Laser Induced Surface Modification of Aluminium Alloys

Author

Subhadarshi Nayak, University of Tennessee - Knoxville

Date of Award

5-2004

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Narendra B. Dahotre

Committee Members

David C. Joy, Carl McHargue, Easo P. George

Abstract

Purpose of this work is to improve surface related properties of aluminum alloys by employing a laser based technique. Two approaches were taken to achieve this goal. First approach involves a rapid solidification induced by laser without any deliberate change in chemical composition. In second approach, laser was used to deposit Fe₃0₄ on A319, producing an Fe₃0₄/Al reaction composite coating.

In-situ high-speed infrared thermographs captured during laser surface melting of A319 enabled estimation of maximum temperature, temperature gradient and cooling rate. In light of limited spatial and time resolutions of the infrared camera, one dimensional heat transfer model was adopted for estimating the cooling rate The cooling rate thus estimated provided a range of cell size that closely matched with the experimentally observed cell size The high cooling rate (>10⁵°C/s) resulted in extensive refinement of micro structure in the laser melted layer. Instrumented indentation technique was used to measure hardness (H) and elastic modulus (E) of the laser-melted layer. Berkovich tip was used to indent the material for 10-nm, 200-nm, 500-nm and 1000-nm depth. The H and the E were found to be 1.22 GPa and 78.2 GPa, respectively, for 1000-mn indentation depths. The variances associated with H and E were minimal, whereas, the same for substrate possessed significant scattering. Also, H and E increased with decreasing depth of indentation. Closer examination suggested that when the heterogeneity in the material was in the scale of indentation depth, significant scattering took place and the hard phase of Si influenced the average hardness. However, the effect of indentation depth on elastic modulus was not statistically significant. The improvement in mechanical properties manifested in better wear resistance.

Aluminum and Fe₃O₄ reacts readily in what is known as Thermite reaction. Infrared thermography was employed for diagnosing the thermal conditions during laser processing. Scanning Auger microscopy, transmission electron microscopy and x-ray diffraction techniques indicated a reaction between oxide particles and aluminum-forming FeAl intermetallic compounds, Al₂O₃, and various intermediate reaction products. Analysis of the coating region, fractured in vacuuo, indicated substantial toughness of the material due to extremely refined microstructure with finely distributed oxide and intermetallic particles and strong interfacial bonding between particles and the matrix. Mechanical properties of the coating were evaluated by nanoindentation techniques employing both Berkovich and cube-corner indenters. Hardness and elastic modulus values were found to be uniform at 1.24 and 76 GPa, respectively. No radial cracking was observed for either the Berkovich or cube-corner indenters. These results indicate that the laser-induced rapidly solidified composite material is tough and fracture resistant.

Recommended Citation

Nayak, Subhadarshi, "Laser Induced Surface Modification of Aluminium Alloys. " PhD diss., University of Tennessee, 2004.
https://trace.tennessee.edu/utk_graddiss/2983