DEVELOPMENT AND USE OF A NEW COMPOSITE MATERIAL FOR ALUMINIUM CONTACT APPLICATIONS

Transcription

1 Development and use of a new composite material for aluminium contact applications Edited by TMS (The Minerals, Metals & Materials Society), 2004 DEVELOPMENT AND USE OF A NEW COMPOSITE MATERIAL FOR ALUMINIUM CONTACT APPLICATIONS Sylvain P. Tremblay, Pyrotek High-Temperature Industrial Products Inc., Chicoutimi (Québec), Canada Mark Vincent, Pyrotek Engineering Materials Ltd, Garamond Drive, Wymbush, Milton Keynes, Buks MK8 8LN, United Kingdom Abstract A new composite material consisting mainly of fiberglass fabric embedded in a calcium silicate CaSiO 3 slurry will be described. Its properties as well as its behavior in contact with molten aluminium will also be presented. Many plant case studies using this composite material, known as RFM will be detailed. Using its outstanding properties, RFM has improved molten metal quality in many applications. Applications such as control pin, AutoPour ladle, continuous rod casting launder and ReMAD will be detailed. Actual plant results in terms of metal quality improvement, leading to a lower cost/ton usage, will be shown in these applications. Introduction Molten aluminium is a very aggressive material, and especially high magnesium alloys (Mg > 3%) when in contact with other materials. There are a lot of good refractory materials available on the market. However, these materials are quite dense and fragile by nature. Pyrotek has developed a new composite material that is chemically inert from molten aluminium attack. It is mechanically stronger than a typical refractory castable due to the incorporation of fiberglass fabric into the matrix. This material called RFM, an acronym for Reinforced Fiberglass Material, is suitable for complex shapes and thin wall parts. In many aluminium contact applications, due to the limited properties of existing refractories, it is difficult to solve engineering and design problems. The use of RFM has opened a new avenue in component designs that come in contact with molten aluminium. This paper will present some of the most significant RFM improvements over materials currently in use. These improvements have led to better metal quality and to lower costs/ton. Four (4) different applications will be described in detail: ReMAD (Reusable Molten Aluminum Distributor) AutoPour ladle Continuous rod casting launder Control pin Direct comparison with the actual material, cost savings and metal quality improvement for each application will also be addressed. RFM Material Description RFM was developed in response to a desire for a new refractory material having the following properties: Light, thin, strong and crack resistant material Low mass and thermal conductivity Excellent aluminium chemical resistance No preheating required Suitable for complex shapes Easy to clean To meet the required properties detailed above a special slurry based on calcium silicate (CaSiO 3 ) had to be developed. The slurry has special non-wetting agents designed for molten aluminium contact. The slurry was engineered with coupling agents to enhance an intimate contact between the refractory slurry and the fiberglass fabric reinforcing material. The fiberglass fabrics used in the construction process depend on the final product requirements. The fabrics can vary from a very open matrix to very closed. The number of strands in the fabric construction can also be varied to increase the final material strength. Finally, the number of fabric layers embedded within the slurry can also be varied. The fiberglass fabric prevents crack propagation when the material is under mechanical and thermal loads. The RFM material can be produced with a thickness range between 2 mm to 25 mm, but typically the thickness is around 5 mm to 10 mm. The typical properties of RFM are shown in Table I. The RFM material can be produced in flat sheets or molded into complex parts. It is a flexible and versatile material. The following plant examples will illustrate the unique nature and properties of the RFM material. Table I: Typical Properties of RFM Material RFM refractory material Density 1600 Kg/m 3 M.O.R MPa Maximum Service Temperature 1100 C Thermal 500 C 0.43 W/(m 0K) Thermal Expansion (mm/mm/ C) x 10E The RFM material and related products manufactured from RFM are covered by various patents and patent applications in major aluminium producing countries. 1

