Hot Hydroforging of Lightweight Bimaterial Gears and Hollow Products
Hot Hydroforging of Lightweight Bimaterial Gears and Hollow Products
ABSTRACT
Feasibility of making lightweight powertrain products with hot hydroforging of steel/low density material hybrid billets is explored. A bimaterial billet is designed such that a steel shell encloses a low density core 100%. Furthermore the low density core is selected among the materials that have lower melting or softening temperature than steel such as aluminum and glass. In hot hydroforging the bimaterial billet is heated to 1000-1200 C range similar to the conventional hot forging of steel. However, in hot hydroforging the core is in liquid or viscous state while steel shell is in solid state similar to the conventional hydroforming. During hot hydroforging the viscous/liquid core has negligible resistance to flow thereby providing a uniform hydrostatic pressure inside the steel and enabling a uniform deformation of the solid steel shell.
Steel/aluminum bimetal billets were prepared. Then, the bimetal billets were hot hydroforged in closed dies in one blow. A uniform steel wall thickness was observed all around the forged part upon cross sectioning. However, there was also a large shrinkage void in the aluminum core. The large shrinkage void is formed due to the CTE mismatch between steel and aluminum and the volume increase of aluminum during phase change. The large shrinkage void can be eliminated if aluminum is replaced by glass that has a matching CTE to that of steel. Furthermore, glass is not fully melted at forging temperatures thereby mitigating the risks of phase change. On the other hand the molten aluminum core can be emptied out of steel shell after forging thereby giving rise to the novel concept of "investment forging". A hollow part with uniform steel shell can be formed for the ultimate weight and cost reductions. For example investment forging of hollow steel valves for engine applications is feasible by hot hydroforging.