For a HIP operation, conventional control of the process variables (T, P) is the
current mode used. The process variables follow a prespecified trajectory
(schedule) assumed to lead to the final quality of product. A second mode is
advanced control, where on-line measurements of a product state variable enable
to monitor the evolution of the product variables and if necessary to compute
corrections to the schedule in order to obtain the desired quality of the final product.
HIP advanced process control requires continuous monitoring of the sample
density during all stages of the process and the on-line identification a process
model. To this end, a dilatometer has been built for in situ monitoring densification
during a HIP operation, the Ashby process model has been validated and on-line
identification has been demonstrated. It is shown that advanced process control of
a HIP operation can be performed in two steps : on-line identification of the
process model and subsequent process path replanning. The first step can only
be performed during the process variables plateau of the preset temperature-
pressure cycle, and process path replanning consists of a new temperature-
pressure plateau.