The Effect of Melt Conditioning on Segregation of Solute Elements and Nucleation of Aluminum Grains in a Twin Roll Cast
- PDF / 2,873,827 Bytes
- 11 Pages / 593.972 x 792 pts Page_size
- 15 Downloads / 175 Views
NTRODUCTION
A fine grain size and structural uniformity has generally favorable mechanical properties of most metals and alloys.[1] In casting, fine equiaxed grains are important for the service performance of cast products and final properties of semifabricated products by the reduced ingot cracking, improved shrinkage porosity, and well-distributed second phases.[2–4] In particular, the fine grains reduce the hot cracking sensitivity of an alloy due to evenly distributed strains.[5] For over half a century, in order to achieve such a fine microstructure in aluminum alloys, inoculants of micrometer size, such as TiB2, TiC, or TiAl3, have been added to aluminum melt with additional titanium to act as substrates for heterogeneous nucleation of aluminum grains during solidification and consequently in order to enhance the nucleation.[6–8] Although the inoculants improve the properties, several problems are encountered with them through the loss of performance over time by agglomeration, settling or poisoning, and formation of undesired particles.[9,10] Recently, however, it has been suggested that a melt conditioning (MC) process can achieve a grain-refined microstructure without adding any exogenous particles. Depending on the method generating the shearing of melt, KEEHYUN KIM, Research Fellow, is with the The EPSRC Centre LiME, School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K. Contact e-mail: k.kim.2@bham. ac.uk Manuscript submitted February 13, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
the process is divided into four main groups: (1) twin screws,[9,11–14] (2) rotor–stator unit,[15,16] (3) electromagnetic stirring,[17–19] and (4) ultrasonic cavitation.[20,21] The strong shearing and high intensity of turbulence in the process can easily break up and disperse fine inclusions and/or fragment dendritic arms, and consequently induce the grain refinement of melt conditioned metals. However, there is hardly any paper showing convincing evidence of heterogeneous nucleation of aluminum grains on the fine particles, because the size and volume fraction of potential particles for the nucleation are too small to analyze in an actual melt conditioned sample.[22] It is also difficult to detect the fragmented aluminum due to its undistinguishable morphology with aluminum matrix. For almost 60 years, twin roll casting (TRC) has been used in the aluminum industry to reduce a movement of liquid and/or solid within the casting and consequently to increase productivity.[23–25] The TRC process, which uses the merit of the high solidification rate, produces thin aluminum sheet of about 1 to 5 mm from the melt.[25,26] However, its productivity and range of alloys which can be cast are limited.[24] The lack of surface scalping causes serious defects at the casting stage, or during subsequent handling of twin roll cast reroll stock.[27] In addition, centerline segregates are inherent to the process. The center region of TRC strips experiences an estimated cooling rate of 100 K/s.[28] This high c
Data Loading...
