Numerical Simulation And Experimental Study Of K447A Nickel-based Superalloy Manufactured By Selective Laser Melting

The service temperature of the gas turbine blade increases year by year,and the harsh working environment sets higher requirements to the structure and materials of the components.With the rapid development of additive manufacturing technology,the Selective Laser Melting(SLM)technology based on powder bed provides a new idea for forming blade.SLM has significant advantages over traditional forming methods,such as high precision,flexible and short production cycle,capacity of producing complex structural parts all at once,etc.K447 A nickel-based superalloy is commonly used as the raw material for hot-end components.Hot cracks are easy to occur during the SLM process due to the excessive temperature gradient.At present,there are few reports about the SLM forming process and crack healing mode of K447 A alloy.Pulse current is an efficient post-treatment method for metallic materials which can heal micro-cracks.In this paper,therefore,an in-depth research has been conducted on the SLM forming process of K447 A alloy and the selection and optimization of process parameters during pulse current post-treatment process.By conducting numerical simulation and experiment,the temperature field,stress field,and deformation during the SLM forming process of K447 A alloy were studied,and the influence of model design,forming process and pulse current post-treatment parameters on the microstructure,defects,and properties of K447 A were analyzed.The crack healing of the formed sample was promoted by the pulse current,and the SLM and electropulse process parameters were selected and optimized.The SLM forming simulation of K447 A gas turbine hot-end components was carried out using the optimized process parameters.The temperature field,stress field,and deformation during the SLM forming process of K447 A alloy were analyzed numerically by the “thermal + structural” coupling method and the inherent strain method.The process was simulated under the laser power of 165W～325W and scanning speed of 910mm/s～1210mm/s for the plate sample(70mm×12mm×4mm).With the increase of laser power,the residual stress decreases first and then increases.When the scanning speed is less than 1010mm/s,the stress does not change significantly.When the scanning speed continues to increase,the stress increases sharply.The overall equivalent stress distribution trend of the sample was that the stress near the surface stress was greater than interior stress,and the fine and dense supporting structure was beneficial to reduce the stress and deformation and protect the parts.Finally,a process window with a laser power of205W～245W and a scanning speed of 1010mm/s was obtained.The microstructure of SLM K447 A alloy is composed of columnar and equiaxed crystalline grains,and the size of the columnar crystal increases with the increasing of laser power.Under the laser powder of 205 W,the defect ratio of the supported sample(3.54%)is lower than that of the unsupported sample(4.44%).The grain size of supported sample is smaller and the growth of columnar crystal is also effectively controlled.Pulse current treatment was introduced to deal with the micro-crack problem that cannot be solved by process optimization,and the healing effect of pulse current on the micro-crack was confirmed by in-situ observation.The effect of pulse current around the crack area caused high temperature gradient and the thermal stress promoted crack healing.The changes in the microstructure and mechanical properties of the parts before and after treatment were analyzed,The results show that the microstructure of K447 A alloy before and after pulse voltage was composed of columnar crystal structure and equiaxed crystal structure.When the pulse voltage increased from 30 V to 45 V,the tensile strength increased from 540 MPa to724MPa,which increases by 34%.When the pulse voltage increased to 60 V,the tensile strength of the sample decreased to 489 MPa.The process parameters of K447 A alloy with better forming precision and performance were obtained through numerical simulation and experiment,which were used to explore and optimize the forming deviation of K447 A complex structural components.On this basis,numerical simulation of reduced scale and full size gas turbine blades were carried out.The distribution range of stress and strain was similar,and the deformation and deviation of the full-size component were larger.The connection position with the supports had large stress and deformation.In order to reduce the forming deviation of K447 A alloy,the model was optimized according to the characteristics of SLM.The forming deviation of full-size component after optimization was within ±0.2mm.The model optimization has no obvious effect on the stress and strain of component.