Stephanie O Connor ATI Wah Chang, an Allegheny Technologies Company Albany, OR

Transcription

1 Stephanie O Connor ATI Wah Chang, an Allegheny Technologies Company Albany, OR INTRODUCTION Design engineers are faced with multiple considerations when it comes to component design and fabrication for military applications, including ease of assembly, weight reduction, structural integrity, corrosion resistance, long term maintenance costs, and overall affordability. These considerations often conflict as design engineers begin to weigh the costs and benefits of each option. In the quest to achieve the best cost/benefit scenario, older technologies can get overlooked in favor of newer technologies. A prime example of an older technology that can meet today s needs is titanium rammed graphite castings for military applications. HISTORY Titanium rammed graphite castings have been used for commercial and military applications since the late 1950s. Originally, the cast titanium parts were desired because of their superior performance in severe service applications like seawater, but design engineers quickly discovered that other benefits could also be realized by utilizing cast parts. A primary benefit was the ability to produce near net shapes that required less material, less machining, and reduced scrap. Titanium s weight advantage compared to steel also became an advantageous characteristic. Over the last 50 years, titanium rammed graphite castings have been used for multiple military applications, such as fire pumps, water pumps, condenser head covers, and hose-to-hose pipe connectors on surface ships, torpedo ejection pumps, large seawater pumps, various sizes of ball, gate and butterfly valves, and other defense components. Figures 1 and 2 show examples of four different titanium components made using the rammed graphite casting process. THE PROCESS Patterns and Molds Titanium rammed graphite castings are made using wood, metal or plastic patterns to produce a mold, as illustrated in Figure 3. Similar to conventional sand castings (see sidebar), rammed graphite castings use the standard cope and drag patterns, with and without cores. Many parts can be cast using the same patterns originally constructed for the casting of other metals. Standard loose or match-plate patterns made of either wood or metal can be used for titanium rammed graphite castings. Standard core boxes can also be used unless they are designed to be used for core blowing only. Most patterns for ferrous and nickel-based alloys will conform dimensionally. Pattern shops can accommodate modifications or new construction. Generally, pattern equipment designed for sand casting processes can also be utilized with modifications to gating and riser systems. Titanium is very reactive in the molten state, and therefore graphite is used as a mold medium. Graphite powder is mixed with water, pitch syrup, and starch, which act as binders. This mixture is pneumatically tamped and rammed around the pattern to form the mold. Figure 1. Cast Titanium Pump Casing (Left) and a Cast Titanium Seal Head (Right). The AMMTIAC Quarterly, Volume 2, Number 1 3

2 Figure 2. Cast Titanium Enclosed Impeller (Left) and Cast Titanium Valve Body & Bonnet (Right). The mold is air dried for approximately 24 hours to achieve a green (uncured) strength before it is baked at a low temperature to prevent steam generation and cracking during the firing step. The amount of baking time depends on the thickness and shape of the mold. Molds are fired in excess of 1500 F to develop the final bonds and to burn off binders, resulting in a hard and rigid final product. The molds are then cleaned and assembled, and gates and risers are cut into the molds (Figure 4). These must be carefully designed to allow the proper flow of molten material during the casting process and to ensure that shrinkage takes place in the gates and risers rather than in the finished casting. The maximum size of a titanium pour is approximately 1800 pounds, yielding an end cast part that weighs about 1300 pounds; however, larger castings have been produced by utilizing multiple pours and molds. The maximum single pour has been 50 inches in diameter by 72 inches in height. Wall thicknesses as thin as inches have been produced using the rammed graphite method. Melting and Casting Titanium rammed graphite castings are currently melted using a vacuum arc skull furnace. This method uses consumable electrodes that are melted into a water-cooled copper crucible; electrodes are either forged billet, consolidated revert, or a combination of the two. The mold assembly is placed on a table in the bottom of the furnace where the titanium can be centrifugally or statically cast depending upon the geometry and specifications of the casting. Extreme care must be used during the melting process since titanium can easily be contaminated by oxygen, hydrogen and nitrogen. The furnace is sealed, a vacuum is drawn and an arc is struck on revert material placed in the crucible, and the titanium is then melted. When the proper amount of material is melted, the crucible is tipped and the molten material is poured into the mold. The mold assembly is left in the furnace under vacuum until the metal has cooled to the proper temperature. After the mold assembly is taken out of the furnace, the graphite is removed by traditional knockout methods, and the gates and risers are cut off by an oxyacetylene torch. The metal surfaces in contact with the mold will have a carbon-contaminated layer, which must be removed by blasting and/or pickling. The pickling solution is generally a mixed acid solution of 15 to 30 percent nitric acid and 3 to 5 percent hydrofluoric acid, with the balance being water. Finishing and Testing Once the knockout has been completed, the casting goes through nondestructive testing (NDT) and inspection to determine any necessary finishing processes for the casting. Depending on the product and customer specification, castings may be inspected using visual, liquid penetrant, and/or radiography methods; castings may be checked for dimensional precision as well. Common Figure 3. Wood Pattern Used to Produce Rammed Graphite Molds. Figure 4. Fired Molds Waiting to Have Gates and Risers Cut Before the Pour. 4 The AMMTIAC Quarterly, Volume 2, Number 1

