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Analysis of an Approach for Detecting Arc Positions During Vacuum Arc Remelting Based on Magnetic flux Density Measurements

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Analysis of an Approach for Detecting Arc Positions During Vacuum Arc Remelting Based on Magnetic flux Density Measurements Analysis of an approach for detecting arc positions during vacuum arc remelting based on magnetic flux density measurements Miguel F. Soler Graduate Research Assistant School of Mechanical, Industrial, and Manufacturing Engineering Oregon State University Corvallis, OR 97331-6001 Email: [email protected] Kyle E. Niemeyer∗ Assistant Professor School of Mechanical, Industrial, and Manufacturing Engineering Oregon State University Corvallis, OR 97331-6001 Email: [email protected] Vacuum arc remelting (VAR) is a melting process for the current into the system, in a vacuum environment. The result production of homogeneous ingots, achieved by applying a is a high-quality metal ingot that exhibits increased homo- direct current to create electrical arcs between the input elec- geneity and decreased defects. The high-quality metals pro- trode and the resultant ingot. Arc behavior drives quality of duced by VAR are typically used for high-performance ap- the end product, but no methodology is currently used in VAR plications such as aerospace systems [1]. VAR is often used furnaces at large scale to track arcs in real time. An arc po- on Ni- and Ti-based alloys [2–5]. sition sensing (APS) technology was recently developed as Figure1 depicts a VAR furnace cross section. The cur- a methodology to predict arc locations using magnetic field rent applied to the system forms electrical arcs between the values measured by sensors. This system couples finite el- melted ingot and the input consumable electrode. Since no ement analysis of VAR furnace magnetostatics with direct ingot exists at the start of the process, common practices in- magnetic field measurements to predict arc locations. How- clude the addition of small metal pieces to the bottom of the ever, the published APS approach did not consider the effect crucible to form an arc. These arcs begin the melting process of various practical issues that could affect the magnetic field of the electrode, which then transfers mass to the bottom of distribution and thus arc location predictions. In this paper, the crucible due to gravity. This mass solidifies at the bot- we studied how altering assumptions made in the finite el- tom of the ingot as the arcs and heat transfer take place at the ement model affect arc location predictions. These include electrode-ingot gap, which travels up the crucible as more the vertical position of the sensor relative to the electrode- mass is transferred from the electrode to the ingot. Arcs can ingot gap, a varying electrode-ingot gap size, ingot shrink- simultaneously form in multiple positions; presently, opera- age, and the use of multiple sensors rather than a single sen- tors can neither visualize nor control the formation of arcs. A sor. Among the parameters studied, only vertical distance water-cooled jacket prevents the copper crucible from melt- between arc and sensor locations causes large sources of er- ing. At the top of the melted ingot a liquid pool of the mate- ror, and should be considered further when applying an APS rial exists. The characteristics of this melt pool have a large system. However, averaging the predicted locations from impact on final quality of the ingot [4–6]. four evenly spaced sensors helps reduce this error to no more Arc behavior drives the remelting process and de- than 16 % for a sensor position varying from 0.508 m below termines ingot quality, but arc positions are challenging and above the electrode-ingot gap height. to quantify due to the VAR system geometry and high- temperature environment. Currently, video cameras directed down the annular gap between the electrode and crucible give 1 Introduction operators qualitative information of arc behavior, as well as Vacuum arc remelting, or VAR, is the metallurgical pro- side arc detection, however these systems cannot track in- cess of remelting metal ingots with the application of a direct stantaneous arc formation and motion. A robust arc detec- tion and tracking system would give insight into the material properties of the final ingot. A common approach for de- ∗Address all correspondence to this author. to soluteingots partitioning [3]. Wang of et the al. alloying [9] developed elements. a Recenttwo-dimensional ax- [3] resultsisymmetric have indicated model both to study through arc characteristics modeling and under different experimentally[4] that significant Fe macrosegregation occursaxial during magnetic the melting fields of using Ti-10V-2Fe-3Al, the commercially and this available soft- macrosegregationware FLUENT. changes Their as model a function focused of the on total magnetohydrody- currentnamics