Castings are susceptible to having small discontinuities such as inclusions, voids, cracks, etc. that are cosmetically unappealing or potentially even a cause of end product failure. In-process weld rework of castings, sometimes also referred to as “cosmetic weld repair”, is a routine and commonplace foundry activity that is used to mend such casting discontinuities. A casting that has been welded and blended will be dimensionally, physically, chemically and metallurgically compliant to drawing requirements.
Foundry customers derive both direct and indirect benefit from weld rework as it helps make castings more affordable and improves foundry on-time delivery. Foundries factor a casting-yield in their production runs which is an allowance for some quantity of scrap. A poorer than predicted yield of castings will jeopardize a customer delivery, burden the foundry with the cost for the unanticipated scrap, and also the additional expense for a re-run of parts. In-process weld rework provides a foundry with an opportunity to salvage parts that might otherwise be scrap and so protect the customer delivery. Foundries won’t perform unnecessary amounts of weld rework as the cost can easily exceed the value of a casting. In effect weld rework stabilizes foundry yields which, in turn, improves the delivery and the overall affordability of a casting.
The practice for in-process weld rework is well established and presents no deleterious risk to the quality of a casting. A properly performed weld, because of its rapid solidification, is likely stronger than the base metal of the casting.* If you think about it prosaically, the welding of a casting is essentially a “local re-liquefaction” of something that was formerly all liquid.
For the heat treatable alloys to be metallurgically compliant the weld rework of castings must be performed prior to any heat treatment. Post treatment welding will destroy the effect of the heat treatment and so is often called “green welding”. Green welding should only be conducted with the express written permission of the customer.
Weld rework of commercial castings is generally performed by foundries without reservation. Military and aerospace drawings, however, are likely to contain an AMS or ASTM material specification that will prohibit weld rework of castings without first having written customer authorization. Drawing notes are frequently used to convey the required permission to the foundry. In instances where an in-process weld authorization is not automatically being flowed a foundry should identify this at contract review issue and formally request a rework authorization from its customer.
Welding of castings should be referred to as an in-process weld “rework” instead of “repair”. In aerospace and perhaps in other industries as well, the word “Repair” alludes to a very specific and closely controlled activity. Unfortunately, the foundry industry does often identify the procedure as a “Weld Repair”. In aerospace vernacular an in-process weld rework being performed prior to heat treatment it is not considered to be a “Repair” and use of the term should be discouraged.
The most commonly referenced specification for the weld rework of investment castings is AMS2694. Titled “In-Process Welding of Castings” the “Purpose” of AMS 2694 is to define “the requirements for in-process correction of foundry discontinuities by manual welding of castings.” A discontinuity is further defined as a casting nonconformance such as a crack, damage, hot tear, cold shut, shrinkage, porosity, gas hole, inclusion, etc. Weld rework may not be used to correct dimensional discrepancies, discontinuities found during machining, or to fabricate portions of castings unless specifically authorized by a customer. AMS2694 requires that weld rework must be performed by qualified welders and that the welds must be blended and inspected to the requirements of the drawing. Residual linear indications are never acceptable.
In-process weld rework is a routine foundry operation that enhances the affordability and delivery of cast products. If you have any questions concerns regarding weld rework please contact your O’Fallon Casting Sales Engineer.
*There are a number of studies that attest to this fact. See Paper presented by Authors Gerald Gagel, Daniel Hoefert, Joseph Hirvela, & Randy Oehrlein at AFS CastExpo 2013 on the “Effect of Weld Repair on Static and Dynamic Tensile Properties of E357-T6 Sand Castings” and concludes that the researchers found that “…repair welding had no detrimental effects on tensile or fatigue properties.”.
Tuesday, 3 November 2015
Tuesday, 27 October 2015
The O’Fallon Casting University
To underscore its commitment to its customers and to contribute to our mutual success, O’Fallon Casting has founded “The O’Fallon Casting University”.
Castings present a cost effective solution for the manufacture of complex shapes that reduce part count and improve the affordability, manufacturability and reliability of an engineered product. However, successful casting design does require a specialized expertise.
The O’Fallon Casting University is a collection of educational resources that will help educate and train casting professionals in the “Specialized Expertise” necessary for successful casting design. Some of these OFC provided resources of the O’Fallon Casting University include:
O’Fallon Casting’s “Investment Casting Design Guide” is a continuously updated and downloadable reference that is posted on ofalloncasting.com. OFalloncasting.com is also home to O’Fallon Casting’s Technical Papers and for Blogs that share experiences and opinions pertaining to investment casting.
