| More and more attention has been paid to the fine-grained material during the past decade since its particular structure and excellent properties. However, it is difficult to obtain ideal three dimensional submicron and nanometer fine-grained samples with high-purity, contamination-free, residual stress-free and sufficiently large sample size due to the limitation of the present preparation techniques. With increasing evidences of novel properties in fine-grained materials, it is reasonable to combination the surface modification with fine-grained material. So the integral properties and behaviors of the materials can be significantly improved by the surface modification. At present, the grain refinement with a certain thickness has been prepared on the metal material surface by laser surface treatment. In other words, the surface of grain refinement has been realized. Nevertheless, up to now, clear scenery of microstructures and properties of the fine-grained surface layer, especially formation of ultra fine sized crystallites from coarse poly-crystals during plastic deformation, is still lacking. Therefore, understanding of the underlying mechanism for grain refinement by heat straining becomes more and more crucialIt can be realized the relatively controllable modification and the reshaping effects by the high-energy pulsed laser irradiation the metal surface changing the morphology and organizational structure. And it is considered a broad prospecting for application engineering. With the increasingly application of laser processing, the interaction of laser and material properties and mechanism are more concerned about.Based on the above idea, the main purpose of the present study is to investigate the surface modification of the pure Al under the Nd:YAG pulsed laser irradiation. In this work, micro-structural characteristics of the fine-grained surface layer in pure Al under different laser processing parameters were investigated by means of X-ray diffraction (XRD) and field scanning scanning electron microscopy (FESEM). The Vickers micro-hardness measurements were carried out using a HXD-1000 micro-hardness tester with loads of 200 g and indentation time 15 s. And the MTS tensile testing system was used to evaluate the mechanical properties. The main results are shown as follows:1. The effects of the laser power density and scanning speed on the surface layer thickness and surface melting region are very significant. When laser power density increases, the surface melting layer thickness increases; when the laser power density and beam diameter are constant, with the scanning speed improve the penetration of the remelted layer becomes smaller. And also, with the increasing of laser power density, the grain size of the Al surface gradually increases.2. The micro-hardness increases obviously due to the grain refinement of the Al surface under the laser treatment. The biggest micro-hardness value is larger about four times than that of the original Al sample. With increasing depth (that is, increasing the grain size), the micro-hardness decreases.3. The tensile tests at RT show that the pure Al has a good combination of high UTS of about 660MPa and enhanced plastic strains of about 17~21.2%, in a wide strain rate range of 5×10-5~0.02s-1 after laser irradiation with the power density of 3.65×109W/m2. With the strain rate increases, the tensile strength increases, but there is no clear plastic change. The tensile strength gradually increases and elongation gradually reduces, but not less than 15%. After fracturing, fracture surface shows fracture toughness characteristic of dimples.4. The wear rate gradually increases with the wear processing. Under the identical friction distances, the wear rate gradually decline with the increasing of the laser power density. Through laser surface treatment, the sample friction coefficient can be stabilized at a lower value compared with the untreated samples, which reduces from the original 0.784 to 0.15~0.52. Compared with the original Al sample, the friction and wear performance increase, while the friction coefficient drops after laser treatment.
|
PDF Full Text Request