A novel model of calculating particle sizes in plasma rotating electrode process for superalloys
Graphical abstract
Introduction
Nickel base superalloy is a series of materials with excellent performances at high temperature, which is applied in the conditions of airplane gas turbines [1]. Nowadays, the widely used Ni-base or Ni-Fe base superalloys (for example Inconel718) applied in higher temperature conditions are added a larger fraction of niobium or tantalum for precipitation of the second phases, which support high hardenability [2]. Complex contents make it difficult to control segregation in casting. In order to reduce the defects of macro segregation and macro shrinkage [3] of casting, near-net-shape process like hot isostatic pressing (HIP) [4] is used, which is the most used powder forming process of the superalloys. EP741NP, the superalloy developed for powder forming, has been reported to be one of the most advanced Ni-base superalloys used in Russian fighters [5].
The superalloys are also suitable for 3D Printing. It has been mentioned that the general superalloys like Inconel718 can be laser-printed from powders with high density and low macro defects [6, 7].
The superalloys spherical powders used for HIP and 3D printing should have applicable size ranges, high flowability, lower impurity and extremely lower micro defects (porosity) [8, 9]. In general, the widely applied spraying methods for producing high purity spherical metal powders are divided into the gas atomization (GA) and the centrifugal atomization [10, 11]. Method of electrode induction melting gas atomization (EIGA) [12] is the mainly used gas atomization process, and the most widely used centrifugal atomization method nowadays is the plasma rotating electrode process (PREP) [13]. For HIP products, generally, PREP powders are more preferred because powders sprayed by centrifugal force have higher roundness degree without gas-formed inner holes, and these defects strongly damage the fatigue performance [13, 14]. However, compared with the GA methods, the fine powder yield of the commercial PREP method is slightly lower than GA methods [15].
It is reported that there are at least 3 different spraying models existing in the centrifugal atomization, including direct drop formation (DDF), ligament disintegration (LD), and film disintegration (FD) [16]. The typical applied equation for calculating the mean diameter of the sprayed powders is based on the view that the drop separates from the molten pool boundary immediately when getting enough centrifugal force against the liquid surface tension itself [16], which is suitable for DDF [17]. For LD particles, the average diameter prediction had been proved, added with viscosity, in Weber's theory [18]. But these models are not easy to compare with the DDF equation until Kamiya and Kayano [19] give a simple semi-empirical law to describe the average diameter of unique LD models for low viscosity liquid.
However, there is few research on the powder yield calculation of the mixed spraying model of DDF and LD, which is useful for the cost control of the PREP. It should concern not only the mean diameter, but also the yield of a selected size range.
This paper aims to derive an applicable particle size distribution (PSD) model of the PREP superalloy powders via different spray model factors, especially the powder yield of a random size range. The model can support/fit a PSD calculating result before atomization under the selected conditions. The tested PSD results and the PSD calculations of the EP741NP/Inconel718 powders produced by PREP were discussed. The model fitted the results well under the conditions that the rotating speeds ranging from 14,000–20,000 rpm.
Section snippets
Atomization mechanism of the PREP
The atomization model for PREP is based on the centrifugal models, which was deeply discussed by the high speed camera and exported 3 elemental spray models: DDF, LD and FD [16, 17], as shown in Fig. 1. A statistical equation to approximately determine the actual spray model can be expressed as the Hinze-Milborn number, Hi [16, 20]:
This equation is suitable for the atomization of metals [20]. In the Eq. (1), the Hi is a dimensionless parameter, the μ (Pa·s) is the
The morphology of the atomized powders of pretest melting
The powders produced by pretest melting were using the mentioned default process parameters with 14,000 rpm rotating speed. The received powders were analyzed by SEM without sieving, shown on Fig. 4, Fig. 5. The selected particles in Fig. 4, Fig. 5 were marked with lengths of the diameters. The figures show that the as-received powders are seemed to be a mixture of the larger particles and the smaller particles. The larger particles measured in Fig. 4 for Inconel718 were about 141–155 μm, and
Conclusion
- (1)
The PSD of the PREP Ni base superalloy powders obeyed the bimodal Rosin-Rammler law, and the sprayed PSD results could be calculated before atomization. The well fitted PSD models were based on the spray factors (Q, ω, D, μ, ρ, γ, εLD) and the fitting factors (n, η, k). A random size range yield could be predicted by those variables. The diameter ratio of the LD particle and the DDF particle, k, was about 0.5–0.6, which fitted the Kamiya-Kayano law.
- (2)
Except the rotating speed, the melting rate
Acknowledgements
The authors would like to acknowledge support by Program for Development of High Quality Inconel 718 Superalloy Spherical Powders for 3D Printing Aerospace Application, (2016KTCQ01-84), Shaanxi Science and Technology Department, China.
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2020, Powder TechnologyCitation Excerpt :The rotational speed of the alloy electrode mainly determined the particle size distribution during supreme-speed PREP [20]. A larger percentage of fine EP741NP/Inconel718 alloy powder was obtained by increasing the rotational speed of the alloy electrode [21]. The average PREP powder size also decreased with increasing diameter of the alloy electrode [22].