Dispersion Strengthened Copper Elaborated by Friction Stir Processing

The current study deals with the elaboration of copper based materials reinforced in surface by particles of second phases through friction stir processing (FSP). Its aim is twofold. First, it consists in elaborating such materials and in characterizing their microstructure. Early attempts towards an optimization of the microstructure lead to investigate the effect of process parameters on the material flow and therefore on the microstructure. Second, mechanical properties of the materials are correlated to the microstructure. Yttria particles with a 4 µm diameter were initially located into two grooves machined at the surface of a copper plate. They were subsequently incorporated inside the base material by friction stir processing. The microstructure of the oxide dispersion strengthened materials is characterized at various scales by light microscopy, electron probe microanalysis and scanning and transmission electron microscopy. Emphasis is given to the analysis of the distribution of th powder particles wich were refined up to a nanometric level by FSP. FSP indeed contributes to the dissociation ot the clusters of primitive powder particles and to their intergranular fracture. The uniformity of the particles distribution was also shown to beenhanced with the number of lap-passes. Despite their low volume fraction, the smallest reinforcements present the highest density. Some hardness and tensile tests performed on as processed samples highlighted the strengthening effect of these finest second phases. In order to gain a better understanding of the material flow generated during multipass FSP and to improve the microstructure and properties of the samples, some specimens were elaborated by changing the weld pitch at each lap pass in pure copper. This route demonstrates the effect of eachh pass on the material flow allowing to observe the difference between the number of passes and the number of complete onions observed in transverse cross-sections. At last, all the results give the way towards process optimization.