3 finishing processes include sandblasting, grinding (Figure 5) and hot isostatic pressing (HIP). Chemical composition and mechanical properties can also be evaluated. During the HIP process, the castings are placed in a vacuum chamber, the temperature is raised to 1500 F and a pressure of about 16,000 psi is applied. The voids are healed by pressing of the material. After the HIP process, penetrant inspection is usually performed. Defects found through penetrant inspection include porosity and voids on the surface of the cast part. If any of these defects are found, the casting is weld repaired. Radiographs of castings can also be performed to detect if there are any possible graphite inclusions that were not healed during the HIP process. Such inclusions are very rare. The combination of HIP and penetrant inspection has greatly reduced the need for radiography. Eliminating or greatly reducing the use of radiography can cut the cost and lead time for a cast part significantly and, therefore, should only be considered for the most critical applications or on a sample basis. THE ADVANTAGES OF RAMMED GRAPHITE CASTINGS Titanium rammed graphite castings have become a viable alternative to the conventional fabrication methods of titanium plates and components. The labor intensive fabrication methods that previously relied on cutting, machining, and fitting plates to be welded are being replaced by simpler near net shape castings. Titanium foundries have successfully replaced welded fabrications on many occasions. This provides engineers with less costly near net shaped castings that have a lower chance for error because there are generally fewer welds on a casting than on cut, fitted plates. The fact that a rammed graphite casting is repeatable is also very desirable to contractors and end users. The low upfront cost of patterns is desirable and affordable when compared to the tooling required for investment castings. Titanium castings use a pattern that is dimensionally correct each and every time, based upon the end users drawing requirements. The casting will maintain the dimensions that are designed into the pattern, so the larger envelopes that often accompany forgings, billet, and plate stock will be eliminated by utilizing casting technologies. A near net shape can eliminate extra hours of machining and milling as well as solve the problem of handling the extra chips and turnings that are generated by machining large amounts of titanium. Rammed graphite castings often provide the advantage of being more readily available than fabricated or machined components. The current titanium market is experiencing unprecedented lead times for mill products such as plate, billet, and forgings. It is not unusual to wait 30 weeks for the arrival of the raw materials before any fabricating or machining can take place. Rammed graphite casting lead times can be significantly shorter because the melt stock raw materials are more readily available than mill products,* and processing lead times for rammed graphite castings are much shorter than that for plate, billet, and forgings. Generally, a prototype casting can be available within a 15- to 20-week lead time, which also includes the production of the patterns. More complex parts may require longer lead times. CASTINGS FOR THE MILITARY Titanium rammed graphite castings have been used in the military for many years, but only recently companies that design tanks and fighting vehicles for the US military have been interested in these products. The lightweight and durable characteristics associated with titanium have caused designers to consider titanium rammed graphite castings for a growing number of applications on military vehicles. Suspension system components have been reviewed and prototype castings have been produced for a number of companies with very good Figure 5. Flash Being Ground Off of a Casting; One of the Many results. The combination of being Finishing Techniques Used. lightweight, having good fracture toughness qualities, and the ability to produce near net shapes have made some engineers sit up and take notice. Idler wheels, idler arms, sprocket carriers, and other components for suspension systems are ideal candidates for titanium rammed graphite castings. Titanium rammed graphite castings are cast from any number of grades of titanium. Grades 2 and 3 are commercially pure and are the most requested grades for chemical processing applications. In applications where corrosion is more severe, grades 7 and 12 are often chosen. Grade 5 (Ti-6Al-4V) is a popular alloy for applications where strength and ballistic resistance properties are an issue. Recently accepted by the ASTM as Grade 38, Ti-4Al-2.5V currently is being considered for military