investment casting processes

HITCHINER Manufacturing Co., Inc. 117 Old Wilton Road Milford, NH 03055 - USA

THE LEADING PROVIDER OF INNOVATIVE TECHNOLOGY SOLUTIONS FOR INVESTMENT CASTINGS

Investment Casting Processes

Sanjay Shendye

Hitchiner Manufacturing Co., Inc.

April 10, 2013

IC Processes

Contents• Investment Casting (IC) Processes• Process Comparison – Advantages and Disadvantages• CG Casting Process Terminology• Foundry Cards• Alloys Cast • Alloy Qualification • Current Melt and Casting Capacity• Summary• Quiz

Ref: Metals Handbook, Vol 1, 10th ed

IC Processes

IC Process Schematic

IC Processes

• Gravity Casting Process• Metal is melted in air and poured into the molds using a ladle

• Counter-gravity Casting Processes• CLA – Counter-gravity Low-pressure Airmelt Process• SSCLA – Sand Supported Counter-gravity Low-pressure Airmelt Process• CLI/SSCLI – Counter-gravity Low-pressure Inert gas Process• CLV – Counter-gravity Low-pressure Vacuum Process• CPV – Counter-gravity Positive-pressure Vacuum Process

• PDC – Pressure Differential Control • VDC – Vacuum Differential Control

• SLIC – Several Layer Investment Casting Process• C3 – Counter-gravity Centrifugal Casting Process

IC Processes

• Gravity Casting Process• More than 2500 years old. Metal is typically melted and poured into molds.

Advances in vacuum technologies have enabled melting and casting of reactive alloys in vacuum

• Metal is induction melted in air or vacuum and poured into the mold. Fill-out of the mold is assisted by gravitational force. Depending on the alloy melted a preformed crucible may be used

• Shell mold thickness is typically 3/8” to ½” or more depending on the size of the part to be cast

• This process is used at ACF for casting some parts, plan to is eliminate casting any parts using this process by the end of 2013

• CLA Process• Metal is melted in air atmosphere and cast using a ceramic fill pipe

(snout) placed in the bottom of the mold chamber prior to casting• Mold is preheated to 1900 F – 2000 F and then transferred into the

casting chamber. Gaskets used at the chamber/snout/collar and collar/top cap junctions. Mold chamber is closed. Oxides/debris on the melt surface is skimmed prior to casting.

• A pressure differential is created in the mold cavity by creating vacuum in the mold chamber while molten metal in the crucible is maintained at atmospheric pressure. The pressure differential fills the liquid metal into the mold. The pressure differential is approx 20” to 25” of mercury depending on the mold height

• There is only one CLA casting machine used to cast stainless and some cobalt-base alloys. It is located in GTO. This will be removed from GTO in the near future

IC ProcessesCLA Casting Process


Vacuum Port

Casting Chamber

Melting Furnace

Ceramic Fill Pipe

Ceramic Shell

Parts on the sprue

IC Processes

• SSCLA Process• This process is same as the CLA process except that sand is filled in the

cavity between the mold and the inner wall of the casting chamber• Sand provides support to the mold during casting and reduces the

chances of leakers• Sand is at room temperature so it also cools the mold

• Mold chamber can be cast static or spun• A sample is analyzed for chemistry prior to cast (first pin) and at the end

of the heat. The first pin results are used for ceritification

IC ProcessesSSCLA Casting Process

Vacuum Port

Casting Chamber

Melting Furnace

Ceramic Fill Pipe

Ceramic Shell

Sand Filled in the Gap Between Casting Chamber and Shell Mold

Parts on the sprue

IC ProcessesSSCLA Casting Process

IC ProcessesSSCLA Casting

Heated mold coming out of the pre heat oven at ACF

Heated mold placed into the casting chamber at ACF


ACF multi station casting unit


Pre heated mold being placed in the mold chamber

at HITSA

IC Processes

• CLI Process• This process is similar in principle to the SSCLA process except

• Metal is melted and cast under inert gas atmosphere instead of air atmosphere. Argon is used as the inert gas as it is readily available and is relatively lower cost compared to other inert gases such as Helium

• The mold cavity is evacuated by applying a small vacuum ~ 15-25 inches of water for 15-30 seconds before snout is inserted into the molten metal. Application of such small vacuum to the casting chamber before filling the mold with metal replaces air in the mold cavity by argon

• Note: argon is slightly denser than air and hence rises less compared to air

• Induction melting of the alloy is typically done using a preformed crucible made of alumina or zirconia depending on the alloy to be cast

• Note: Alumina crucible is typically used to melt Ni-based superalloys, while zirconia crucible is used to melt Co-based superalloys

• Usage of preformed crucible reduces the chance of contamination of the alloy which could occur if a rammed crucible is used instead of preformed crucible

• CLI process is used for reactive alloy melting and casting e.g. Ni and Co-based superalloys such as IN713, C-263, IN718, etc.

