New Mold Wrap Improves Manufacturing Casting Efficiencies and Reduces Environmental Impacts
By Dr. Craig Freeman, Research and Development Manager, Morgan Advanced Materials
Morgan Advanced Materials has a long history of working closely with many companies in the aerospace, industrial gas turbine (IGT), and automotive industries that use investment casting to manufacture specialized metal components. Many such companies currently use Refractory Ceramic Fiber (RCF) to provide thermal insulation in the investment casting process. However, the use of RCF has recently been affected by strict regulation in the European Union, forcing EU companies to find alternative insulation materials, especially for use as mold wraps.
In this article, we describe a recent project Morgan carried out in collaboration with a major engine original equipment manufacturer (OEM) and a casting foundry, to develop a next generation low bio-persistent Alkaline Earth Silicate (AES) wool insulation that can replace RCF in the casting of gas turbine blades. Recent casting trials have confirmed that using AES during product manufacturing results in the same high quality as RCF mold wrap and that there are no other detrimental effects if RCF is substituted with AES. Following these successful trials, the conversion process was completed and both companies are now using Morgan’s Superwool® HT Blade Wrap in mold wrap applications.
High Temperature Insulating Wool is used widely for heat containment in investment casting.
The use of superalloys to manufacture highly critical precision components for the aerospace and IGT industries has increased dramatically in recent decades, because their temperature capabilities allow turbines to run hotter and more efficiently, thereby reducing fuel consumption. Mold wrap is an essential element in process control for the investment casting process used to manufacture superalloy components. It is used to insulate the pre-heated mold, ensuring efficient mold fill, and to provide controlled cooling during the critical solidification of the metal. High Temperature Insulating Wool (HTIW) blanket, with carefully controlled specifications, is the normal choice for mold wraps. It has excellent thermal shock resistance, low thermal mass, and a fibrous structure that makes it highly insulating at high temperature.
Traditionally the majority of aerospace and IGT casting processes used blown RCF blanket. However, such products have been classified hazardous by the European Union and are strictly regulated. In addition, under the EU REACH (Registration, Evaluation and Authorisation of Chemicals) regulation, RCF has been proposed for inclusion in a list of chemicals that may require an authorization covering the mold wrap application. The changing nature of RCF classification creates uncertainty about future availability and use of these products.
Non-RCF alternatives for investment casting applications
As a result of the increasing regulation and resulting uncertainty, a number of companies have been developing non-RCF alternatives. Once proven to meet the high temperature needs of investment casting applications, such alternatives can replace RCF and remove the risk of a future break in supply.
Morgan Advanced Materials has developed alternative HTIW materials with excellent high temperature insulating performance that also offer low bio-persistence, thereby avoiding classification as a carcinogen. These fibers, sold under the brand name Superwool®, are exonerated from carcinogen classification in Europe under the provisions of Note Q of European Directive 97/69/EC (adapting Council Directive 67/548/EEC). Superwool HT Blade Wrap has been designed to deliver equivalent insulation characteristics to those previously experienced with RCF mold wrap and can thereby be readily used as a substitute.
Engine OEM and foundry seek alternative to RCF
A major engine OEM and a casting foundry, both global leaders in the supply of gas turbines into aerospace, marine and power applications, considered the use of RCF to be a business continuity risk, based on the uncertainty surrounding their classification. As a result, they decided to investigate transitioning away from RCF for investment casting applications, including the production of turbine blades. The two companies, began a development project with Morgan as its primary development partner to pursue the use of high temperature AES as the alternative to RCF. The work was largely carried out for the aerospace business they supplied, which requires castings with equiaxed or randomly orientated grain structures using the mold wrap to control the cooling process. The companies required a consistent, thin blanket product with sufficient mechanical strength to be manipulated around complex molds and provide equivalent thermal insulation.
Morgan conducted casting trials using its Superwool® HT, the highest classification temperature AES fiber commercially available (1300˚C). The material is in blanket form, primarily 6 and 13 mm thick with 128 kg/m3 density. The material’s advanced low bio-persistence fiber manufacturing technology significantly lowers shot content, improving insulation performance and handleability.
The first phase of casting trials on a broad range of aerospace and IGT products within the engine manufacturer’s foundry confirmed that the castings are manufactured to the same high quality and exhibit no detrimental effects compared to castings made using RCF mold wrap. The trials determined that the insulation can accommodate the high temperatures required during the mold pre-heat stage while retaining mechanical integrity. The Superwool HT also offers the advantage of not being on the limit of current pre-heat temperatures, allowing the flexibility to increase the temperature further if required.
Next, testing measured thermal gradients inside pre-heated molds as they cool to compare the RCF to the Superwool HT wrap materials. Two thermocouples were positioned on the legs of the mold, and then loaded into a furnace held at 1050°C. The temperature was monitored using a data logger and the mold allowed to soak for 5 minutes once it reached temperature. The mold was then removed and allowed to cool while the temperature continued to be logged. These experiments allowed the selection of the optimum specification of Superwool HT material.
The team then conducted metal-filled mold trials in the foundries and determined the effects on metallurgy. The results confirmed that the Superwool HT material compared favorably to the existing RCF mold wrap product. Both companies have now fully transitioned to the use of Superwool HT Blade Wrap. Morgan’s Thermal Ceramics business continues to help customers achieve manufacturing process efficiencies to help enhance the companies’ processes, improve environmental impact, and reduce costs.