Optimizing Induction Heating of WNiCo Billets Processed via Intensive Plastic Deformation

1. Introduction

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Within the last five decades, induction heating has become a favorable method for the heating of metallic workpieces throughout various industrial branches (e.g., transportation, aerospace, marine industry, etc.) [ 1 ]. It is mostly used to (pre)heat ferromagnetic materials, such as steel [ 2 ], since they intensify the magnetic field and thus induce the induction effect already at relatively low frequencies. However, the selection of the particular frequency depends on the geometry of the workpiece and its material properties, since both the electric conductivity and magnetic permeability can be (highly) temperature-dependent. Nevertheless, induction heating is also preferred for other applications, such as the melting of metals [ 3 ], the annealing and quenching of metallic components and surfaces [ 4 ], the preheating of workpieces intended for subsequent deformation processing [ 5 ], surface heating prior to specific surface treatments [ 6 ], and more. Under optimized conditions, induction heating can be precisely controlled to enable accurate gradual heating at relatively low costs. On the other hand, unsuitable settings of the heating process result in insufficient and/or inhomogeneous heating of the workpiece, and increase the possibility of the occurrence of microstructure inhomogeneities and defects within the material (e.g., local melting or overheating resulting in undesired abnormal grain growth), as well as the development of an unfavorable stress state, which can result in the occurrence of residual stress during subsequent processing [ 7 9 ].

The design of induction heating processes in the industry should be performed considering the technological, as well as metallurgical, viewpoints, especially for materials prepared via specific technological procedures, such as via powder metallurgy. Such specific materials should be processed with the highest care, since unnecessary increases in the heating time can non-negligibly affect their structures, as well as provoke surface oxidation [ 10 ]. The oxidation of workpieces during heating, which can hardly be avoided in industrial conditions, is an important issue in general. Increasing the heating time significantly affects not only the structures, but also the level of surface oxidation of the workpieces. While the level of surface oxidation can more or less be affected via the modification of furnace atmosphere or the usage of protective coatings during heating in conventional furnaces [ 11 ], possible structure changes are usually complex and their control is complicated (for example, the grains grow exponentially with increasing heating time [ 7 ]). The application of a rapid induction heating is thus very favorable, not only for challenging materials.

The optimization of the induction heating process can be advantageously performed with the help of numerical modeling. Others have presented numerous models applicable for induction heating in specific cases. For example, Bay et al. [ 12 ] developed numerical and mathematical models of induction heating for several specific inductor geometries, Liu et al. [ 13 ] proposed a numerical method to predict the temperature field of induction heating and investigated the distribution of temperature, and Riccio et al. [ 14 ] compared different numerical models and performed electromagnetic analyses of the bonding of reinforced polymer components with induction heating. Some authors even proposed the induction heating process as a supportive method to fabricate long tubular parts from tungsten heavy alloys (THAs) [ 15 ]. Nevertheless, studies focusing on the induction heating of THAs intended to be plastically deformed after heating are scarce, possibly primarily due to the paramagnetic behavior of these materials. Despite the fact that their pre-process heating via induction is a challenge, the usage of paramagnetic materials in the industrial and commercial spheres is on the rise.

3) and high strength (up to 1900 MPa after optimized deformation processing) [2, Ar2, vacuum) and sintered within the temperature range from 1000 °C to 1550 °C [

THAs feature exceptional mechanical and physical properties, among which are, e.g., high density (17–19 g/cm) and high strength (up to 1900 MPa after optimized deformation processing) [ 16 17 ]. They are typically used for radiation shielding, as therapeutic devices in oncology, or as kinetic energy penetrators [ 8 18 ]. THAs are usually fabricated via methods of powder metallurgy [ 19 ]. The mixed powders are isostatically pressed under a protective atmosphere (H, Ar, vacuum) and sintered within the temperature range from 1000 °C to 1550 °C [ 16 20 ]. The powder mixture typically consists of more than 90 wt. % of tungsten plus a mixture of other elements with lower melting temperatures (Ni, Co, Fe). Within the mixtures, the grain boundaries of the elements featuring lower melting temperatures locally melt during sintering, and consequently form a matrix binding the agglomerated tungsten particles together [ 18 21 ]. The alloying elements thus provide the plasticity of the final material, whereas tungsten primarily increases the ultimate tensile strength (UTS).

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Based on the aforementioned, the optimum combination of strength/plasticity for THAs can be achieved by optimizing the chemical composition and sintering conditions. Nevertheless, the deformation processing performed after sintering can also non-negligibly affect the performance of the final product [ 16 22 ]. Intensive deformation processing contributing to the structure refinement and enhancement of the final properties can advantageously be performed via the severe plastic deformation (SPD) methods [ 23 ], such as equal channel angular pressing (ECAP) [ 24 26 ], twist channel (multi) angular pressing (TCAP, TCMAP) [ 27 28 ], ECAP with (partial) back pressure (ECAP-BP, ECAP-PBP) [ 29 30 ], high pressure torsion (HPT) [ 31 32 ], etc. These processes are discontinuous and can only be used to process samples of limited dimensions. On the other hand, continuous methods of intensive plastic deformation, such as ECAP-Conform [ 33 ], accumulative roll bonding (ARB) [ 34 ] or rotary swaging (RS) [ 35 ], can be used to process long axisymmetric workpieces and, moreover, are industrially applicable. RS is used to gradually reduce the cross-sections of workpieces and increase their lengths via the repeated action of the swaging dies [ 36 ]. It induces changes in the shapes and dimensions of the workpieces, and is thus favorable for the production of final products featuring the required shapes. RS can be performed under cold, warm, and hot conditions to process various materials, including challenging ones, such as powder-based alloys, laminated materials, biomaterials, or composites [ 37 41 ].

The aim of the study is to design an optimized induction process applicable in the process of the manufacturing of high-quality billets from pre-sintered WNiCo pseudoalloys consisting of a consequent combination of induction heating and rotary swaging. The first part of the study focuses on the characterization of the induction heating process, including a brief description of the electromagnetic parameters within the inductor and workpiece, and the design of the optimum induction heating model. For these purposes, numerical modeling using two individual 2D and 3D software packages was performed. The second part of the study then deals with the 3D numerical analysis performed with the aim of proposing suitable induction heating parameters and a deformation temperature for rotary swaging of the particular WNiCo alloy. Finally, the experimental realization of the induction heating and subsequent warm rotary swaging of the sintered workpieces is performed and the experimental results are discussed together with the predicted ones.