Abstract

Through powder metallurgy techniques, it is intended to obtain a metal matrix composite (MMC) of 7075 aluminum alloy reinforced with 1% of multiple wall carbon nanotubes (MWCNTs), which after hot extrusion present mechanical properties similar or even superior to the conventional AA7075 alloy in condition T6. This MMC should be able to be welded by friction stir welding (FSW), thus avoiding the disadvantages presented by the hardenable alloys by precipitation in the welding process. This work focuses on the optimization of the dispersion stage of the MWCNTs in the prealloyed powders of the matrix. For this, three mechanical ball milling strategies are compared. In the first, the pre-alloyed powders and MWCNTs are milled together following a cycle of variation of low energy rotation speeds (LE) (300-200rpm). The second also performs the milling of the mix of powders and nanotubes  by varying the rotation speed in a high energy cycle (HE) (1300-1200rpm). In the third one (H + LE), powders of 7075 are initially milled at high folowing a cycle of high rotation speed (high energy), followed by a low rotation speed stage (low energy) in which the MWCNTs are mixed. From the morphological characterization by SEM and determination of the crystallite size of the powders (XRD), a complete discussion of each of said pathways is carried out.

Abstract

An aluminium matrix nano-composite (AlMNC) was obtained by a powder metallurgy (P/M) process, consisting of an ex situ reinforcement of pre-alloyed AA6005A powders with 3% vol. of TiC nanoparticles (nTiC), high-energy mechanical milling (HEMM) and hot extrusion powder (HEP). AA6005A powder was chosen as matrix because it is a heat-treatable medium strength Al alloy with excellent corrosion resistance, extrusion performance and good weldability; and the nTiC particles due to their low density, good chemical stability and excellent combination of hardness and elastic modulus. HEMM allowed obtaining an ultra-fine grained (UFG) matrix, a uniform dispersion of the nano-reinforcement and its proper incorporation in the ductile matrix. Simultaneously, the repeated plastic deformation of matrix powder led to a high density of crystal defects, and the large number of dislocation barriers produced by nTiC particles are responsible of the obtained nanostructured composites. HEP allowed obtaining high quality extrudates with an a homogeneous distribution of nanoparticles. The hardness of the nanocomposite is almost 43 % higher than that of the unreinforced AA6005A, showing important increases in yield strength and ultimate tensile strength with a 5% elongation. The extruded profiles were Friction Stir Welded (FSW) obtaining sound welds. The grain size refinement on the stir zone (SZ), led to a slight increase in hardness in the welds.