AC 33.15-1A AC, should be maintained for the appropriate period of time, per §21.137(k), and be available for review as needed.
7.5 New melters, raw material suppliers, and their processes should be adequately qualified.
7.6 A system of handling deviations and non-conformances with respect to product or part limits should be established between the PAH and the suppliers.
8 RAW MATERIALS AND STORAGE.
8.1 Raw Material Requirements.
8.1.1 Raw Material Controls.
Melt suppliers should maintain effective specifications and procedures for procurement, storage, and processing of charge materials for VAR, CHM or skull melting. Raw material suppliers should maintain effective production controls and inspect material on a sampling plan sufficient to assure compliance with the melt supplier raw material requirements. Melt suppliers should additionally inspect/verify conformance to requirements and ensure that raw material suppliers (and any sub-tiers of the raw material suppliers) exercise effective diligence to ensure the absence of inclusions/extraneous material in the raw materials and associated processing equipment, that might persist into the final product based on the production route.
8.1.1.1 Charge Composition.
The charge materials should be composed of only approved raw materials, such as sponge, master alloys, elemental additions, titanium oxide, and recycled material (where permitted). The weight percent, chemical composition, and batch or lot number of each type of raw material used in the charge materials should be recorded and maintained for each heat of the material. Charge materials containing alloy elements should be added in a form that allows complete dissolution and homogenization following melting.
8.1.1.2 Titanium Sponge.
Titanium sponge suppliers should have established procedures and limits for the production of high purity premium quality titanium sponge. A representative sample of each sponge lot should be visually inspected to established standards to ensure its freedom from burned particles capable of causing HIDs, and from other contaminants. Sponge particles removed during this inspection should be evaluated to determine if the entire sponge lot should be subject to disqualification. The titanium sponge supplier should establish specific acceptance standards, including chemical analysis of suspect sponge particles. Sponge batches or lots that have been involved in a fire at any stage of their processing should not be considered for premium quality rotor application.
AC 33.15-1A
8.1.1.3 Zirconium Sponge.
Zirconium sponge suppliers should have established procedures and limits for the production of high purity zirconium sponges. Each zirconium sponge lot should have a representative sample visually inspected to compare to established standards to identify and remove particles suspected of containing either burned sponge or other contaminants considered to cause HIDs. Suspect particles from this inspection should be removed and evaluated to determine if the entire sponge lot should be subject to disqualification. The zirconium sponge supplier should establish specific acceptance standards, including chemical analysis of suspect sponge particles. Any zirconium sponge batch and lot which has been involved in a fire after inspection should not be considered for premium quality rotor application.
8.1.1.4 Master Alloys.
Master alloy suppliers should have established procedures and limits for the production of high purity master alloys. Master alloys should be inspected to established standards to identify and remove detrimental foreign material, oxides, nitrides, and other contaminants considered to cause HIDs or other deleterious inclusions.
8.1.1.5 Elemental Additions.
Elemental additions should be processed to preclude contaminants considered to cause HIDs or other deleterious inclusions.
8.1.1.6 Titanium Oxide.
Oxides of titanium used as raw material charge should be in powder form and be processed to preclude the presence of detrimental foreign material.
8.1.1.7 Recycled Material.
All recycled materials should be cleaned and be free from contamination.
The following limitations should be considered when recycling previously melted alloy:
• Multiple VAR Processed Material.
In general, only turnings should be permitted to be directly recycled in multiple VAR processed premium quality titanium alloy. When used as recycled material, turnings should be segregated as to alloy, crushed, and cleaned, and should be 100 percent radiographically inspected for particles considered to cause HDIs and the particles removed. Bulk weldables should be prohibited for use in multiple VAR processed material, except when previously melted (consolidated) by an approved CHM process. The restrictions on use of bulk weldables in such a CHM or skull consolidation melt should follow the limitations set forth 12
AC 33.15-1A in the following paragraph which applies to material that has been PAM or EBM melted as the primary melting operation.
• CHM or Skull Plus VAR.
Previously melted materials may be recycled through the primary CHM or skull melting process, provided the material is clean, free of scale and slag. Recycled material should be of the same alloy, or compatible alloy composition. Preparation and cleaning procedures should be established for each form of recycled material. Each form of recycled material should be cleaned and inspected to established standards. A radiographic inspection may be considered unnecessary for turnings. Material forms with surface connected internal cavities, exfoliations and other features that prevent complete visual inspection, should not be recycled.
