With deep studying on the process of laser rapid forming (LRF) the researchers gradually meet the knowledge that it is very important to understand the mechanism of interaction between the laser and the powder particles since it is the key point to realize the effective control of the LRF process. The high-speed photography has been employed to realize in situ observation on the delivery process of powdered materials for the first time. A group of parameters -- delivery parameters of powder is put forward to characterize the delivery process in quantitative by dealing with the digital images obtained. On the basis of quantitative description of the powder delivery, an analytical model is presented to study the attenuation of the laser power caused by the cloud of the power particles. Another analytical model is also presented to study the temperature rise of the particle irradiated by the laser. It can be found that the attenuation ratio is determined together by the powder specifications, the powder feeding parameters and the powder delivery parameters. With the off axial powder nozzle being employed in the paper, the diameter of the powder steam was always bigger than the diameter of laser spot, thus the laser processing parameters have no effect on the laser attenuation. The temperature rise of the particle is determined by the powder specifications and the powder delivery parameters too. Meanwhile the laser processing parameters also affects the temperature rise of the particle. With the decreasing of the particle radius, the irradiation heating effect increases remarkably.
The Laser Rapid Forming (LRF) has been used to build up default free and nonoxidation Ti- 6Al- 4V samples in atmosphere controlled LRF equipment. The microstructure and mechanical properties of as-deposited and heat-treatment are analyzed. It has been found that the macrostructure of as-deposited Ti- 6Al- 4V alloys takes the shape of huge columnar prior beta grains (PβG) with continuous boundary and epitaxial growth along the vertical direction (Z) of the laser scanning. Between cladding layers there are bands of coarse structures resulting from the reheating by laser beam when the prior cladding layers forming. The substructure in prior beta grains is mainly of the fine acicular α and basketweave matrix of α+β. After high temperature annealing treatment, little basketweave matrix of α+β remains and the acicular α changes into α laths with clear β outlines. After quenching-aging treatment, the substructures are mainly of the α laths and basketweave matrix of α+β. Multi-quenching -aging heat treatments produce bi-modal structure. The physical property test shows that the tensile strength and plasticity at the vertical direction (Z) of the laser scanning are higher than those at the direction of parallel (X). After high temperature-annealing treatment, the Ti-6Al-4V alloy has lower level of tensile strength and plasticity while quenching-aging treatment decreases tensile strength and increases plastic slightly.
Laser direct forming experiments were carried out systematically with 316L stainless steel and nickel-base alloy to investigate the technical characterizations deeply and some metal components were fabricated. It is found that, the height of single cladding layer, which was affected by almost all the processing parameters and was quite hard to be precisely controlled, was very important to laser direct forming for not only the accuracy of vertical direction but also the fabrication stability. The variation of the width of single clad, which was mainly affected by laser power, spot diameter and scanning velocity, was similar to that in laser surface melting. The surface quality was another important characterization for laser direct forming and was remarkably affected by oxidation and powder attachment. In order to improve the surface quality, the flow flux ofshielding gas should not less than 10l/mm.