Concurrent Engineering is a one-on-one active review of customer casting designs by OFC Engineers to catch errors and provide design recommendations, hopefully, “before the ink is dry”. Concurrent Engineering activities also serves to mentor and improve the skill of customer casting designers.
Good casting design translates into both direct savings from lower manufacturing cost and reduced part count and also from indirect savings from higher levels of quality, more reliable deliveries and lower administrative costs.
The O’Fallon Casting University is a free service to the customers of O’Fallon Casting. Detailed information about all of these resources is available on ofalloncasting.com.
To commence your educational benefit, please contact your O’Fallon Casting Sales Engineer or email to: sales@ofalloncasting.com
Castings present a cost effective solution for the manufacture of complex shapes that reduce part count and improve the affordability, manufacturability and reliability of an engineered product. However, successful casting design does require a specialized expertise.
The O’Fallon Casting University is a collection of educational resources that will help educate and train casting professionals in the “Specialized Expertise” necessary for successful casting design. Some of these OFC provided resources of the O’Fallon Casting University include:
- O’Fallon Casting’s Investment Casting Design Guide
- IC-101 Class on the considerations for investment casting design
- IC-201 Course on the manufacture of investment castings
- Concurrent Engineering Service
- Ofalloncasting.com
- Technical Papers
- Blogs
O’Fallon Casting’s “Investment Casting Design Guide” is a continuously updated and downloadable reference that is posted on ofalloncasting.com. OFalloncasting.com is also home to O’Fallon Casting’s Technical Papers and for Blogs that share experiences and opinions pertaining to investment casting.
Concurrent Engineering is a one-on-one active review of customer casting designs by OFC Engineers to catch errors and provide design recommendations, hopefully, “before the ink is dry”. Concurrent Engineering activities also serves to mentor and improve the skill of customer casting designers.
Good casting design translates into both direct savings from lower manufacturing cost and reduced part count and also from indirect savings from higher levels of quality, more reliable deliveries and lower administrative costs.
The O’Fallon Casting University is a free service to the customers of O’Fallon Casting. Detailed information about all of these resources is available on ofalloncasting.com.
To commence your educational benefit, please contact your O’Fallon Casting Sales Engineer or email to: sales@ofalloncasting.com
Wednesday, 14 October 2015
Edge & Corner Radii
Edges & Corners are external features such as you would see on any cube such as a child’s Alphabet Block. Structurally Edges & Corners have little impact on the strength of a casting. However, Edges and Corners are still an important consideration of casting design.
Investment Castings will naturally exhibit a .008” - .012” radius along an edge even when tooled “sharp”. I often say that foundries “don’t cast razor blades” (but then even razor blades are not truly “sharp” either). The freezing of the wave front and back pressure from air in the cavity prevent the liquid metal from achieving a truly “sharp” edge. Casting Designers should provide for this condition on their designs with an allowance of at least a .015” R Max along cast edges & corners.
Foundries, however, generally prefer not to have “sharp” edges and corners on cast parts. In Investment Casting a sharp edge can create a weak point in the ceramic shell mold. (If you’ve ever tried to paint a sharp corner you’ll understand that it can be very difficult to build thickness along a sharp edge.) A vulnerable “thin” area in the ceramic shell that may cause the shell to crack from the stress of the De-Wax operation or fail as the mass of the hot metal is being cast. O’Fallon Casting recommends that a radius roughly equivalent to the wall thickness be allowed on all external Edges & Corners.
OFC also recommends that dimensions for Corner & Edge radii have an R-Max tolerance applied. An R-Max tolerance provides flexibility to the pattern mold designer to strategically omit the Edge Radii along parting planes in the mold. Omitting an edge Radii allows the parting plane in the pattern mold be located at the top of features which simplifies the design of the mold and makes it less expensive to design and build. Cosmetically, in this manner, the parting lines on the casting will blend with the “Sharp” edge. (Edges along non-parting surfaces in the pattern mold will be cut with the full radii.) An R-Max allows for a Sharp Edge at parting planes that will reduce the cost of the Pattern Mold and also improve the cosmetic appearance of a casting.
If for the function of the casting it is necessary for edges and corners have a radius this can certainly be accommodated. A design includes a ± tolerance (rather than an R-Max) for the Edge Radii the parting planes then need to be placed adjacent to the root of the Edge Radius rather than at the top of features. This requirement will increase both the design and construction time of a pattern mold and so also its expense. Also a casting produced from a tool cut with an edge radius at the parting surfaces will exhibit a residual parting line adjacent the root of the edge radius and that will likely grow heavier as the pattern mold wears.