• Alloy may be in the form of pre-alloyed ingot or made from component raw materials• Skim oxides/debris on the melt surface prior to casting

CLI Processes

IC ProcessesCLI Furnace at ACF

IC Processes

• CLV Process• This process is similar in principle to the CLI process except following

• There is no sand back up between the mold and the inner wall of the casting chamber. Mold is not spun during casting

• A mold base with a center hole for inserting the snout is used. Preheated mold is placed on top of the snout and a top cap is placed on top of the mold. A mold bonnet is placed on the mold base thus creating a mold chamber. This mold chamber is then transferred on top of the metal chamber

• Metal is induction melted under vacuum (vacuum is below 50 millitorr during melting) in the metal chamber and cast using argon assist

• Initially both metal in the crucible and mold chamber are maintained under vacuum. The interlock between the mold and metal chamber is removed. Just before filling the mold cavity argon is pumped into both the metal and the mold chamber until the mold chamber reaches 1 atmosphere pressure. The crucible is raised so that the fill pipe at the bottom of the mold chamber is immersed into the liquid metal. In order to fill the mold cavity, the argon in the mold chamber is evacuated while maintaining the argon pressure in the metal chamber thus creating a pressure differential between the metal and mold chamber. This results in the metal being “filled” into the mold cavity

• Alloy used is always in the form of pre-alloyed ingot. A chill plate is used to remove oxides/debris floating on the melt surface before casting the mold

CLV Processes

IC ProcessesCLV Furnaces at GTO

IC ProcessesCLV Mold Preheat Furnace

IC Processes

• CPV Process• This process is similar in principle to the CLV process except following

• Metal is filled into the mold cavity by 2 different methods – Vacuum Differential Control (VDC) or Pressure Differential Control (PDC)

• VDC process is the same as in the CLV process• PDC – Both the metal and mold chamber are evacuated and held under vacuum (50

torr or better) until the mold is ready to be cast. Just before the beginning of the cast cycle argon is introduced into the metal chamber while maintaining the vacuum in the mold chamber. This pressurizes the molten metal thus forcing it to fill into the mold cavity

• This process can result in the fill out of very thin sections e.g. thin edges of blade that are 0.015” to 0.025” thick

• Note: Using too much pressure to pressurize the molten metal can cause the mold to break thus creating metal leakage which could result in increased scrap

• The CPV II furnace is equipped with 2 melt crucibles. One bigger crucible from which the molds are cast and another small crucible (~ 60 lb melt capacity) which is used to melt ingots while molds are cast from the bigger crucible

IC ProcessesPre heat Oven for CPV 2 Furnace

IC ProcessesCPV 2 Mold Loading

IC ProcessesCPV 2 Furnace

IC Processes

• C3 Process• This process is similar in principle to the SSCLA/CLI process except the

following• Mold is spun centrifugally along the vertical axis (center sprue) at a speed of up to 300

rpm after it is filled with metal. Vacuum is released after the mold speed reaches the desired rpm thus releasing the molten metal in the center sprue back in to the crucible

• Spinning of the mold chamber reduces the vacuum hold time• Too long of a vacuum hold (dwell) increases the chance of solidifcation of the center sprue

• This process is not used in conjunction with CLV or CPV processes as these processes do not use sand back up. Sand back up stops excessive leakage of the mold if it were to happen during spinning

C3 Process

IC Processes

• SLIC Process• This process is similar in principle to the SSCLA process except for the

following• Only a few shell layers are required to make a mold

• 3 layers in the SLIC process vs 7-8 in the SSCLA process

• Parts can be made from either wax or foam• Mold with wax patterns is loaded in a chamber with the snout at the bottom. Gap

between mold and the chamber ID is filled with sand similar to that in the SSCLA process. The chamber (mold/snout/sand assembly) is transferred to a burnout station for insitu burnout of the patterns i.e. this process does not use the std autoclave dewax process

