99.Microstructure and refining performance of an Al-5Ti-0.25C refiner before and after equal-channel angular pressing

Microstructure and Refining Performance of an Al-5Ti-0.25C Refiner before and after Equal-Channel Angular Pressing

Zuogui Zhang¹⁾, Yoshimi Watanabe¹⁾, Ick Soo Kim ¹⁾, Xiangfa Liu ²⁾ and Xiufang Bian ²⁾
1) Department of Functional Machinery and Mechanics, Shinshu University
3-15-1 Tokida, Ueda 386-8567, Japan.
2) Key Laboratory of Liquid Structure and Heredity of Material, Ministry of Education
Shandong University, Jingshi Road 73, Jinan 250061, China

Since Al-Ti-C master alloys were usually fabricated by a direct cast or the direct cast combining with a hot rolling methods, the Al₃Ti particles appear as large platelet shape and the TiC particles have a concentrated distribution trend in some degree, which leads to the limitation of grain refining performance of the Al-Ti-C refiner. In the present study, to clarify the influences of size and distribution of TiC and Al₃Ti particles on the nucleating of Al crystals, some Al-5Ti-0.25C refiner samples were severely plastically deformed by an equal-channel angular pressing (ECAP) technique. The grain refining effects and Vickers microhardness (Hv) of the pure Al samples refined by the Al-5Ti-0.25C refiner after ECAP processing were proved to have a significant improvement over that of before ECAP processing. It is expected that the application of ECAP technique to Al-Ti-C grain refiner would be a very useful practice in refining industrial Al alloys.

The Al-5Ti-0.25C master alloy was prepared from compound containing Ti, commercial pure Al and graphite powder with 20-100 mm. A certain of commercial pure Al were melted respectively by using medium frequency induction furnace. Al melt was heated to a temperature of about 1273 K, adding the mixture of the three kinds of resources and holding 2-3 min, then pouring into a steel mold. In this study, some rod-shaped Al-5Ti-0.25C refiner samples with 10 mm in diameter and 55 mm in length were prepared by machining. ECAP processing was conducted at room temperature by using a pressing speed of 0.33 mms^-1 with MoS₂ as a lubricant. Grain refining experiments were carried out in 5 kW graphite crucible furnaces with different refiner addition level, melt temperature and holding time. The melt was poured into a ring steel mold with outside diameter of 70 mm and inside diameter of 50 mm. For the determination of grain size, each test sample was cut horizontally by 5 mm distance from the bottom. The mean size of Al grains was calculated by using a mean linear intercept technique.

Figure 1 shows the variations of mean Al grain size with holding times for pure Al samples refined by the Al-5Ti-0.25C refiner. In the present study, the mean size of Al grains in original high pure Al sample is estimated at 1.1 mm when the pure Al sample is not refined. From Figure 1, it is clear that the mean size of the Al grains obviously decrease with increasing refiner addition levels. When the holding times are less than 3 min, the refiner has a poor refining effect.

Figure 2 shows the variations of TiC and Al₃Ti particle sizes in the Al-5Ti-0.25C refiner samples with increasing ECAP passes. From this Figure, it is found that with increasing ECAP processing the mean size of Al₃Ti and TiC particles gradually decreased and after eight passes of ECAP processing the mean size of TiC particles has decreased to about 1.08 m from about 1.53 m of before ECAP processing. Simultaneously, the plate-shaped Al₃Ti particles are significantly cracked and the mean size of the Al₃Ti particles is reduced to about 10 m from about 35 m of before ECAP processing. We can also find that even though the Al-5Ti-0.25C refiner was severely deformed by only two passes ECAP processing, the reduction ranges of the Al₃Ti and TiC particles size are large, but with the further increase of repetitive ECAP passes the decrease trend is not so apparent. This is because that during the first two passes ECAP processing, huge intense shear strains were introduced into the sample and it resulted in that the Al₃Ti and TiC particles were cracked and fragmented in great degree.

Figure 3 shows the varied curves of mean Al grain sizes and Hv of pure Al samples refined by 0.8 % addition of Al-5Ti-0.25C refiner with increasing repetitive ECAP processing. It is demonstrated that under the above-mentioned optimal refining conditions, Al grains could be further refined to about 0.1 mm when using the Al-5Ti-0.25C refiner samples after eight passes ECAP processing. Using the same refiner samples before ECAP processing, the optimal Al mean grains size refined is only limited to about 0.2 mm. It is also found that the Hv values of pure Al samples refined by the refiner after eight passes ECAP processing have increased to 73 MPa from 53 MPa of before ECAP processing. In this study, after 2 passes ECAP processing the microstructures of Al-Ti-C refiner obviously were changed and the Al₃Ti and TiC particles were crushed in large degree. Therefore it is considered that the minimum 2 passes ECAP processing for the refiner that should be given to assure good grain refining effects. The application of ECAP technique to Al-Ti-C grain refiner will be expected to have a very useful practical application in refining industrial Al alloys.

[Published in Metall. Mater. Trans. A, 36A, 837-844 (2005)]

Fig. 1 Variations of mean Al grain size with holding times for pure Al samples refined by the Al-5Ti-0.25C refiner with different holding time at 1003 K.

Fig. 2 Variations of TiC and Al₃Ti particle sizes in the Al-5Ti-0.25C refiner samples with increasing ECAP passes.

Fig. 3 Variations of mean a-Al grain sizes and Hv of pure Al samples refined by Al-5Ti-0.25C refiner with ECAP passes.