MICROSTRUCTURAL AND MECHANICAL ASPECTS OF REINFORCEMENT WELDS FOR LIGHTWEIGHT COMPONENTS PRODUCED BY FRICTION HYDRO PILLAR PROCESSING

摘要

The development of new creep resistant and cost effective die casting magnesium alloys such as AE, MRI, MEZ, ACM, AXJ, AJ, WE have emerged as an alternative to fulfil the actual demands in structural relevant applications as engines blocks, gear and converter boxes. However, magnesium components are in most of the cases screwed with aluminium and steel bolts, which lead the screwed joint to lose the preload force due to relaxation. This barrier limits thus the broad use of magnesium within this segment and should somehow find an adequate solution to be implemented and to help overcoming this limitation. In this context Friction Welding (FW) and particularly Friction Hydro Pillar Processing (FHPP), which can be described as a drill and fill process, appears as an alternative to widespread the use of magnesium. In this context, FHPP is intended to be used to locally reinforce mechanical fastened magnesium components. In the present work a preliminary experimental matrix was defined and used to determine optimal welding conditions. Furthermore elaborate experimental techniques have been used to describe the process parameters-microstructure-properties relationships and the consequent mechanisms leading to bonding in FHPP welds in dissimilar configurations. The welds have been performed using a hydraulic powered friction welding machine, originally designed and built as a portable stud welding unit, delivering up to 40 kN welding force and 8000 rpm. All welds were monitored, analysed and evaluated using a purpose built data recording system. AZ91 and AXJ magnesium cast ingots have been used in the experimental programme. The results obtained in the course of this study have shown the feasibility of FHPP to produce high strength welds with mechanical properties comparable to those from base material. Defects, like porosity or lack of bonding, were not observed. It could be demonstrated that for dissimilar AXJ to AZ91D welds the consumable member is fully plasticized across the bore of the hole and through the thickness of the workpiece. Mechanical properties of the welded joints have shown values similar to those from AZ91 base material. An increased upsetting indicates no clear variation of tensile strength, with values, in both cases, significantly superior to those from AXJ base material due to the formation of a completely different microstructure in the extruded zone after welding. Hardness values achieve in some points values up to 80HV, which means that in the extruded AXJ material an overmatching condition was created.