• A propane gas flame is fired into the mold through the snout which enables removal of the patterns (foam burns out and wax patterns melt. Molten wax comes out through the bottom snout

• The burnout process heats the mold. The preheated mold is then cast similar to the SSCLA process. Mold can also be spun using the C3 process

Process Comparison Advantages and Disadvantages

• Advantages of Counter-gravity Casting Processes• Greater part density on the sprue compared to the gravity casting process• Lesser shell layers required to make the mold so that adequate porosity of the

shell can be maintained for mold fill• Reduces cost of shell, less land fill

• Cleaner metal can be utilized to fill the mold cavity as the snout enters the center of the molten metal pool in the crucible. The convex miniscus of the molten metal enables lighter density materials formed in the liquid metal (ceramic/oxide inclusions) to float toward the edge where they have a greater chance to bond with the crucible

• Thinner wall components can be cast. Counter-gravity processes typically enable casting features that are 0.015” thick. Such a small wall thickness typically can’t be cast using the gravity process

• Fill process can be controlled to reduce turbulent flow. Turbulent flow will typically result in more inclusions and gas porosity in the castings

• Counter-gravity processes typically utilize a higher part wt/gate wt ratio. Ratios as high as 0.60 are common for counter-gravity processes vs. 0.25 for gravity process

Process ComparisonAdvantages and Disadvantages

• Advantages of Counter-gravity Casting Processes (contd.)• Since the center sprue is hollow after the cast cycle is complete, parts are not

attached to the sprue when they soldify. Part removal and cut off from the sprue is efficient. This reduces the chance of dimensional distortion of the parts

• Because the center sprue is dropped back into the crucible there is more metal available for casting. This results in reduced cycle time and improved throughput

• Metal temperature used for fill out of the mold is typically 100-150 F lower compared to the gravity process. This results in finer grains in the casting which results in improved mechanical properties


• Disadvantages of Counter-gravity Casting Processes• Insufficient amount of molten metal in the crucible can result in mold aspiration

i.e. air or argon would get entrapped in the parts causing increased scrap • At the end of the cast campaign there is always some amount of metal left in the

crucible called “heel” which has to be reverted into scrap• Large parts with very high wt. are difficult to cast


• Advantages of Gravity Casting Process• Process is relatively simple – metal is melted and poured in to the mold• Large parts can be cast if large qty of metal can be melted• Process is suitable for both air and vacuum melt alloys

• Disadvantages of Gravity Casting Process• Smaller number of parts assembled on the sprue• Gate/part ratio is typically 4:1 i.e 4lb of alloy required for gate for every 1

lb of part wt.• Only relatively thick walled parts can be cast e.g. 0.060” or thicker• Higher potential for trapping gas and inclusions in the part• Parts solidify with the gates attached to the center sprue and therefore

gates need to be cut off to remove the parts• This can damage the parts and also potentially dimensionally distort them

• All of the above can potentially increase the part cost

CG Casting Process Terminology

• Rate of Rise (RoR)• Rate at which the metal rises into the mold cavity. RoR varies depending on the

part, alloy and the process used. RoR values range from 0 sec to 5 sec• Vacuum Dwell or Dwell

• Time for which vacuum is held on the mold during casting cycle. This can vary depending on the part/alloy/process. Can be 45 sec to several minutes long

• Vacuum Curve• A chart showing the vacuum applied to the mold as a function of time

Time in Seconds

Va

cuu

m

measured command


• Oven to Pour Time• Time taken from opening the burnout oven door to casting the mold. This time is

critical. A longer oven to pour time will result in cooling of the mold which will affect the part quality. Typical oven to pour time is 2-3 minutes

• Spin Time/Spin Speed• This is applicable only to the C3 process. The time for which the mold chamber is

spun. Typical spin speed is 300 rpm and the time up to 5 minutes• PDC Mode

• Pressure differential control mode. This is used only in the CPV process• VDC Mode

• Vacuum differential control mode. When the CPV process operates similar to the CLV process it is referred to as CLV mode


• Stick • GTO uses 4 sizes of sticks – 12”, 16”, 20” and 24” in length only 3” in dia• ACF uses 2 sizes of sticks – 18” and 27” in length, 3” and 5” in dia• HITSA uses 1 size of stick – 22” in length and 5” in dia

