Acoustic emission energy transfer rate: A method for monitoring abrasive waterjet milling

Abstract

Abrasive waterjet (AWJ) milling is an enabling technology for generating complex geometries; however, the lack of methods for online monitoring of jet penetration (i.e. area of abraded footprint) makes difficult to control the quality of the process. By introducing a new concept, Transfer Rate of Energy that links the input jet energy, area of abraded footprint and jet feed velocity and exploiting its property to remain constant for particular set of pump pressure and abrasive mass flow, the paper presents a method to control the jet penetration on AWJ milling. The input jet energy that produces the part erosion is monitored using an acoustic emission sensor mounted on the target workpiece surface, while the jet feed velocity is acquired online from the machine axis encoders. With the pre-evaluation of TRE as specific response of the target material (i.e. Ti6Al4V) to the AWJ milling set of parameters, the area of abraded jet footprint can be calculated on line. To make the method more powerful the input jet energy has been related to the process operating parameters (pump pressure, abrasive mass flow) while their constant values during AWJ milling process have been monitored via a pressure gauge and second acoustic emission sensor mounted on the focussing tube. The strength of the proposed monitoring method relies on the fact that via TRE it is possible to know the quantity of adjustments of the jet feed velocity to enable keeping constant the jet penetration in case of any process disturbances occur. This monitoring methodology has been demonstrated over a wide range of process parameters and it opens avenues for closed-loop control strategies of AWJ milling so that complex features can be generated with minimum human intervention.