2 1. ReMAD Case Study The ReMAD was presented at TMS 2002 [1]. ReMAD is an acronym for Reusable Molten Aluminium Distributor. The ReMAD is a molten aluminium distributor system comprised of a refractory frame coupled with an inner fiberglass bag as illustrated in Figure 1. The inner bag is rigidified using a special heated mold to the exact internal dimensions of the RFM frame. The RFM frame has excellent non-wetting properties, further enhanced by the use of the special boron nitride coating. The boron nitride coating ensures the ReMAD is easy to clean and deskull. Minor cracks usually appear in the ReMAD after the first cast, but these cracks do not propagate with time Depending on alloys cast and casting conditions, it has been possible to get an RFM frame life of up to 30 casts. Figure 2 shows the RFM frame after a cast while Figure 3 illustrates the RFM frame after 25 casts. Figure 1. Inner bag & RFM rigid frame The ReMAD distributor was developed to distribute molten aluminium inside sheet ingot molds. This is in conjunction with the major mold ingot casting technologies. Typically, the RFM wall thickness used within the ReMAD application is 6 mm. The desired properties of a reusable frame material are as follows: Figure 2. RFM frame after a typical cast Light, thin & mechanically strong Aluminium resistant No preheating required Thermal shock resistant To be used with the current bag set-up Easy to clean The list of desired properties above have been met by the RFM material. Without discussing the RFM frame design in detail, it should be noted that a substantial amount of the fine-tuning design was achieved by the use of Pyrotek's water modeling capability. This paper will concentrate on the performance of the RFM frame. The ReMAD has been tested in excess of five different plants casting sheet ingot. In order to ease the cleaning of the frame at the end of the cast, all surfaces were coated with a special boron nitride coating. Depending on the metal flow and alloy type, the coating can last between 15 and 20 drops before reapplication. Typically, the casting duration is approximately 2 hours for double length ingots. The ReMAD was not preheated prior to casting. The ReMAD was installed under the spout/pin system and the metal flowed directly into the RFM frame. Different alloys such as AA-3003, AA-3004, AA-5182 and AA-7000 have been cast using the RFM frame. Some of these alloys such as the AA-5182 and the AA-7000 series are very chemically aggressive. Figure 3. RFM frame after 25 casts In conclusion, the RFM material demonstrated the following characteristics: It is thin and mechanically strong It is thermal shock resistant It does not require preheating It has been used up to 30 casts It is not wetted by molten aluminium alloys It is easy to clean 2. AutoPour Ladle Case Study The AutoPour ladle is typically employed within an aluminium foundry. Its purpose is to transport an exact quantity of liquid aluminium from a holding furnace to the point of casting. Typically the processes served are gravity-feed or high-pressure die-casting. 2

3 The process is demanding on the AutoPour ladle, as it has to operate under a number of dynamic and fixed parameters. Table II describes the operating parameters and their technical demands. Figure 4shows an RFM AutoPour ladle collecting its molten aluminium payload. It is equipped with an embedded peripheral support ring and robot location and fixing point. Table II. AutoPour ladle operating parameters No. Parameter Technical Demand 1 Thermal Shock Tolerant of rapid temperature increase 2 Thermal Tolerant of temperature Cycling fluctuation 3 Torque / Tolerant of stress loading during Tension rotation 4 Mechanical Impact Tolerant of mechanical impact 5 None Wetting Resistance to molten aluminum 6 Thermal Mass Minimal cooling effect of molten metal 7 Contamination Inert Traditional Technology Employed Traditionally cast iron has been used for this application as it meets most of the operating parameters required. However, cast iron does not fully fulfill the requirements of numbers, 5, 6 and 7 of Table II. Cast iron is wetted by liquid aluminium and as such needs to be coated on regular intervals. Cast iron has a much higher density than RFM at ~7000 Kg/m 3 versus 1600 Kg/m 3 for RFM. This higher density has a direct chilling effect on the collected and static metal inside the ladle. Because cast iron is wetted by molten aluminum, it is also inevitable that iron contamination will occur due to iron erosion during service. RFM AutoPour Ladle Properties Thermal Shock and Cycling Performance Because RFM is a composite body, constructed of CaSiO 3 slurry embedding interwoven layers of fiberglass fabric, the resultant composite structure provides excellent thermal shock and cycling properties. These are essential properties as the operational life of an AutoPour ladle places such demands on the material body over long operational time periods. Torque / Tension AutoPour ladles are usually mounted onto an automated collection and delivery system such as a robot arm. During a complete collection, travel and delivery cycle, the AutoPour ladle is subjected to a number of axis rotations around a fixed location point. The fixed location point is usually singular, but on larger ladles can be multiple. To accommodate the operational stresses of this particular discipline the RFM AutoPour