How to Cost-Effectively Manufacture Superalloy Aerospace Combustor and Turbine Components
Mike Shappell, K. Philip Varghese and Bruce Gustafson | July 06, 2018
Figure 1: Superalloy jet engine components have complex geometries, precision tolerances and rigorous surface integrity requirements. Source: Norton Abrasives
The combustor and turbine components in aircraft engines operate at red-hot temperatures while rotating at high speeds. They are made of nickel, cobalt or iron superalloys to withstand the extreme temperatures and cyclic stresses. Forty percent of the engine weight typically consists of superalloys.
Superalloys are engineered for high-temperature strength, creep, fatigue and toughness characteristics. These properties make component manufacturing extremely difficult. New rapidly solidified powder metal superalloys allow increased alloying additions to further enhance properties, which also exacerbates manufacturing challenges.
Figure 2: Factors enhancing surface integrity during superalloy component manufacturing using abrasive methods. Source: IEEE GlobalSpec
Engineering Surface Integrity
Maintaining fatigue strength is a critical consideration for engine components rotating at high speeds. Fatigue properties are dependent on surface integrity (finish, residual stress, surface damage). Engineers have traditionally chosen grinding and abrasive finishing for their inherent surface integrity enhancement ability, which is the result of superior surface finish, induced compressive residual stress and lack of damaged layers. Abrasive processes are often employed to enhance the integrity of parts made by machining, rolling, forging or casting by removing damaged layers and refining the finish. In fact, NASA researchers recently used low-stress grinding to enhance additively manufactured Inconel 718, which typically has poor fatigue properties compared to wrought processing.
While abrasive grinding, finishing and polishing have long been the go-to processes for surface integrity enhancement, newer abrasive material removal and finishing methods are displacing less efficient machining on a productivity basis. Superalloy component manufacturing begins with a wrought, cast or powder raw material.
Figure 3: Process steps for superalloy jet engine component manufacturing using abrasive processes. Source: IEEE GlobalSpec
While each raw material has microstructural characteristics greatly impacting machinability, grindability differences are often minimal. For instance, solidification during casting can produce coarse particles, which reduces cutting tool life. Powder metal superalloy processes allow higher alloy content and smaller grain size. The severe work hardening and tool adhesion of powder metallurgy superalloys makes machining slow and expensive.
Figure 4: 718 Inconel metal removal rate (MRR) of Norton Quantum and competitive ceramic cutoff wheels. Source: Norton Abrasives
Advanced abrasive technologies can be utilized in various superalloy part manufacturing steps to enhance productivity and quality. The first step, pre-grinding (cut-off sawing and rough grinding), is used to cut stock to length or remove casting gates, forging flash and additive manufacturing support material. Automation of flashing or gate cut-off can be optimized to maximize productivity and scrap recovery. Engineered high-flexibility cut-off wheels enable robots to consistently saw off gates much closer to a casting. This reduces less efficient belt grinding, minimizes unrecoverable swarf and makes belt grinding more automatable.
Figure 5: Performance of Norton Abrasive investment casting points (ICP) compared to competitive mounted points. Source: Norton Abrasives
Additional cost savings arise through the selection of abrasive products with enhanced performance or life. Norton Abrasives Quantum Fast Cut products typically outperform competitive abrasive saw blades in metal removal rate (MRR). Norton Abrasives ICP points allow removal of material in recesses unreachable with a wheel or belt.
Precision Grinding – Generating Part Geometry and Integrity
After pre-grinding, the part geometry is manufactured to the required tolerances. Modern abrasives such as ceramic alumina (like Norton Quantum), engineered shape ceramic (TG2) and cubic boron nitride (cBN) have made grinding the most efficient option for superalloy manufacturing. Grinding can manufacture components with smaller features at tighter tolerances compared to machining.
Figure 6: Grinding outperforms end mill machining in slotting 718 Inconel. Source: Norton Abrasives
Grinding has higher MRR with lower or no white layer thickness and finer finishes. Certain highly alloyed superalloys are barely machinable due to their extreme hardness, wear resistance and strength. Hard phases within the new superalloys can cause unpredictable cutting tool failures and lifespan. As a result, grinding is the only viable option for some highly alloyed superalloys. The higher MRR of grinding reduces cycle times and increases throughputs compared to machining.
