Study On The Mechanical Properties Of Laser Rapid Forming Of Rene'88DT Superalloy

Powder Metallurgy technique is the major preparation approach for manufacturing aerial turbine disk. But there are some technological obstacles to overcome in our country at the present time. While Laser Rapid Forming is a new prospective technology for making superalloy parts with high properties, which integrates Rapid prototyping with Laser cladding, and can realize the near net shaping of dense metal parts with complex structures and high properties. If it is applied to the preparation of Rene'88DT turbine disk, Another imaginable way will be explored for manufacturing pivotal metal parts, such as turbine disk of high thrust-weight ratio aero-engine.Cracks easily form during the Laser Rapid Forming process of high alloying superalloy, which becomes an limitation for the technique coming into use. The same problem happens in the process of laser formed Rene'88DT superalloy. So the forming and expansion mechanism for the cracks produced in the laser rapid forming (LRF) processing of Rene88DT superalloy, has been investigated. The results show that the cracks belong to the liquefying crack, which presents a characteristic of cracking along grains. The liquid films along grains, form due to y/y' eutectic composition in the pre-layer already solidified by laser deposition, which are pulled apart by residual tensile stress increasing with the laser deposition process. Columar dendrites are parallel to the laser deposition direction at the bottom of the present laser depositon layer, while at the top they haul to the direction perpendicular to it for the change of heat flux direction. The hauling dendrites exist at the overlapping zone of the subsequent laser depositon layer due to unmelting. As the microstructure of LRF shows epitaxial growth, grain boundry forms in the overlapping zone between laser deposition paths for the unmelting hauling dendrites. y/y' eutectic also solidifies in the area, which makes the zone cracking susceptibly. Once the liquefying crack forms, it will rapidly expand along the penetrating columnar grain boundry between laser deposition layers. So cracks enlarge generally along the central line of the overlapping zone. Based on the the forming and expansionmechanism theory, Processing parameters of Laser Rapid Forming was systematically investigated for controlling cracks, it was found that the the following processing parameters influencing cracking, are the single deposition path width, the single deposition layer height, the single layer climbing height and overlapping rate. Through optimizing the above-mentioned feature parameters, Laser formed Rene'88DT bulk sample without cracks is obtained. At the same time, Relative rational processing parameters are also acquired for laser formed Rene'88DT components.For y' precipitation nickel-based superalloy Rene'88DT, Heat treament is the most critical processing step that has ultimate influences on the microstructure, such as the size and distribution of γ' precipitation , and hence, on the ultimate mechanical properties. So another objective of this paper is to preliminarily study the effect of the size and distribution of γ' precipitation on tensile test. Test specimen of Rene'88DT superalloy are laser formed and heat-treated. The variation of microstructure before and after heat treatment is analyzed. The results show that γ' precipitation become more fine and homogeneous than directly laser deposition after 1165℃, 2 hours solution treatment. When the temperature reaches the complete solid solution of γ' precipitation, there will be no change of the size and distribution of γ' precipitation as the temperature increases further. Using 760℃ aging treatment, γ' precipitation grow a little bigger, and γ' precipitation distribution becomes more homogeneous as the aging time increases, so the tensile strength enhance. The cooing rate after solution treatment may be too high in this resaerch, so the γ' precipitation growth is restrained. And the size range is from 10nm to 50nm. However, Tensile strength after heat treatment increases 100~200MPa, compared to as-deposition strength. The yeild strength is even equal to 90% of that of the Powder Metallurgy standard. Cooling rates and aging time are supposed to be the critical factors for strength enhancing.