Vacuum arc remelting (VAR) furnaces are used to produce metal ingots with high chemical and mechanical homogeneity for demanding applications found in the aircraft and aerospace industries as well as energy, nuclear, chemical/petro-chemical, medical and automotive industries. The VAR process operates under a vacuum or inert atmosphere and uses direct current (DC) to remelt ingots of specialty metals that may normally react with oxygen, nitrogen or hydrogen. While VAR can improve the cleanliness and refine the structure of metals, it can also introduce defects that can adversely affect the fatigue and fracture toughness of final products made from these ingots if the process is not properly controlled. This article provides a brief overview of the VAR process and describes how the Ampere Scientific VARmetric™ system can be used to improve quality, efficiency, yields and safety in the production of specialty metal ingots.

The VAR process

High quality ingots of specialty metals, such as superalloys, high strength steels, titanium and zirconium alloys are processed to create critical components in the aircraft and aerospace industries. These components – such as the disks that hold turbine blades in aircraft engines – must have a high degree of reliability, since the failure of these components may have catastrophic results. Microscopic defects, such as dissolved gasses, voids, contaminants, or inclusions can become sources of crack initiation when the component is subjected to thermal, structural, dynamic, or fatigue loads.

The VAR process is used to improve the purity and refine the structure of specialty metal ingots through the removal of potential defects and crack initiation sites. VAR utilizes a consumable electrode that generates heat from a DC arc formed between the electrode and ingot. One of the factors that governs the formation of defects in an ingot is the arc distribution. That is, an uneven arc distribution may lead to undesired solidification, reducing the effectiveness of the process for removing microscopic defects and leading to non-homogeneous metallurgical structures, which may result in macroscopic metallurgical defects. One key quality control measure is to ensure an even arc distribution and to center the arcs on the ingots during the VAR process to increase yield and improve ingot quality.

The VARmetric™ System advantages

One of the problems with controlling the position and distribution of the arcs is that these characteristics can only be measured by monitoring the electromagnetic fields associated with the arcs during the remelting and refining process. Ampere Scientific (AmpSci) has addressed this problem with the development of the VARmetric™ measurement system (Figure 1), which is available to retrofit on existing VAR furnaces or fully integrated on new Consarc VAR furnaces. VARmetric™ is an advanced, noninvasive measurement system for VAR furnaces that utilizes the self-generated magnetic fields of the normal VAR process to image the arc dynamics during operation. It is much like the magnetic resonance imaging (MRI) systems commonly used on humans in medical diagnostics. This system can be used to see what the human eye cannot. It can be used to improve product quality and efficiency in real time and can provide a real-time safety capability in the case of side arcs and arcing to the stinger during the processing, which may result in catastrophic damage to the equipment. When coupled with a control system, VARmetric™ can improve process efficiency leading to lower electrical consumption and greater cost savings.

Figure 1. A Consarc VAR furnace with VARmetric sensors. Source: InductothermFigure 1. A Consarc VAR furnace with VARmetric sensors. Source: Inductotherm

The VARmetric™ measurement system provides an unparalleled amount of information to system operators, which may allow them to predict ingot defects in real time during processing. It is the only system available today that provides an integrated chain-of-custody dataset for an ingot that includes the current state-of-the-art measurements with arc location and magnetic field variations, and other parameters important to understanding the overall melting characteristics that define ingot quality (Table 1). The monitoring, measurement and analysis of these characteristics enable operators to react to off-normal events and adjust process parameters to ensure defect-free ingots.

Table 1. Summary of VARmetric capabilities. Source: InductothermTable 1. Summary of VARmetric capabilities. Source: Inductotherm

Figure 2. Left: Magnetic field data taken from an industrial VAR showing a constricted arc followed by a glow. Right: simulations showing the impact of a 10-minute constricted arc on the probability of defective formation. Source: Ampere ScientificFigure 2. Left: Magnetic field data taken from an industrial VAR showing a constricted arc followed by a glow. Right: simulations showing the impact of a 10-minute constricted arc on the probability of defective formation. Source: Ampere Scientific

About Ampere Scientific and Consarc

Ampere Scientific (AmpSci) is a small business headquartered in Albany, Oregon, that is dedicated to the improvement of production processes for specialty metals. VARmetric has its roots in the U.S. Department of Energy national labs and was originally developed on funding provided by the Specialty Metals Processing Consortium (SMPC). More recently, AmpSci has been commercializing the VARmetric™ system while developing software and analysis tools used by their customers to understand arc dynamics and enhance product quality over a wide range of operating conditions. Their collection of sensor and control systems together with their innovative software suite allows AmpSci to offer the most advanced VAR process monitoring and control systems in the industry.

Consarc, an Inductotherm Group company, is a strategic part of one of the world’s largest groups that are developing technologies for the melting and processing of high-performance metals and alloys. Consarc makes a wide variety of furnaces and related systems for vacuum induction melting (VIM), VAR, electroslag remelting (ESR), vacuum precision investment casting (VPIC), induction skull melting (ISM), solar silicon melting furnaces and specialty furnaces for proprietary processes. Consarc is ISO 9001 certified, and with operations worldwide, is well equipped to tackle a wide variety of furnace projects on a global basis and can tailor furnace designs to suit specific customer needs.

Consarc Corporation and Ampere Scientific have formed a strategic alliance to bring the most advanced solutions available to vacuum metallurgy. New VAR furnaces with fully integrated AmpSci VARmetric™ measurement systems are available exclusively on Consarc’s industry-leading VAR furnaces. Further, Consarc and AmpSci offer control upgrades to existing furnaces. The integrated measurement and control system provided by Consarc and AmpSci provides the safest and most advanced remelting system. More information on how the VARmetric™ measurement system can increase product quality, process efficiency and safety in the production of specialty metal ingots can be found on the VARmetric™ website.