• Snout • GTO uses a more dense silica snout with a pocket for inserting a filter. A filter is

made of ceramic material. We use only reticulated filter not the foam filter. A snout is used only once at GTO

• Filter can potentially reduce the chance of slag/oxides in the molten metal from entering the mold cavity thus reducing inclusion related scrap

• ACF uses a slightly lower grade of snout and it does not have a pocket to place a filter as in case of GTO snout. None of the ACF parts are cast with filters. A snout is only used once at ACF

• HITSA uses a 9” long tube as a snout that is glued on to the ceramic collar before cast. After cast ,the snout is removed cleaned and reused several times before it is discarded

• Size of the snout is different in all 3 facilities


• Crucible• GTO uses only preformed crucibles ~ 10” L x 10” dia x 1” thick wall for

melting and casting• Made of alumina (for Ni-base alloys) and zirconia (for Co-base alloys)• CPV 2 uses smaller crucibles for premelting in addition to the larger crucibles for

melting and casting• ACF uses preformed crucibles (alumina for CLI cast alloys) and rammed

magnesia for steel/stainless steel/some nickel-base/cobalt-base alloys cast using SSCLA/SSCLA-C3

• HITSA uses only rammed magnesia crucibles. They don’t use any preformed crucibles

• Insulation• Made from ceramic fiber material. Kaowool is a brand name• Typical size is ½” thick. This material comes in the form of a roll


• Alloy Makeup• ACF and HITSA use alloy makeup techniques. Component raw materials

are mixed and melted in a crucible• GTO uses only pre-alloyed ingot ~ 3.5” dia and ~ 50 lb wt• Approx metal wt requirement calculations

• Metal required to fill snout + center sprue • 5” dia x 27” long stick + 16” L snout (2.5” dia) = ~ 184 lb• 3” dia x 27” long stick + 16” L snout (2.5” dia) = ~ 81 lb• 3” dia x 20” long stick + 12” L snout (2.25” dia) = ~ 57 lb• 3” dia x 18” long stick + 12” L snout (2.25” dia) = ~ 53 lb

• Metal required to cast parts is in addition to metal required to fill snout and the center sprue

• Mold Base• CLV and CPV processes don’t use a mold chamber as in SSCLA. Instead a

mold base is used to load the pre heated mold. The mold base is made of A36 steel. Diameter of the mold base determines the size of the mold that can be cast

CG Casting Process TerminologyPrealloyed Ingot

Prealloyed ingot

CG Casting Process TerminologyAlloy Makeup

Alloy makeup at ACF Alloy makeup at HITSA


• Ceramic Collars • Size of the GTO, ACF and HITSA collars is different and they are not interchangeable. A mold

made at ACF with ACF style collars can’t be cast at GTO and vice versa

• Gaskets• Gaskets provide the seal between the mating surfaces

• Between the snout and the casting chamber (CLA/SSCLA/CLI) or the base plate (CLV/CPV)

• Between the snout and the mold collar• Between the ceramic collar and the top cap

• GTO uses graphite rings 1/16” thick as gaskets• ACF and HITSA use ceramic fiber gaskets between snout and chamber and between snout

and ceramic collar. Top cap is glued on to the ceramic collar and there is no gasket between the top cap and the mold collar

Some parts cast using insulation wrap at GTO. HITSA does not use insulation wrap on their molds. ACF uses insulation

on some parts



GTO uses 4 sizes of sprue sticks – 12”, 16”, 20” and 24”

ACF uses 2 sizes of sprue sticks – 18” and 27” in 3” and

5” diameter


Mold base, snout, top cap and graphite gaskets used in the CLV process


3” dia x 18” long Sticks at ACF

Tube snouts used at HITSA


Tube snouts cleaned and ready for reuse at HITSA. Pieces of white square shape insulation seen around

the tube dia


Tube snout being removed from the bottom of the chamber in the HITSA foundry. Snouts are cleaned and resued


Standardized shell molds in the HITSA foundry. Molds are 22” tall and use only 5” stick