ladle has been engineered to accept a steel peripheral support ring and attachment point. The steel support ring is built into the composite body during the fabrication process. Special processes enable the RFM and steel support ring to operate in concert. Utilizing this system is advantageous as it offers a high degree of security and support to the RFM body and aluminium payload. The technique also allows the traditional cast iron fixing position and design to be retained. This is a key feature as it allows straightforward integration of the RFM AutoPour ladle into the current working technology. Figure 4. RFM AutoPour ladle in action Mechanical Impact It is an operational fact of life that an AutoPour ladle will from time-to-time be subjected to some form of mechanical abuse. This can be from impacting a misplaced mold, a programming error of the robot, or human handling. RFM AutoPour ladles are well equipped to tolerate mild mechanical abuse. The composite nature of the body allows impact to be absorbed without catastrophic failure of the ladle. Monolithic refractory ladles often fail catastrophically due to mechanical impact because there is no built-in mechanism to prevent crack propagation. The RFM body, being built of multiple layers of interconnected and fiberglass, is able to absorb such impacts. This ability within a thin wall section of typically 12 mm offers a safe operational product. Non-Wetting The ability to resist molten aluminium reaction is advantageous in many applications and especially in a semiautomated process where the AutoPour ladle is employed. Having to continually stop the process to pull adhered aluminium skull from the ladle is time consuming and costly. Failure to adequately keep the ladle free of skull will result in a large build-up of aluminium and oxide deposits. This can lead to operational problems and product quality issues. Traditional materials such as cast iron need constant supervision and off-line maintenance as the process takes its toll on the material. However, RFM AutoPour ladles are non-wetting and require little maintenance to keep them in excellent operational condition. It is not necessary to remove the ladle for maintenance procedures as a light coating of a release material, such as boron nitride, can be applied online typically every fourth day of operation. Thermal Mass Having the ability to transfer and pour liquid aluminium very close to the holding furnace temperature is advantageous. However, there are a number of operational situations that lead to higher or lower levels of temperature loss. These include: AutoPour ladle material AutoPour ladle mass versus aluminium payload Distance to travel from charging point to pouring point Dwell time while the ladle is full of aluminium 3

4 Product freeze time in the mold / die A material having a density of 7000 Kg/m 3 will chill the retained metal faster than one with a density of 1600 Kg/m 3. RFM AutoPour ladles have a much lower chilling effect on the aluminium payload. Because of this, holding furnace temperatures can be reduced. The degree of reduction depends on many of the items listed above, but typically reductions between 5 C and 15 C are realized. Contamination The RFM AutoPour ladle is non-wetted by liquid aluminium. This has a direct impact on the purity of the metal cast. If the body of the ladle is reacting and eroding in contact with liquid aluminium, then impurities will be included in the cast body. This is not the case with RFM AutoPour ladles, thus leading to higher quality cast parts. Figure 5. Schematic of the Properzi casting process [2]. Conclusions AutoPour ladles: Dramatically reduce the amount of maintenance required to keep them in top operational condition when compared to cast iron ladles. Allow the reduction of holding furnace temperatures. Reduce the amount of scrap produced due to lower metal temperature and reduced time to solidification of the final part. Improve the quality of the finished product. Reduce loading on automated systems such as robot arms due to a much lower ladle weight when compared to cast iron. Have little to no skull sticking, thus improving cycle times. Have an excellent resistance to thermal shock and cycling. Have a good tolerance of mechanical impact when compared to monolithic refractory alternatives. Allow for traditional location and fixing positions. Per casting machine, annual operating cost savings of $6000 (derived from energy savings, consumable savings, etc.) have been reported when compared to cast iron ladles. 