High-performance abrasive products fabricate superalloys using low specific energy or horsepower. This allows small, low-cost machine tool spindles and reduced capital expenditures (capex). Using products specifically designed for superalloy grinding such as Norton Abrasives’ grinding wheels with TG2 extruded grain can maximize abrasive machining efficiency and performance.
Figure 7: G-ratio of crushed and engineered shape ceramic abrasive grinding wheels. Source: Norton Abrasives
When a cutting tool or grinding wheel wears, part surface integrity often suffers. Dressing the wheel at recommended intervals ensures that wheels run at optimal efficiency. Unlike cutting tools, wheels can be resharpened on the machine during the cycle. Older fused aluminum oxide wheels need to be continually dressed while grinding to maintain sharpness, but ceramic abrasives self-sharpen during grinding due to their ability to micro-fracture. Ceramic abrasive wheels only need truing or shaping at intermittent intervals during the cycle. Norton grinding wheels with a TG2 engineered shape abrasive ceramic grain are extremely durable, exhibiting high G-ratio (volume of material removed ÷ volume of wheel loss).
Automating Abrasive Deburring and Finishing
After precision geometry has been generated, abrasive finishing processes can be applied to remove burrs, refine surface finish and blend to a uniform appearance using abrasive brushes, belts and nonwoven wheels. Manual superalloy blade finishing is a slow process requiring many repetitions of roughness checking and refinishing. Skilled operators willing to perform this arduous offhand task are scarce.
Figure 8: Performance mapping of four Nonwoven Vortex Rapid Prep discs on 718 Inconel for product selection based on MRR aggressiveness and surface finish generation. Source: Norton Abrasives
For maximum productivity and part consistency, abrasive finishing processes can be automated using robots, vision systems and predictive algorithms to reduce the overall abrasive cycle time from nine hours to 45 minutes. Machine compliance using a controlled force robotic arm with engineered conformability abrasives provides a controlled MRR with automated compensation for abrasive wear. Finish measurements using vision systems indicate the specific areas requiring refinishing. Robotic automation increases finish consistency and part quality while reducing operator injuries, workforce shortages and rework.
Figure 9: Total cost benefit factors for superalloy manufacturing using abrasive grinding and finishing. Source: IEEE Globalspec
Abrasive processing provides productivity enhancement and cost reduction opportunities for superalloy parts manufacturers. Abrasive superalloy manufacturing provides a total cost benefit when factoring in surface integrity, lower capex, increased productivity, reduced waste, improved quality, enhanced consistency and reduced cycle times.
Norton Abrasives engineers have the expertise to improve, develop and tailor application parameters such as abrasive surface speeds, part feed rates and coolant delivery to optimize superalloy manufacturing. Product specifications can be adjusted such as abrasive grain types, bonds, backings, grit size, flexibility, porosity and shapes.
The Norton Abrasives group has a vast palette of tens of thousands of abrasive products to select from as well as the knowledge and resources to tailor and develop new products to maximize manufacturing efficiency.
Figure 10: Engineers at Norton Abrasives Higgins Grinding Technology Center in Northboro, Massachusetts, work with customers and machine tool OEMs to select, develop and optimize abrasive products and processes. Source: Norton Abrasives
The aerospace industry is adopting new materials such as ceramic matrix composites (CMCs) and titanium aluminides to further enhance engine performance and economy. While these materials have improved high-temperature strength, high specific strengths and good oxidation resistance, their hardness, low thermal conductivity and brittle nature pose additional challenges in maintaining both surface integrity and productivity during manufacturing. Norton engineers are studying how to cost-effectively manufacture next-generation jet engine materials.
Norton Abrasives Products for the Aerospace Market, Aerospace Commercial Finishing Abrasive Product Guide, Norton Quantum Cutoff Wheels provide more information on Norton Abrasives’ processes for aerospace component manufacturing. To discuss the optimal abrasive processes for your application, contact Norton | Saint-Gobain Applications Engineers.