by Howard Pickard, LBBC Technologies
EDITOR’S NOTE: This article is based on a longer paper presented at the investment Casting institute’s 60th Anniversary Conference and Expo in Pittsburgh, PA.
Ceramic cores have been used in castings for many years to assist in achieving ever more complex internal geometry. In the investment casting industry, ceramic cores have played an important part in the development of improved performance of blades and vanes in gas turbines.
In order to form the passageways within the components, cores with the same intended geometry as the cooling channels are positioned in an investment casting shell. Once the component is cast and the majority of the shell removed, a process known as caustic leaching is widely used for dissolution of the encapsulated core, thus leaving behind the empty cooling channels. Although widely used for many years, the fundamentals of the process physics and chemistry are little understood.
Following some initial experimental work performed in LBBC Technologies’ facility in 2010, Rolls-Royce PLC and LBBC Technologies agreed to cosponsor a Post Graduate Doctoral Studentship through the University of Birmingham’s four year engineering doctorate program with the project aim to investigate the physics and chemistry of the present investment casting core removal process in order to enhance its industrial application.
To recap on the process operation, components are loaded within a basket and the basket is placed in the autoclave. The autoclave is filled with a caustic base solution with approximate concentrations of typically 20% sodium hydroxide (NaOH) or 40% potassium hydroxide (KOH). This breaks down the bonds in the ceramic core which is typically made up of silica (70-100%) and zircon etc (0-30%). The solution is heated to around 320°F (160°C) under a pressure of around 90 psi (6-7 bar) to suppress boiling at this temperature. Through venting, dwelling and repressurizing of the autoclave at regular intervals the solution boils and creates a ‘flow’ of solution within the autoclave. This flow enables the alkaline solution to be replenished at the interface with the core as well as physically removing the core material that has softened or ‘broken away’.
Key Components of the Process
To understand what is happening during the leaching process, consider the three main areas of the process:
1. Caustic solution– Caustics are strong alkaline chemicals that destroy soft body tissues resulting in a deep penetrating type of burn, in contrast to corrosives, that result in a more superficial type of damage via chemical means or inflammation. Caustics are usually hydroxides of light metals. The
alkali metal hydroxides are the most basic of all hydroxides; sodium hydroxide (NaOH or caustic soda) and potassium hydroxide (KOH or caustic potash) being the most widely used caustic agents in industry.
2. Ceramic Core– The most commonly used core material in turbine blade manufacture is fused silica due to its chemical removability, well documented high temperature properties, abundance, and economic cost. The material development and complexity of future superalloys formed via directional
solidification or single crystal techniques is limited by the capability of conventional silica-based core and shell materials to withstand longer casting times at high temperatures (~1700 °C). Preformed cores fashioned from alumina, Al2O3, are suitable for investment casting application as they exhibit good dimensional tolerance, high strength and high refractory properties. The leaching of alumina is however often economically not feasible for production and manufacturing operations due to longer
dissolution rates relative to silica.
3. Boiling at Diffusion Layer– When a liquid is heated in an open vessel, the liquid vaporizes from its surface. The condition of free vaporization throughout the liquid is called boiling. The temperature at which the vapor pressure of a liquid is equal to the external pressure is called the boiling point at that pressure. Boiling does not occur when a liquid is heated in a rigid, closed vessel. Instead, the vapor pressure, and hence the density of the vapor, rises as the temperature rises. Factors which control the rate of a reaction are:
- Concentration of reactants in solution.
- Surface area of any solid reactants.
Initial experimental work was focused on determining the most effective chemical reagent and its optimum concentration to facilitate the dissolution of cristobalite and zircon. Observations have been;
- The higher leaching capability of NaOH compared to KOH.means or inflammation. Caustics are usually hydroxides of light metals. The alkali metal hydroxides are the most basic of all hydroxides; sodium hydroxide (NaOH or caustic soda) and potassium
- The plateau/dip in reactivity after concentrations 20 % w/v of NaOH. hydroxide (KOH or caustic potash) being the most widely used caustic agents in industry
Initial trials to investigate the optimum cycle parameters have been performed using an LBBC test unit. Studies were conducted using the core test piece shown below with 33 % potassium hydroxide (KOH) solution. The extent of core dissolution was determined by measuring the change in core depth on a number of different components with six identical cores, each with a diameter of two mm to give an average over 18 samples.
Experimental work has been undertaken to study the effect that different high and low pressure dwell times have on core removal. Observations have been;
- Varying the low and high pressure dwell time has little impact on leach effect.
- Varying the vent time has the greatest impact on leach effect .
- Lower temperatures reduce core leach but not as significantly as expected.
- Reducing suppression pressures only begins to effect leaching below 55psi when using this concentration of KOH.
The caustic leaching of ceramic cores involves a wide range of physics and chemistry concepts. Key aspects to the process have been illustrated here. Identifying and analyzing specific areas of caustic bases, core materials such as silica and zircon, boiling action, etc., is expected to result in a better understanding the process of caustic core leaching. Initial experimental work has begun to show the aspects of the process that need to be further investigated.
For more information, contact the author at [email protected]
From Incast Magazine, June 2014