entering and the plasma ingot. Thebehavior modeling in the study electrode-ingot assumed gap, as- an axisymmetric and Gaussian distribution of the arc, whethersuming this thatis valid plasma is one consisted of the aims of onlyof the electrons current and ions and work.its flow can be described with a hydrodynamic approxima- Perhapstion. Theythe defect concluded of most concern that the with effect Ti-6Al-4V of current is density dis- so called hard-alpha interstitial inclusions, a type 1 low- densitytribution, inclusion arc (LDI), distribution, because was these significant inclusions can for VAR because becomeit directly crack initiation correlates sites to the leading heat to flux premature density at the anode. fatigueWoodside failure. Theet al. fact [2, that12] hard-alpha used the multiphysics material can finite-element have a similar melting point and similar density as comparedmodeling to the (FEM) alloy makes software it di COMSOLfficult to remove to simulatevia the magne- VAR.tostatics The term ofhard-alpha a VAR furnace. actually They refers assumed to material an axisymmetric overfurnace, a range homogeneous in the Ti-N phase material diagram, properties, sometimes and a single non- referreddiffuse to as arc nitrides, in a three-dimensional and within this range, model. there Model is results were considerable variation in the fracture toughness.[5] Ti-N inclusionused ‘‘survival’’ to develop times a relationship in a VAR between melt pool measured as a magnetic functionfield of readings particle at size a notional and density Hall have sensor been position mod- and arc loca- [6] eled.tionsAs [ mentioned,2]. Nair et the al. [ arc10] current used the drives FEM the software fluid Opera3d to motion. An understanding of the fluid dynamics in the poolstudy is critical the to use predicting of magnetic the ability source of VAR tomography to reduce to understand thesearc defects. behavior[7] Therefore, in VAR knowing systems. the VARThey arc modeled distri- electrostatics butionwhile is in assuming turn critical homogeneous to making accurate material predictions properties, and in- Fig. 1—Cross section of the VAR furnace. Sketch is courtesy of ATI of the dissipation of hard alpha inclusions within the FigureAllvac, 1: Cross with a modification section of to show VAR instrumentation. furnace. Image taken from VARcluded melt pool. both Producing single and ingots double free non-diffuse of high-density arcs. Nair et al. Woodside et al. [2]. inclusionsconcluded (HDI), that a arc type locations 1 defect can consisting be predicted of a based on mea- melting rates are much higher than utilized during nickel refractorysurements element of suchmagnetic as tungsten, flux density is also outside important the furnace with alloy melting. Thus, the molten pool is quite deep, but is expected to be less dependent on arc distribution. sometimes being described as having a ‘‘soda can’’ Thissufficient is because accuracy these inclusions under tendthe right to rapidly circumstances. sink to According tailedshape. study This of VAR means furnaces the localized is numerical heat flux from modeling the arcs [2–9].the bottomto literature, of the melt arc pool, locations so changes and characteristics in the arc driven directly affect The applicationwill have less of impactlarge currents on the through solidification the system front as resultsfluidingot dynamics characteristics. are less important. [2–4,7–10] Various studies showed that compared to nickel melting because of the difference in It is also possible that the arc distribution impacts the in a strongdistances magnetic from arc field to solidification surrounding front. the furnace,However, on arc whichphysicalarc locations structure of can the be ingot predicted sidewall accurately surface. This using is magnetic flux severalconstrictions studies focused toward the [2, side9, 10 wall]. can The potentially arc position lead to in thesignificantdensity because measurements the sidewall around integrity VARfurnaces and grain combined with ingot-electrodeshelf remelting, gap causing is a key material parameter to fall that into affects the pool, the mag-structureaccurate can innumerical turn affect models. subsequent [2, 10 forging] However, opera- these method- thus the commonly used term ‘‘fall-in.’’ This material tions and product yield. Multiple techniques are used neticcan field. remain Arcs intact concentrate in the melt the pool electrical and can be current a source passingwithinologies industry made to assumptionsimprove ingot and surface simplifications quality, but that should be throughfor point the system defects as and its impact composition the distribution will often differ of from the mag-determiningexamined the further effect of to encouragethe arc distribution their application on the in industry. neticthe field. nominal alloy composition. An example is a type 2 ingot surfaceThe has purpose been di offfi thiscult.
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