Recycling of grinding products, dust, and sludge should be prohibited. Recycling of material that has been involved in fire at any stage of charge material preparation should be prohibited.
Flame-cut materials may be used in CHM or Skull Plus VAR processed material provided that the affected areas are cleaned and inspected.
8.2 Bottom Charge Materials.
If a bottom charge is used for VAR, the composition, form, preparation, and inspection procedures should be established. Sponge should not be used as the bottom charge.
8.3 Raw Material Storage.
Procedures to prevent damage and deterioration of each product and article during handling, storage, preservation, and packaging should ensure all virgin raw material, turnings and bottom charges are stored in covered containers, in a secure area, immediately after inspection to preclude the extraneous addition of foreign or uninspected material.
9 CONSOLIDATION AND MELTING.
This section details some of the more significant process parameters, controls and procedures recommended in the areas of input material consolidation and ingot melting for the production of premium quality titanium. These items should be viewed as guidelines which, when incorporated into the suppliers’ specific method of manufacture, should result in a well-controlled process that minimizes HDI and HID occurrence, and produce sound ingots with acceptable homogeneity.
9.1 Consolidation.
Fixed blending and consolidation practices should be established by the supplier and approved by the PAH. Proper preparation, blending, and consolidation practices not only minimize HID and HDI introduction sources, but can also assist in the elimination of HIDs from sponge type input materials.
AC 33.15-1A
9.1.1 Multiple VAR Process.
Double or triple VAR processes typically use blended input materials consolidated either by compaction and welding, or by consolidation melting.
9.1.1.1 Blending.
The supplier should establish procedures for selecting and preparing the input materials with respect to size and shape. Blending practices should demonstrate acceptable chemical homogeneity in the final ingot product.
Procedures should also be established to preclude excessive deblending during handling and compaction prior to the first melt. Caution and inspection, as required, should be applied to preclude fires from material crushing, handling and blending, and to exclude any material exposed to such a fire.
9.1.1.2 Compaction.
Procedures should be established for compacting the input materials in a form that facilitates the supplier’s consolidation practice. Compact size and shape, and compaction pressure, should be a fixed practice. Caution and inspection, as required, should be applied to assure no local fires are created during compaction, and to exclude any material exposed to such a fire.
9.1.1.3 Primary Electrode Fabrication Welding.
When a primary electrode is fabricated by welding, all welding should be conducted in an inert gas filled or a vacuum chamber. Tungsten Inert Gas or graphite electrode weld processes should not be used for the electrode construction. Welds should be visually inspected at a minimum for atmospheric contamination; contaminated welds should be removed and replaced prior to primary melting. When appropriate, limits and procedures for the chamber weld should be established for the following:
• chamber leak-up rate,
• chamber pressure,
• plasma torch gas, and purge,
• metal-inert gas (MIG) weld wire,
• electrode cooling before venting chamber.
9.1.1.4 Consolidation Melt Fabrication.
Charge material may be prepared using an established consolidation melt procedure before the primary VAR step. Effective controls and limits for the consolidation process should be maintained for producing material free from contamination known to cause HIDs and HDIs. The consolidation melt should be performed in an inert gas-filled or vacuum chamber. For the non-consumable electrode melting processes, the electrode should be 14
AC 33.15-1A constructed to avoid the introduction of HDIs or other deleterious materials into the melt. Tungsten or graphite electrodes should not be used. When a CHM refining process is used as the consolidation melt, additional practices, procedures, and limits should be established as identified in 9.1.2, CHM Plus VAR Process. The consolidated ingot should be inspected prior to VAR for surface contamination caused by coolant or air leaks. Limits and procedures should be established for the consolidation melt chamber with respect to the following:
• chamber leak-up rate,
• chamber pressure, and
• input material feed system.
9.1.1.5 Electrode Mechanical/Electrical Attachment.
When the possibility exists that the electrode attachment material may be consumed during melting, the attachment material should have the same nominal composition and quality level as the electrode. It should be prepared and inspected using established procedures. The attachment should be joined to the electrode using established inert gas in-chamber welding procedures, or plunge-arc welding in a vacuum chamber. Limits and procedures for weld chamber leak detection should be established.
When appropriate, limits and procedures for the chamber welds should be established for the following:
• leak-up rate,
• chamber pressure,
• plasma torch gas and purge,
• metal inert gas (MIG) weld wire, and
• weld cooling prior to venting chamber.
When plunge welding is used, procedures should be established for the following:
• depth of plunge,
• voltage and amperage,
• rundown, and
• leak-up rate.