applications. Grade 38 has proven to be very castable and much easier to pour than Grade 5, which historically has been used for military applications. In their short existence, Grade 38 titanium castings have found their way into a number of military applications and in some cases have replaced Grade 5 castings. The advantage in specifying Grade 38 titanium castings is cost savings. While the properties between Grade 5 and Grade 38 are similar, Grade 38 has a slight pricing advantage over Grade 5 because the raw material cost is lower. There is also an added corrosion benefit with Grade 38 titanium over Grade 5. To date, Grade 38 titanium has been selected for suspension system applications, cast hatch covers, and other components for fighting vehicles. Ballistic tests on cast Grade 38 materials are currently underway to determine its level of threat resistance capabilities. Table 1 shows the chemical compositions of the titanium alloys commonly used in rammed graphite castings. Rammed graphite castings play a very important role in the casting marketplace by providing a technically sound near net shaped cast part to the end customer. By selecting and specifying titanium rammed graphite castings for military and defense applications, near net shape technology can be utilized, eliminating unnecessary scrap accumulation and the machining of oversized sections. The AMMTIAC Quarterly, Volume 2, Number 1 5

4 Table 1. Chemical Compositions of Various Grades of Titanium. Chemical Composition, max wt.% ASTM Designation Ti (min) C H O N Fe Others Grade Grade Grade 5 Remainder Al, 4.50 V Grade Pd Grade 12 Remainder Mo, Ni Grade 38 Remainder Al, 2.5 V CHARACTERISTICS OF CAST TITANIUM In Corrosive Environments The majority of titanium alloys (wrought or cast) are very resistant to corrosive attack and virtually immune to many oxidizing and reducing environments. This is due primarily to a tenacious oxide film that is formed when titanium is exposed to the atmosphere. The oxide film acts as a barrier to the surrounding corrosive environment and thereby protects the titanium alloy from further oxidation and corrosion. By combining titanium with small amounts of palladium or molybdenum and nickel, the corrosion resistant properties of the titanium alloy can be improved further. Strength and Structural Integrity Unlike aluminum and steel alloys, which tend to lose structural integrity and strength when cast, titanium tends to maintain structural integrity and strength that is comparable to wrought titanium products. Table 2 illustrates this point by comparing tensile and yield minimums for wrought and cast titanium alloys commonly used in chemical processing and defense applications. Dimensional Control of Titanium Rammed Graphite Castings Rammed graphite castings made with titanium alloys are either static or centrifugally cast, depending on the shape and size of the cast part. Common tolerances for titanium rammed graphite castings are listed below; tighter tolerances can be achieved with additional trials and correction efforts: Minimum Section Thickness: 3/16", 1/8" if less than one square inch of surface Base Tolerance: Up to one linear inch: ± 1/32" 1 inch up to 10 inches: ± 1/16" 10 inches up to 20 inches: ± 1/8" 20 inches up to 60 inches: ± 3/16" Additional Tolerances: For dimensions across the parting line: ± 1/8" Added tolerance for dimensions affected by parting line and parallel to the parting line (mismatch): Less than 10 inches: ± 1/16" 10 inches and above: ± 1/8" Radius Dimensions: General radius conforms to base dimensions Sharp to 1/8" radius ± 1/32" (convex) Up to 1/2" fillet radius ± 1/32" (concave) Finish Stock: Nominal 1/8" on all machined surfaces on dimensions less than 12" Nominal 3/16" 1/4" on all machined surfaces greater than 12" Angles: ± 1 degree The Basics of Metal Casting Benjamin D. Craig AMMTIAC Rome, NY Casting is a manufacturing process that has been in use for several millennia; cast metals, for example, have been dated back to 4000 B.C. There have been advancements in the process over the years, but the basic principles have remained. A molten or liquid material (either a metal, polymer, or ceramic) is poured into a mold, where it conforms to the shape of the cavity, and then is solidified to create a finished or nearly finished product. The process is often used to efficiently form complex parts and is typically a cheaper route compared to the alternative fabrication methods, such as forming or machining from a solid. The size of the castings can range from hand held pieces (and smaller) to multi-ton items such as ship propellers. A pattern is the three dimensional model that is designed and used to create a mold cavity, which will ultimately form the final casting product. Patterns can be reusable or expendable. The reusable patterns are typically made from wood, metal, plastic, or a composite material and can be used to make duplicate molds. Expendable patterns, on the other hand, are made of materials such as wax, polystyrene, or other plastics and are disposed of after making the mold. In the case of the lost Styrofoam method, the pattern is vaporized during casting process.