Grain refinement limit and mechanical properties of 6063 alloy inoculated by Al-Ti-C (B) master alloys

The grain refinement limit of 6063 alloy inoculated by Al-5Ti-0.25C and Al-5Ti-1B master alloys and the impact of grain size on the mechanical properties, including the tensile strength and hardness, are investigated in the present article. It is revealed that the smallest average grain size of 6063 alloys refined by these two master alloys is the same under the present experimental condition, thus the grain refinement limit of 6063 alloy is 40 μm. However, the limit addition of the two master alloys corresponding to the grain refinement varies at 2.0 wt.% and 3.0 wt.% for Al-5Ti-0.25C and Al-5Ti-1B, respectively, and the grain refining performance of the former is much better than that of the later one before reaching the refinement limit. Furthermore, the mechanical properties of 6063 alloy are also improved first with the grain refinement, but turn to decrease after the same 1.5 wt.% addition both for Al-5Ti-0.25C and Al-5Ti-1B. The highest ultimate tensile strength (UTS) and hardness (HB) of 6063 alloy obtained by the refinement of Al-5Ti-0.25C and Al-5Ti-1B are 271 MPa, 81.7 and 250 MPa, 80, respectively. Thus the UTS and hardness of 6063 alloy refined by the former master alloy are also much higher than those refined by the later one. It is worthy to note that the 6063 alloys with the smallest average grain size did not behave the highest mechanical properties and the possible mechanisms are also discussed.