All welds should be visually inspected to established standards.
9.1.2 CHM Plus VAR Process.
EBM or PAM CHM processes typically use blended input materials compacted as small briquettes for particulate feed or consolidated using either VAR or one of the nonconsumable electrode melting processes. Input material requirements for CHM are the 15
AC 33.15-1A same as the multiple VAR process, except that turnings may not require X-ray inspection because the CHM process effectively removes HDIs and the multiple VAR process prohibits use of bulk weldables.
9.1.2.1 Blending.
The supplier should establish procedures to select and prepare the input materials with respect to size and shape. Blending practices that achieve acceptable chemical homogeneity in the final ingot product should be demonstrated. Procedures should also be established to preclude excessive deblending during handling and compaction prior to the first melt. Caution and inspections should be applied to prevent fires during material crushing, handling, blending, and to exclude any material exposed to such a fire.
9.1.2.2 Compaction.
Procedures should be established for compacting the input materials into a form that facilitates the supplier’s consolidation melt or CHM practice.
Compact size and shape, and compaction pressure, should be a fixed practice. At a minimum, the sponge, master alloy, elemental additions, titanium dioxide and turnings should be compacted. Caution and inspections should be applied to assure no local fires are created during compaction, and to exclude any material exposed to such a fire.
9.1.2.3 Consolidation Melt Prior to CHM.
Some or all the charge material may be prepared using an established consolidation melt procedure before charging the CHM furnace. Effective controls and limits for the consolidation process should be maintained for producing material free from contamination known to cause HIDs and HDIs. The consolidation melt should be performed in an inert gas filled or a vacuum chamber. For the non-consumable electrode melting processes, the electrode should be constructed to avoid the introduction of high density or other harmful materials into the melt. Tungsten or graphite electrodes should not be used. For the VAR consolidation process, procedures identified in 9.1.1, multiple VAR process for input material consolidation, and for melting, should be followed. The consolidated ingot should be visually inspected prior to CHM for surface contamination caused by water or air leaks.
9.1.2.4 Mechanical Attachment.
When the possibility exists that the pushrod mechanical attachment may be consumed during melting, the push rod should have the same nominal composition and quality level as the charge material and should be prepared and inspected using established procedures. If welding is used, the push rod should be joined to the consolidation melt using established inert gas in-chamber welding procedures or plunge arc welding in a vacuum chamber. Limits and procedures for weld chamber leak detection 16
AC 33.15-1A should be established. When appropriate, limits and procedures for the mechanical attachment chamber welds should be established for the following:
• leak-up rate,
• chamber pressure,
• plasma torch gas and purge,
• metal-inert-gas (MIG) weld wire, and
• procedures for venting chamber.
When plunge welding is used, procedures should be established for the following:
• depth of plunge,
• voltage and amperage,
• rundown, and
• leak-up rate.
The weld should be visually inspected to established standards. Since electrical contact of the push rod is not required, mechanical methods of joining the push rod may be employed.
9.1.3 Skull Plus VAR Pr ocess.
Charging. 9.1.3.1 The supplier should establish procedures to select and prepare the input materials with respect to size and shape. Melting practices, including the positioning of charge materials in the hearth, should be demonstrated to achieve acceptable chemical homogeneity in the final ingot product.
Caution and inspections should be applied, as required, to prevent fires during material crushing, handling, blending, and excluding any material exposed to such a fire.
Compaction . 9.1.3.2 Procedures should be established for compacting the input materials into a form that facilitates the supplier’s skull melt practice. Compact size and shape, and compaction pressure, should be a fixed practice. At a minimum, the sponge, master alloy, elemental additions and titanium The skull process typically uses revert material in the form of solid and scrap which is placed in the hearth prior to sealing the melting chamber. Input material requirements for the skull process are the same as the multiple VAR process, except that turnings may not require X-ray inspection because the skull process effectively removes HDIs. In addition, blended and compacted virgin material may be used to balance the chemistry of the heat.
AC 33.15-1A dioxide should be compacted. Caution and inspections, as required, should be applied to assure no local fires are created during compaction, and to exclude any material exposed to such a fire.
9.2 Primary Melting.
Primary melting may be accomplished by either the VAR of an electrode which has been consolidated as described in 9.1.1, Multiple VAR Process, by the CHM of input materials which have been processed as described in 9.1.2, CHM Plus VAR Process, or by the skull melting of input materials which have been processed as described in 9.1.3 Skull Plus VAR Process.