3. Properzi RFM Launder Case Study Production of industrial wire and bar made from aluminium for electrical engineering applications were developed by Properzi in the 1950s. Since that time, many improvements have been brought to the caster, as well as the introduction of similar casting technologies. The principles of the casting process are illustrated in Figure 5. The mold is formed between the grooved periphery of the rod casting wheel and the endless steel belt. The casting wheel, usually copper, is water-cooled. The molten aluminium solidifies between the belt and the casting wheel. The cast bar has a triangular or trapezoidal cross section and a temperature of about 350 C after leaving the casting wheel. The molten aluminium prior to casting is processed to the required standard and then fed onto the wheel by a small launder. The launder is usually made from steel coated with a ceramic paper as illustrated in Figure 6. Sodium silicate is used to glue the paper to the steel frame. The ceramic paper tip can be rigidified using colloidal silica. Figure 6. Typical steel launder Typically the steel launder is removed and changed every 8-12 hours of continuous casting. The main mode of failure is the breakdown of the ceramic paper. The ceramic paper is also prone to oil absorption, which after some time, causes failure of the paper. Broken ceramic paper is a source of molten aluminium inclusions; the paper inclusions increase the ratio of wire breakage during the subsequent downstream draw operations. Ceramic paper can also react to and be attacked by molten aluminium. This can cause additional inclusions in the molten aluminium. During a launder change, even if the furnace is not completely stopped, that minute stop, plus some metal drainage, contributes to decrease the caster productivity. Table III shows the major differences between the steel coated launder and the RFM launder. Table III. Steel and RFM launder differences Property Steel launder + RFM launder paper Launder preparation Extensive Not needed Typical life 8-12 hours hours Reusable No Yes Oil penetration High Moderate Repairable No Yes Quality Variable High Productivity Variable High Figure 7 shows typical RFM launders. Our customers using RFM launders, have made the following observations: Longer casting runs without bar breaks Less oxide release compared to ceramic paper design Less cost ($/ton) than hand fabricated steel/ceramic paper launder Consistent performance every time at start up Better surface finish on bar Better casting yield and caster up-time due to no tundish changes during run 4

5 Lower oil absorption, which can be further decreased with boron nitride coating operational needs often results. The list below identifies the necessary operational requirements of the control pin. Thermal shock and cycling tolerant Non-wetting to molten aluminium Stable and inert in molten aluminium Resistant to chemical attack / reaction Minimal chilling effect on the molten aluminium Straight and true Health and safety issues Figure 7. Typical RFM launders The idea of a continuous caster is to cast for as long as possible. The use of an RFM delivery launder being a key component to meet that criteria without compromising bar or wire quality. The critical path now becoming the steel belt changes, not the delivery launder. This is due to the superior capabilities of the RFM material. 4. RFM Control Pin Case Study The control pin is used to regulate the flow of molten aluminium within direct chill casting technology. The control pin is used in conjunction with a downspout. The downspout retains and directs the molten aluminium into the distribution system while the control pin regulates the molten aluminium flow within the downspout. It is sometimes necessary to stop the flow of molten aluminium completely and this is achieved by direct contact between the control pin and down spout location seat. Figure 8 is a cutaway view showing a control pin at the top engaging with a downspout at the bottom. Figure 8. Cross section of a pin & spout Unfortunately, the control pin is usually geometrically challenged. It is often between 350 mm and 900 mm in length, but typically 750 mm. Its outer diameter is usually between 15 mm and 75 mm, but typically 40 mm. Therefore, this typical design is 750 mm long with a 40 mm outer diameter. Any product with a diameter to length ratio in excess of 7:1 will be fragile and more vulnerable to mechanical damage. The control pin needs to have a number of key properties to achieve operational success. It is not always possible for typically available materials to support these key features entirely. Therefore, a trade off between must have and desirable There are a number of materials accepted within the industry that have been proven to work satisfactorily. The most common of these materials are D.F.S., Dense Fused Silica, and C.B.R. Cement Bonded Refractory. These materials both have advantages and disadvantages over each other. Again, the choice between the materials is often made on the control pin geometry and operational environment. Pins made from either D.F.S. or C.B.R. are traditionally solid in construction. This is not always the case but is typical. These materials usually have a bar density of ~2000 Kg/m 