Modification of Cast Al-Mg2Si Metal Matrix Composite by Li

Abstract

The effects of both Li modification and cooling rate on the microstructure and tensile properties of an in-situ prepared Al-15 pct Mg 2 Si composite were investigated. Adding 0.3 pct Li reduced the average size of Mg 2 Si primary particles from~30 to~6 lm. The effect of cooling rate was investigated by the use of a mold with different section thicknesses from 3 to 9 mm. The results show a refinement of primary particle size as a result of both Li additions and cooling rate increases, and their effects are additive. Similarly, both effects increased ultimate tensile stress (UTS) and elongation values. The thin sections show somewhat unexpectedly low and scattered tensile results attributed to the casting defects observed in fracture surfaces. The Li-modified alloy displays serrated yielding behavior that is not fully explained here. The refinement by Li and enhanced cooling rate is explained in terms of an analogy with the effect of Sr and cooling rate in Al-Si alloys, and is ultimately attributed to the effect of the alkali and alkaline earth metals deactivating oxide double films (bifilms) suspended in Al melts as favored substrates for intermetallics.

The sequence of intermetallic formation and solidification pathway of an Al–13Mg–7Si–2Cu in-situ composite

“…It is difficult to form perfect octahedral primary Mg 2 Si due to the complexity of the solidification conditions. In a practical solidification process, primary Mg 2 Si particles normally form imperfect octahedrons [22] or a hollow shape [25] similar to coarse primary silicon in the Al-Si system, marked as B in Fig. 6a.…”

Effect of process variables on Mg-Si particles and extrudability of 6xxx series aluminum extrusions

“…Figure 1 shows the Al-Mg 2 Si phase diagram. 10 The constituents form a eutectic at 595°C. There is an appreciable solid solubility in aluminum reaching 0.85 wt.% Mg and 1.10 wt.% Si at the eutectic temperature.…”

Evaluation of cutting force and surface roughness in the dry turning of Al–Mg2Si in-situ metal matrix composite inoculated with bismuth using DOE approach

“…Mg 2 Si compounds are prone to the formation of undesirable coarse Chinese script and brittle dendritic morphologies, which would deteriorate the mechanical properties of the materials. Thus, several researches have been done to modify the deleterious morphology of Mg 2 Si particles with addition of strontium (Sr) [11][12][13], phosphorous (P) [14], yttrium (Y) [15], antimony (Sb) [13,16], manganese (Mn) [17], lithium (Li) [18], mischmetal [19], and cerium (Ce) [20]. Moreover, bismuth (Bi) is used as an alloying element in wrought aluminium alloys to promote chip breaking and helping tool lubrication [21].…”