9.2.1 Multiple VAR Process.
The first melt for the multiple VAR process should be conducted using a fixed melt practice established by the supplier and approved by the PAH. A consumable electrode process should be used, which incorporates effective controls and limits shown to minimize the occurrence of HIDs and HDIs. Care should be taken to terminate melting before any non-premium material is melted.
9.2.1.1 Critical Parameters.
When using the VAR process to produce primary melt ingot, processing procedures, parameter limits and verification methods should be established by the supplier to control the following–
• bottom charge (initial charge),
• pressure/leak rate,
• power levels,
• melt rate/gap control,
• arc focusing controls,
• end of melt control/indicators,
• crucible cooling,
• annulus control,
• interruptions (power, pressure),
• chamber, crucible and base plate cleaning,
• coolant leaks, and
• electrode orientation practice.
9.2.1.2 Multiple Electrode Melting – Precautions.
Multiple electrode melting is an acceptable, but not preferred, practice.
When multiple electrode melting is used to produce a primary ingot of sufficient size for subsequent melts, care should be taken to assure cooling to appropriate temperatures prior to any furnace opening. Piggyback 18
AC 33.15-1A melting of a second electrode on top of the first ingot should be avoided.
The primary ingots should be removed from the VAR furnace, cleaned, and chamber-weld joined prior to the second VAR step. Multiple electrode melting should not be used beyond the manufacture of the primary ingot.
9.2.1.3 Primary Electrode Post-Melting – Precautions.
The potential risk related to primary VAR electrode burning during air exposure following melting should be minimized, and subsequent primary electrode cleaning methods should be sufficiently robust to remove any contaminated surface to prevent HID(s) remaining in the final product.
9.2.2 Cold Hearth Melting.
When the CHM practice is used to produce a primary melt ingot, a fixed melt process should be established at the CHM supplier and approved by the PAH. During the CHM melting process, the heating source should be designed to preclude it from being the source of HDIs or other deleterious materials entering the melt.
9.2.2.1 Critical Parameters for EBM and PAM.
When using either a well-controlled EBM or PAM CHM method to produce a primary melt ingot, processing procedures, parameter limits and verification methods should be established by the CHM source to control the following:
• hearth design and assembly and set up of the associated equipment,
• method of removing the skull from the hearth, storage of the skull to prevent contamination, provisions for retiring the skull and traceability of the skull back to previous melts,
• molten pool dimensions during steady-state melting,
• molten pool temperature during steady-state melting,
• chamber leak-up rate,
• chamber pressure,
• melting (ingot casting) rate,
• cooling cycle and atmosphere,
• melt interruptions,
• chamber and raw material feed cleaning,
• input material feed practice, and
• precautions to prevent accidental transfer of unmelted material into the refining hearth or withdrawal mold.
9.2.2.2 Additional Electron Beam Melt Parameter Controls.
AC 33.15-1A In addition to the controls listed in 9.2.2.1, Critical Parameters for EBM and PAM, the EBM CHM source should also establish procedures to control the following:
• electron beam power, deflection pattern, beam diffusion and scanning frequency,
• duration of beam interruption, and the restart procedure, and
• condensate control and fall-in.
9.2.2.3 Additional Plasma Arc Melt Parameter Controls.
In addition to the controls listed in 9.2.2.1, Critical Parameters for EBM and PAM, the PAM CHM source should also establish procedures to control the following:
• torch gas type and flow rate,
• torch voltage, current, motion pattern, velocity, and position,
• hydrogen, oxygen and moisture control in the chamber and torch gas, and
• duration of arc interruption, and the restart procedure.
9.2.3 Skull Melting.
When the skull practice is used to produce a primary melt ingot, a fixed melt process should be established at the skull supplier and approved by the PAH.
9.2.3.1 Critical parameters for Skull Melting.
When using a well-controlled skull melting method to produce primary melt ingot, processing procedures, parameter limits and verification methods should be established by the skull melt source to control the following:
• chemistry of consumable electrode,
• charge materials, including positioning,
• pressure/leak rate,
• current,
• voltage,
• end of melt control/indicators,
• electrode cooling,
• interruptions (Power, pressure), including duration and restart,
• chamber, hearth, mold and associated fixturing cleaning, and
• coolant leaks.
AC 33.15-1A
9.3 Intermediate and Final Melts.
The final melt should be by the VAR process following either a primary melt by (1) VAR process, (2) CHM process, (3) skull process, or (4) an intermediate VAR melt in the case of a three VAR process.