* Traditionally, the molds for casting metals have been made with prepared sand (called green sand casting because it is unfired), although other materials can be used. Typically these mold materials include a binding material (clay) and a small percentage of water so that after molding the sand retains the exact inverse of the pattern shape. The prepared sand retains an imprint of the pattern to create the casting cavity that will eventually host the molten metal. A common practice of casting metals is to create the mold in two parts. The top part of a mold is called the cope and the bottom part is called the drag. These two components are separated at what is referred to as a parting line. Molds are held in place and supported during the casting process using a frame called a flask, which is typically made of wood or metal. A core is a separate solid material within the mold cavity that is used to make an internal void or passageway in the final product. The pattern is modified to allow for the positioning and retention of the core in an exact location. Figure 1 illustrates some of the common components of a mold. * Styrofoam is a registered trademark of the Dow Chemical Company. 6 The AMMTIAC Quarterly, Volume 2, Number 1

5 Table 2. Comparison of Strength Specifications for Cast and Wrought Forms of Various Grades of Titanium [1], [2]. Titanium Grade Tensile Strength, min. Yield Strength 0.2% Offset, min. Ksi MPa Ksi MPa Cast Grade Wrought Grade Cast Grade Wrought Grade Cast Grade Wrought Grade Cast Grade Wrought Grade Size and Weights While the size and weight of titanium rammed graphite castings can vary depending on the manufacturer, 1 to 1300 pounds is a common weight range for cast titanium alloys. Larger castings can be produced by fabricating several castings into a single component. In certain configurations, casting weights in excess of 1300 pounds are possible but they should be evaluated on a case by case basis. CONCLUSION While titanium rammed graphite castings are not new to the military, they are often overlooked as an alternative to traditional manufacturing processes and wrought products utilized in the production of defense components. Today, design engineers can realize many military objectives by utilizing titanium rammed graphite castings to ease the burden of assembly, reduce component weight, maintain structural integrity, increase corrosion resistance, cut back lead times and reduce long term maintenance costs. While titanium rammed graphite castings may not be the newest technology available, it certainly could be the best choice especially when weighed against the military objectives of today and tomorrow. NOTE AND REFERENCES * Melt stock raw materials are more readily available than mill products because mill products must be manufactured from the ground up, whereas melt stock raw materials can be taken from mill products that have already been produced and are in the inventory. [1] ASTM Designation B , Standard Specification for Titanium and Titanium Alloy Castings, ASTM. [2] ASTM Designation B b, Standard Specification for Titanium and Titanium Alloy Strip, Sheet, and Plate, ASTM. GENERAL REFERENCES M. Guclu, I. Ucok, and J.R. Pickens, Effect of Oxygen Content on Properties of Cast Alloy Ti-6Al-4V, Symposium on Cost-Affordable Titanium, Charlotte, NC, March 2004, The Minerals, Metals & Materials Society, Inc., pp , DTIC Doc.: AD-P D. Eylon, F.H. Froes, and L. Levin, Effect of Hot Isostatic Pressing and Heat Treatment on Fatigue Properties of Ti-6Al-4V Castings, Proceedings of the Fifth International Conference on Titanium, 1985, pp , DTIC Doc.: AD-D R.D. Williams and J. Dippel, Comparisons of Titanium Investment and Rammed Graphite Castings, Titanium Net Shape Technologies, The Metallurgical Society of AIME, 1984, pp , DTIC Doc.: AD-D More Titanium Rammed Graphite Castings reports available through Private STINET and TEMS. Qualified users can access these databases through the DDR&E Research Portal ( The mold cavity in the cope and drag can be formed using different types of patterns, such as a standard loose pattern or a match plate pattern. The match plate has a pattern mounted on both sides of the pattern plate and conforms to the parting line between the cope and the drag parts of the mold. The match plate system typically uses an interlocking feature that surrounds the exterior edge of the pattern. This ensures a very accurate registration of the mold halves and restricts movement of the mold during solidification of the metal. During the casting process, shrinkage of the casting material can occur during solidification. To account for this, risers are used which act as a molten metal reservoir to fill any voids created when the casting shrinks inside the mold cavity. Risers that do not have an outlet on the topside of a cope are called blind risers. Vents are used to allow for gases to escape in order to reduce the occurrence of voids in the casting. A sprue is the vertical channel in the mold that connects the pouring basin to the gating system. Some molds do not have a pouring basin, in which case the molten metal is poured directly into the sprue. The gating system is a series of passageways that distributes the molten metal between the sprue and the casting cavity. Design of a proper gating system is important because it allows the liquid to flow and enter the casting cavity properly in order to avoid defects in the final casting. The use of so called green (unfired) or fused sand allows for the easy removal of the sand from the solidified material. Additionally, the sand can be reprocessed, and when clay and water are added it can be used for the next series of castings. Parting Line Flask Sprue Pouring Basin Gating System Blind Riser Compacted Green Sand Vent Hole Figure 1. Components in a Green Sand Casting Mold. Cope Core Mold Cavity Drag The AMMTIAC Quarterly, Volume 2, Number 1 7