Foundry CardACF Foundry Card Example

3 sec to 299” water

300 rpm in 30 sec

Argon backfill start at 45 sec

end at 125 sec

Test bars can be heat

treated separately from parts

Vacuum dwell for 45

sec

Foundry Card GTO Foundry Card Example

Foundry CardGTO Foundry Card Example

Alloys Cast

• CLA/SSCLA/SLIC• Steels, stainless steels, Ni-base alloys such as Nimocast 80 and

Nimocast 90, Hasteloy X, cobalt alloys including Co 21, Co 31, L 605 and alloy 400

• Molds may be spun or not depending on the part configuration• CLV/CPV

• Ni and Co-base superalloys including• IN718, IN939, IN625, Hasteloy X, Haynes 230, Mar-M-509, mar-M-247, Rene 41,

Rene 77, Rene 80, Rene 108, IN738, IN713, IN100

• CLI• Ni-base alloys including

• IN718, IN625, Hasteloy X, Haynes 230, IN713• Molds can be spun or not depending on the part configuration

Alloy Qualification

• Chemical Testing• We use optical emission spectrometer and combustion methods to analyze chemistry

• Mechanical Testing • Hardness, tensile, stress/creep rupture• Test pieces are either in as-cast or heat treated (and/or HIPed) condition

• Heat treatment: Parts are heated to temperature held at temperature in air/argon/vacuum and cooled at certain rate to room temperature

• HIP – Hot Isostatic Pressing. Parts heated to high temperature under high argon pressure for several hours and then cooled to room temperature

• Microstructure Analysis• Some specifications require that microstructure analysis be performed every time a

part is cast from a new metal heat/lot• We S/C some of the work to outside laboratories depending on customer

requirements• Unless the alloy used to cast the parts is qualified parts can’t be shipped.

Not all alloys require all tests referred above

Current Melting and Casting Capacity

• Current Melt and Chamber Size • SSCLA – 500 lb and 2000 lb. Chamber size 14” ID shell diameter is limited

to 12” allowing sand to be filled between mold and inner wall of the casting chamber

• There are 6 furnaces in HITSA and 3 in ACF with 2000 lb capacity• There are 2 furnaces in ACF with 500 lb capacity

• CLI – 300 lb melt capacity and 18” and 21” chamber ID• There are 2 CLI units in Milford. One at ACF and other at MCT• There are no CLI melting units in Mexico

• CLV – 200 lb melt capacity and chamber size of 22” ID. This means mold size in shell can’t exceed 21”

• There are 2 CLV units in GTO• There are no CLV units in HITSA or at ACF

Current Melting and Casting Capacity

• Current Melt and Chamber Size (contd.)• CPV – 250 lb capacity crucible and mold chamber ID of 26”. Shell mold

can’t exceed 24” OD including insulations wrap if any• ACF and Mexico do not have any CPV units

• SLIC – Furnace capacity is same as SSCLA • This process is currently used only in MCT• Airmelt alloys are cast in SLIC

• There is no C3 capability in GTO or Mexico

Summary

• Acronyms and Terminologies – CLA, SSCLA, CLV, CPV, CLI, C3, PDC, VDC, SLIC, Stick, Sprue, Snout, Collar, Crucible, Filter, Gasket, Ingot, Dwell, RoR, Spin, Oven to Pour Time…..

• ACF and HITSA predominantly cast airmelt alloys, GTO Ni and Co-base superalloys

• 2000 lb max melt capacity in ACF and HITSA, 230 lb max capacity at GTO

• Molds are max 27” tall at ACF, 22” tall at HITSA, 24” tall at GTO• ACF used 3” and 5” dia sticks, HITSA uses only 5” dia sticks, GTO

uses only 3” dia sticks

Quiz

• Given that the HITSA plant in Mexico is located at about 8000 feet and ACF plant is at ~ 50 ft above sea level• why is the mold height 22” max at HITSA and 27” at ACF?

• There is a mold with 16 parts. Each part is 12” long and weighs 2lb. Overall mold size is 21” diameter. Alloy to be cast is a Ni-base superalloy. This part has very thin trailing edge ~ 0.020” radius• Which process should be used and why?

• Assume liquid density of nickel alloys is 50% that of mercury. Assume loss of vacuum ~ 3” mercury through the snout/gasket/collar and collar/top cap assembly. Assume snout length ~ 12”• If we draw a vacuum of 15” of mercury, how high a 3” dia x 18” stick mold be

filled assuming sufficient qty of liquid metal is available?

investment casting processes

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