9.3.1 Multiple VAR, CHM Plus VAR or Skull Plus VAR Process.
Controls should be applied to the melting process to ensure consistency in the melting parameters and, hence, in the ingot product. It is advisable to restrict furnaces and crucibles used for primary melt from being used for the second and third melt. Cleaning and inspection of crucibles is important for all VAR melts and should be performed to established procedures (reference 9.5.4, VAR Furnaces, and 9.5.5, CHM Furnaces).
9.3.1.1 Weld Joining of Electrodes.
Melting through a weld should not be permitted in final melting.
Therefore, if two ingots are welded together to form a longer electrode, that material shall be melted twice. Controls on welding should be per paragraph 9.2.1.2, Multiple Electrode Melting – Precautions.
9.3.1.2 Electrode Cleaning/Descale/Crown.
Prior to remelting, all electrodes should be visually inspected and cleaned.
The cleaning method used should thoroughly remove all loose material, foreign material, or any evidence of burnt titanium. In addition, any crown or splatter at the top of the electrode should be removed. Identification and recording of electrode orientation for each melt should be maintained.
9.3.1.3 Electrode Mechanical/Electrical Attachment.
It should be in accordance with 9.1.1.5, Electrode Mechanical/Electrical Attachment or 9.1.2.4, Mechanical Attachment, with the exception that the push rod should be joined to the primary electrode, or intermediate electrode in the case of triple VAR process, instead of a consolidation melt per 9.1.1.4.
9.3.1.4 Critical Melt Parameters.
The melt supplier should establish a melting sequence to minimize the chemical segregation of the ingot. Once a sequence has been adopted, it should be consistently applied.
All parameters listed in 9.2.1.1, Critical Parameters are applicable to the intermediate and final melts. In addition, for final melting, a hot topping procedure should be established. Input power should be reduced in a controlled fashion over a period of time, and the hot top sequence should be recorded. The hot topping should be consistently applied from heat to heat for each alloy.
1. A system should be established to identify the start of the hot topping sequence. If electrode markers are attached to the electrode, or in any manner introduced into the furnace, they should be manufactured 21
AC 33.15-1A using premium quality titanium of a similar composition and melting point to the alloy being melted. Attachment should prevent electrode markers from falling into the melt. Welding of electrode markers for final melt should be prohibited. If machined grooves are used, the tools should be selected to avoid HDIs.
2. The hot topping sequence for the final melt should be arranged to terminate before the introduction of any non-premium material into the melt and consumption of any welds. This could be done by ensuring that a ‘wafer’ or ‘platter’ of the electrode is always left unmelted. The furnace should be opened in a controlled manner to avoid surface contamination.
9.3.2 Inclusion Risk Comparison for CHM Plus VAR versus Multiple VAR Processes.
A well-controlled CHM Plus VAR process has demonstrated the capability of reducing HID and HDI rates compared with a well-controlled triple VAR process. The presence of the hearth melt step in a well-controlled CHM Plus VAR process allows for a significantly longer residence time during CHM to dissolve inclusions and allow HDIs to density separate compared with a well-controlled VAR process.
9.3.2.1 Seeded Heat Inclusion-Elimination Trial to Qualify a New Hearth.
It has been demonstrated that designing the cold hearth process to be capable of removing select inclusion sizes, types, and quantities is an important contributor to the overall success of the much lower inclusion rates in well-controlled CHM Plus VAR materials. New hearths should be qualified per PAH direction.
9.3.2.2 Streamlined Post-CHM PQ-Focused VAR Melt Shop.
It has been demonstrated that having and maintaining a PQ-focused VAR melt shop is important to preserve the quality benefits associated with a well-controlled CHM process.
9.4 Inspection.
Ingot inspection procedures should be established by the supplier and approved by the PAH. Inspection procedures relevant to the possible introduction of HIDs during the melt procedure are discussed below. It should be noted that at the ingot stage, inspection methods are relatively limited and simple. Therefore, the primary emphasis should be placed on melt process control. Detailed billet and final forging inspection procedures are covered in Section 10, Billet Conversion, Disk Forging, and NDT.
9.4.1 Wafer/Platter Inspection.
The residual electrode ‘wafer’/‘platter’ should be inspected for thickness and color after each VAR melt and compared to physical or photographic standards. Melting through a weld during the final melt should not be permitted. Any indication of abnormal color due to oxidation should be investigated, and sources of furnace leaks identified and repaired before processing additional material in the affected furnace. The heat of 22