Edge and Corner radii are an important consideration for casting design. Even when tooled sharp, castings will display a slight radius that should be noted on a drawing as .015 R Max. Foundries prefer that casting designs permit an Edge Radius equivalent to the wall thickness and recommend that they be designated with an R-Max tolerance so that the radius may be omitted from parting surfaces in the pattern mold.
Investment Castings will naturally exhibit a .008” - .012” radius along an edge even when tooled “sharp”. I often say that foundries “don’t cast razor blades” (but then even razor blades are not truly “sharp” either). The freezing of the wave front and back pressure from air in the cavity prevent the liquid metal from achieving a truly “sharp” edge. Casting Designers should provide for this condition on their designs with an allowance of at least a .015” R Max along cast edges & corners.
Foundries, however, generally prefer not to have “sharp” edges and corners on cast parts. In Investment Casting a sharp edge can create a weak point in the ceramic shell mold. (If you’ve ever tried to paint a sharp corner you’ll understand that it can be very difficult to build thickness along a sharp edge.) A vulnerable “thin” area in the ceramic shell that may cause the shell to crack from the stress of the De-Wax operation or fail as the mass of the hot metal is being cast. O’Fallon Casting recommends that a radius roughly equivalent to the wall thickness be allowed on all external Edges & Corners.
OFC also recommends that dimensions for Corner & Edge radii have an R-Max tolerance applied. An R-Max tolerance provides flexibility to the pattern mold designer to strategically omit the Edge Radii along parting planes in the mold. Omitting an edge Radii allows the parting plane in the pattern mold be located at the top of features which simplifies the design of the mold and makes it less expensive to design and build. Cosmetically, in this manner, the parting lines on the casting will blend with the “Sharp” edge. (Edges along non-parting surfaces in the pattern mold will be cut with the full radii.) An R-Max allows for a Sharp Edge at parting planes that will reduce the cost of the Pattern Mold and also improve the cosmetic appearance of a casting.
If for the function of the casting it is necessary for edges and corners have a radius this can certainly be accommodated. A design includes a ± tolerance (rather than an R-Max) for the Edge Radii the parting planes then need to be placed adjacent to the root of the Edge Radius rather than at the top of features. This requirement will increase both the design and construction time of a pattern mold and so also its expense. Also a casting produced from a tool cut with an edge radius at the parting surfaces will exhibit a residual parting line adjacent the root of the edge radius and that will likely grow heavier as the pattern mold wears.
Edge and Corner radii are an important consideration for casting design. Even when tooled sharp, castings will display a slight radius that should be noted on a drawing as .015 R Max. Foundries prefer that casting designs permit an Edge Radius equivalent to the wall thickness and recommend that they be designated with an R-Max tolerance so that the radius may be omitted from parting surfaces in the pattern mold.
Thursday, 8 October 2015
Why Castings need a Fillet Radii
By definition a Fillet Radii is a rounding of an interior corner and are employed on castings to increase their load bearing strength and to improve both manufacturability and quality. For those reasons a fillet radius should be a standard allowance on every casting design.
A Fillet Radius makes a structure stronger because it redirects stresses from being concentrated at a sharp interior corner and distributes them over the broader volume of the fillet. The effect of the Fillet Radius is to mitigate a potential weak point of cast structure making transitions within a structure, especially between right angled walls, stronger.
Because of this faculty to mitigate stress concentrations a Fillet Radius also improves the manufacturability of a casting. Castings often require a mechanical “Straightening” operation to restore flatness, perpendicularity and parallelism to the part and a filleted corner is less prone to cracking during this operation. Better casting manufacturability provides a foundry with control of its costs and promotes more consistent deliveries.
The foundry industry works diligently to avoid turbulence in the flow of hot metal as their products are being cast. Turbulence can cause a mixing of the metal with air to trap gas or lead to the formation of oxides that may become embedded in the casting. Fillet radii help to minimize the turbulence in the metal as it flows through the cavities of the mold thus inhibiting the formation defects from oxides and trapped gas and so contributes to the better quality of a cast product.
In the Investment Casting process a thickness of ceramic shell is formed about a pattern assembly by repeated dips of the assembly into slurry. Sharp internal corners in a part configuration can provide nucleation sites onto which bubbles of air form and adhere to the pattern while the slurry is being applied. Air bubbles on the pattern will become small voids in the ceramic shell that will subsequently fill with metal and become positives on the surface of the casting. A fillet radius improves the ability for the pattern to shed air bubbles as the slurry is applied thus reducing the propensity of casting to exhibit positive metal.
A general “Rule of Thumb” for an aluminum investment casting is to apply a fillet radius equal to 1x to 1.5x of the wall thickness with a recommended minimum of .06”R. It is also advisable to specify fillet radii with a Max tolerance.
