK HARDENING, (G) PARTIALLY ANNEALING SAID PANEL WITHIN THE CRITICAL TEMPERATURE RANGE OF FROM 500* TO 655*F. FOR A PERIOD OF FROM 5 MINUTES TO 10 HOURS IN WHICH SAID SHEETS HAVING SAID COARSE PARTICLE SIZE DISPERSION UNDERGOES CONSIDERABLE LOSS OF STRENGTH BY RECOVERY BUT IN WHICH SAID SHEET HAVING SAID FINE PARTICLE SIZE DISPERSION UNDERGOES ONLY SLIGHT LOSS OF STRENGTH BY RECOVERY THEREBY DEVELOPING A DIFFERENTIAL IN YIELD STRENGTH BETWEEN SAID SHEETS, AND (H) INFLATING SAID COMPOSITE PANEL IN SAID UNJOINED PORTIONS BY THE APPLICATION THEREIN OF A FLUID UNDER PRESSURE.