3. The Table IV details the common advantages and disadvantages of each material. Table IV. Advantages & disadvantages of materials Property R.F.M. D.F.S. C.B.R. Thermal Shock Excellent Good Good Thermal Cycling Excellent Good Good Bar Density ~ 1600 Kg/m 3 ~ 2000 Kg/m 3 ~ 2000 Kg/m 3 Pre-Heat Temperature ~ 150 C ~ 700 C ~ 700 C Mechanical Resistance Good Good Poor None Wetting Excellent Poor Good Straightness Excellent Good Good Toughness Good Good Poor Corundum Catalyst Low Medium Low Different companies apply different pre-heating practices, and as a result, the traditional control pin is subjected to a range of different thermal shock & cycling situations. The scope of this paper does not cover or detail the entire range of different scenarios but the following examples can be typically found within the industry D.F.S. and C.B.R. materials. For placement of the control pin into a pre-heating oven, the pin can be pre-heated up to 800 C before placement into the spout. For placement of the control pin into the downspout prior to introduction of the molten aluminium, a high velocity gas torch placed at the entry to the spout achieves pre-heating. The purpose of pre-heating is to: a) Ensure there is no moisture present b) Ensure the molten aluminium does not freeze when in contact with the pin 5

6 RFM control pins are manufactured as a tube, not a solid rod of material. As such the volume of material present within the control pin is dramatically reduced. This has a direct impact on the chill effect propagating into the molten metal from the control pin as illustrated in Figure 9. Figure 9. RFM control pin tube It is not necessary to actually pre-heat an RFM control pin because of its freezing effect on the molten aluminium. RFM control pins are pre-heated to 150 C simply to ensure any atmospheric moisture is driven off. This fact means that energy costs are reduced and safety within the cast house is improved. There is no need to transport > 700 C control pins from preheating ovens to the production line. RFM is inert in contact with molten aluminium. This is extremely advantageous in this application. Having a material that cleans effortlessly after use is a bonus, but the true benefit is actually during its operation. The cavity between the outside diameter of the control pin and the inside diameter of the downspout is often quite small. There should be absolutely no restrictions or surface defects that could disrupt the flow of metal. Surface porosity or defects can offer areas for molten metal to become attached and cause turbulent flow. Aborted casts due to problems within the downspout can sometimes be attributed to corundum growth interrupting the controlled outlet orifice. The frequency of this phenomenon can be increased when pre-heating of the control pin and downspout is carried out with a high velocity gas burner. To further enhance the operational performance of the RFM control pin, a cement bonded refractory stopper section is inserted. Because the end of the control pin has to seal against the downspout seat, a material with a close bar density is desirable in this area. This ensures there is no premature wear of the control tip of the RFM control pin when in contact with a more dense material. RFM is extremely tolerant of such an insert exhibiting similar thermal expansion properties. The combination further increases the composite design bringing the best materials and combinations together. Figure 10 shows the inserted control tip into the RFM tube body. Figure 10. RFM control pin ceramic tip The advantages of the RFM material combine to result in increased life performance. A beta test site has shown that RFM control pins achieve in excess of 55 drops, when traditional pins achieve a maximum of 30. Summary The RFM body being of a composite nature and inert in molten aluminium offers this application many operational advantages. The step improvements offered by the RFM control pin against traditional materials is significant. It addresses many of the issues unable to be improved upon by traditional materials, without compromise in proven areas. Conclusions RFM composite material is a unique material. Its non-wetting properties against molten aluminium, its outstanding mechanical properties, as well as its high workability make RFM material the perfect material for aluminium contact applications. Other applications being alpha tested include: RFM skim dam RFM basin float RFM troughs RFM cross-feeders Preliminary test results are very encouraging. If a metal contact application faces a material challenge with existing materials, consider RFM material as a possible solution. Bibliography 1. S. Tremblay and M. Lapointe, The manufacturing, design and use of a new reusable molten aluminium distributor for sheet ingot casting, Light Metals, TMS, 2002, pp Catrin Kammer Gosler, Continuous casting of aluminium, TALAT Lecture 3210, 27 p. 6