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Investigations of Process Optimization, Microstructure Modification and Material Performance Improvement in High Deposition-rate Laser Metal Deposition of Inconel 718

Antragsteller Dr.-Ing. Andres Gasser
Fachliche Zuordnung Produktionsautomatisierung und Montagetechnik
Förderung Förderung von 2016 bis 2020
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 317809677
 
Erstellungsjahr 2020

Zusammenfassung der Projektergebnisse

The project aims to significantly increase the deposition rate of laser metal deposition (LMD) of IN718 (from ~ 0.5kg/h in traditional LMD to above 5kg/h) to achieve rapid manufacture of large high-performance high-value metal structural parts. The room temperature and high temperature mechanical properties of HDR-LMD IN718 were improved through process optimization and microstructure control. The research results are as follows: (1) A HDR-LMD powder nozzle was developed and the powdery materials were characterized. Based on the modified coaxial nozzle, an additional HDR-LMD nozzle, which allows a powder mass flow of more than 6 kg/h has been developed. In comparison to the GA-powder, the particles of the PREP powder are nearly ideally spherical in shape. Little to no hollow particles or particles featuring satellites have been observed. (2) The influence of the process parameters on porosity, track geometry, track area, dilution zone, deposition rate as well as powder efficiency were studied and the proper process parameters for a deposition rate of 1.8kg/h were developed (laser power is 3kW; laser spot diameter 4mm; scanning speed 1500mm/min; powder mass flow 2.3kg/h). (3) The effects of powder characteristics on the microstructure and tensile properties were investigated. The deposition by HDR-LMD with GA powder and PREP powder was investigated and compared. Compared to PREP powder, the porosity is higher, grains are finer and the volume fraction of Laves phase is lower with the GA powder. Using the same heat treatment, all the average tensile values of the PREP samples are slightly lower compared to the ones of the GA samples: yield strength, 5.4%; tensile strength, 2.0%; elongation, 9.1%. (4) The effects of the gas environment on the microstructure were investigated. HDR-LMD Inconel 718 deposited in air and argon environment was compared. The results show that the use of a global protective environment (argon) had no significant benefit for the deposition of IN718 alloy. (5) The powder stream concentration and temperature evolution were characterized and the particle temperature in the laser beam was analyzed. By using the self-radiation effect from the powder stream heated by the laser irradiation, grey images of the high temperature powder stream were obtained by high-speed photography. The grey value was corrected considering the divergent characteristics of the powder stream to correspond to the temperature change. A novel experimental method for characterizing the temperature evolution of the powder stream under the laser irradiation was developed. Meanwhile, the mathematical analysis of the particle temperature evolution in the laser beam was conducted successfully. (5) The nonlinear temperature evolution behaviour of the powder particles in the laser beam was calculated considering different process conditions. (i) It shows that by decreasing the speed and size of powder particles or increasing the laser power, the incident particle temperature reaching the deposition surface can be sufficiently increased. (ii) With the decrease of the particle speed from 9.3 m s-1 to 3.9 m s-1, the particle temperature at theoretical deposition surface is increased by 144%. (iii) In particular, the increase of the powder incident angle can significantly increase the incident temperature of powder particle. The temperature of the 65° incident particle is increased by 54.4% relative to that of the 50° incident particle. (iv) Due to the effects of the laser energy distribution along the moving trajectories, the powder particles that cross the centre axis of the laser beam and fall near the edge of the laser spot on theoretical deposition surface have the highest incident temperature. (v) Single-pass deposition experiments show that the enhancing interaction effect between laser beam and powder stream can effectively improve deposition quality and save energy. 20% and 30% laser power were saved to obtain good deposition quality by increasing powder stream incident angle and decreasing the particle speed, respectively. (6) The influence of heat treatment on the microstructure was investigated. The two employed solution heat treatment regimes are 1100 ℃×0.5 h/WQ+720 ℃ ×8 h/FC to 620 ℃/8 h/AC (Type-S) and 1100 ℃×1h/WQ+720 ℃×8 h/FC to 620 ℃/8 h/AC (Type-L). The recrystallization degree of type-L sample is higher than that of type-S and twins only exist in the former one. In addition, niobium segregation and Laves phases. Moreover, the remained granular sub-micron particles in both sample types are preserved. The tensile tests result show that both types of samples have similar yield strength and tensile strength, whereas the elongation of type-L is superior to that of type-S. The higher elongation of type-L sample is due to the existence of twins and the relatively uniform grain in it. The fracture mechanism of both samples is micro-voids coalescence ductile rupture. The separation of the granular sub-micron particles and g matrix is the main nucleus of the micro-void. (7) The different heat treatment systems were designed to investigate the effect of the heat treatment on the mechanical properties of the block sample. The result showed that the tensile test results indicate that by heat treatment, tensile strength and 0.2% yield strength of IN718 formed by HDR-LMD were improved, while the ductility was reduced, but all of these indexes exceeded AMS properties of casting and wrought IN718. Combining strength and plasticity values, the S15 +DA is the most suitable heat treatment system for room temperature tensile properties of HDR-LMD IN718. The S30+DA has the best strength and plasticity and its tensile properties at 650℃ can be comparable to the AMS wrought standard. The stress rupture testing was performed on RC1230 durable performance test machine at 650 ℃ and 725 MPa. The plasticity has been improved for HDR-LMD IN718 and exceeded the wrought standard (15%) by the heat treatment. However, the stress rupture life is still lower than the wrought standard. The creep testing was performed at 595℃ and 825MPa and found that the S30 +DA sample has better high temperature creep properties.

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