Customers of O’Fallon Casting are provided a free Concurrent Engineering service to assist in their design and improve their use of castings. If you have questions regarding Fillet Radii or other investment casting design considerations call your O’Fallon Casting Sales Engineer for assistance.
A Fillet Radius makes a structure stronger because it redirects stresses from being concentrated at a sharp interior corner and distributes them over the broader volume of the fillet. The effect of the Fillet Radius is to mitigate a potential weak point of cast structure making transitions within a structure, especially between right angled walls, stronger.
Because of this faculty to mitigate stress concentrations a Fillet Radius also improves the manufacturability of a casting. Castings often require a mechanical “Straightening” operation to restore flatness, perpendicularity and parallelism to the part and a filleted corner is less prone to cracking during this operation. Better casting manufacturability provides a foundry with control of its costs and promotes more consistent deliveries.
The foundry industry works diligently to avoid turbulence in the flow of hot metal as their products are being cast. Turbulence can cause a mixing of the metal with air to trap gas or lead to the formation of oxides that may become embedded in the casting. Fillet radii help to minimize the turbulence in the metal as it flows through the cavities of the mold thus inhibiting the formation defects from oxides and trapped gas and so contributes to the better quality of a cast product.
In the Investment Casting process a thickness of ceramic shell is formed about a pattern assembly by repeated dips of the assembly into slurry. Sharp internal corners in a part configuration can provide nucleation sites onto which bubbles of air form and adhere to the pattern while the slurry is being applied. Air bubbles on the pattern will become small voids in the ceramic shell that will subsequently fill with metal and become positives on the surface of the casting. A fillet radius improves the ability for the pattern to shed air bubbles as the slurry is applied thus reducing the propensity of casting to exhibit positive metal.
A general “Rule of Thumb” for an aluminum investment casting is to apply a fillet radius equal to 1x to 1.5x of the wall thickness with a recommended minimum of .06”R. It is also advisable to specify fillet radii with a Max tolerance.
Customers of O’Fallon Casting are provided a free Concurrent Engineering service to assist in their design and improve their use of castings. If you have questions regarding Fillet Radii or other investment casting design considerations call your O’Fallon Casting Sales Engineer for assistance.
Saturday, 3 October 2015
How an Investment Casting is Manufactured
Investment Casting is a cost effective method for the production of precise, near-net-shape and complex metal parts. Although Investment Casting is relatively ancient in origin it is today a highly sophisticated manufacturing process.
By definition Investment Casting is a foundry process by which a metal casting is produced from a ceramic mold that was formed by a disposable pattern. Much as Sand Castings are produced from sand molds and Die Castings from metal dies, the Investment Casting process name originates from the ceramic “Investment” in which parts are cast.
There are eight basic steps for the manufacture of an Investment Casting:
By definition Investment Casting is a foundry process by which a metal casting is produced from a ceramic mold that was formed by a disposable pattern. Much as Sand Castings are produced from sand molds and Die Castings from metal dies, the Investment Casting process name originates from the ceramic “Investment” in which parts are cast.
There are eight basic steps for the manufacture of an Investment Casting:
- The first step of the process is the production of a Disposable Pattern. Generally Investment Casting patterns are made of Wax and produced by an aluminum injection mold. However, Investment Casting patterns might alternatively be produced from a rubber or soft metal molds or formed by an Additive Manufacturing process such as SLA, SLS, or Voxeljet.
- Once the wax pattern has been formed it is attached to a Runner System. The runner system is also comprised of wax that is cut to size from injected or extruded stock and is “wax-welded” together. A complete and “Assembled” part & runner system is referred to as a “Sprue” or “Tree” and may be comprised of many multiple Wax Patterns.
- The Assembled wax Sprue is then ready to be “Invested” or coated with ceramic. The most prevalent method to “Invest” a wax sprue is the Ceramic Shell Process. In the Ceramic Shell Process a sprue will dipped multiple times into vats of specialized slurry and refractory sand to form a Shell. The shell is allowed dry time after each layer is applied and once a sufficient layers of ceramic have been applied to the sprue the completed shell is allowed to dry fully.
- Once the shell is dry a DeWax operation is performed, generally with a steam autoclave, to melt and eliminate the wax from the sprue.
- With the wax removed the shell is Fired in an oven to create a crystalline ceramic structure that will withstand the weight and thermal shock of the metal to be cast into it.
- Once the shell has been fired molten metal is then Cast to fill the cavities in the sprue that had been formed by the wax pattern & runner system.
- After the metal has solidified the ceramic is removed from the Sprue with high pressure water or some other form of knockout system.
- With the ceramic removed the metal parts are cut–off from their runner system with a saw or other cutting method such as a laser.
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