Improved simulation and analysis tools are helping develop more and better once-exotic alloys, plus good old aluminum, for lighter aerospace designs with less waste.
by Ann R. Thryft, Senior Technical Editor, Materials & Assembly. Originally published in DesignWeek.com on May 2016.
The number and types of lightweight metals available for building military and aerospace components are expanding. Not only are aircraft structures and components making use of lightweight aluminum alloys, but also alloys of titanium, and other metals previously considered too exotic like beryllium. These materials help vehicles meet tougher fuel consumption standards by shedding hundreds of pounds.
About 50% of the aerospace materials market by volume consists of aluminum, according to a Marketsand-Market research report. The potential weight that can be shed by using titanium for aircraft components has spurred the growing use of this material in next-generation planes, which the report forecasts will drive up titanium demand through 2019.
Lightweight alloys of several kinds are the focus of the Lightweight Innovations of Tomorrow (LIFT) consortium operated by the American Lightweight Materials Manufacturing Innovation Institute (ALMMII). The institute’s goal is to accelerate the transition of technologies already developed by industry and research organizations from the applied development stage to the fully fleged production-line-ready process.
LIFT Consortium Looks to Improve Multiple Metals
One LIFT project led by Boeing and The Ohio State University aims to cut manufacturing costs and part weight in transportation applications by advancing technologies used to die-cast and heat-treat aluminum parts. It will focus on a high-speed, vacuum-aided, aluminum die-casting process to produce access panels on an airplane wing, cutting both weight and cost, and then heat-treat them to increase their hardness.
This project will also take advantage of integrated computational materials engineering (ICME), which combines design and production parameters with information about the metal’s microstructure to predict the performance of aluminum die-cast parts. This could reduce the amount of time needed for designing and testing new components using new materials and processes.
ICME is even more important in the consortium’s most recent projects. With GE Aviation and The Ohio State University as lead partners, LIFT’s third project focuses on advancing computer analytics so engineers can better understand and predict the performance of titanium alloys in aircraft engines and other aerospace designs. One important use of titanium is in the turbine fans in aircraft engines: these are critical to maintaining those engines’ high performance and they must operate in demanding conditions.
Computer models that can better predict how a design will perform will let engineers do less “making and breaking” of multiple test parts before they know the design is right for a specific critical component. Titanium’s expense means the cost of designing and testing new parts using conventional methods has been high. The project’s ICME focus can be applied across several related manufacturing processes to reduce the cost of both materials and testing, as well as a cut typically long lead times for developing new aerospace designs.
This project will look closely at FEA (finite element analysis) tools, said John Allison, a professor of materials science and engineering at the University of Michigan and LIFT’s materials modeling and simulation technology lead. To date, when analyzing stress and temperature distributions in structures, FEA tools assume that materials properties are constant and uniform throughout a part. Yet those properties can be very different depending on both manufacturing processes and history. Different processes, such as a casting versus forging, can produce different properties in two different parts, and variables in their separate manufacturing histories can produce different materials properties.
The consortium’s most recent work focuses on improving the understanding of, and predicting the performance of, aluminum-lithium alloys in formed parts for next-generation jet engines and other aerospace applications. These alloys are both lighter and stronger than aluminum alone.
Earlier generations of aluminum-lithium alloys something suffered from cracking or experienced issues performing in high-temperature environments, said Allison. The latest generation is much improved, but predicting these alloys’ performance at multiple steps – all the way from atomic structure to finished component remains to be done by developing more integrated computer models. Lead partners United Technologies Research Centre and the University of Michigan will develop material process modeling and simulation of how the materials’ properties evolve during industrial operations.
Pratt & Whitney, a United Technologies Corp. company, has developed a new blade design and materials processing technologies for a lightweight, low-cost aluminum-lithium fan blade, but they’re the first to do so in a commercial jet engine and the practice isn’t at all common. “Some aircraft structures are already using aluminum-lithium, but aircraft engines generally don’t employ this alloy for fan blades,” said Allison. “So we’ll be improving on their capabilities.”
Aluminum-lithium alloys often have a microstructure that’s analogous to wood grains, which behaves differently when it’s bent in different directions. “So when we’re forging a part, we need to either align the grains in the direction we want, or at least not in directions that cause problems,” he said. “Because we didn’t quite know how to deal with some of these directional properties before, they were very slow to be used.”
The team will therefore develop crystal plasticity modeling to predict an alloy’s final microstructure, which, in turn, defines the alloy’s mechanical properties as it’s formed into a part. “Tools for modeling crystal plasticity have been around for a while, but we’ve gotten more efficient and have more accurate models of the way metals flow,” said Allison. “Now we have to marry them with forging models and software, the traditional finite element analysis models. None of the commercial versions of finite element software models the materials at the microstructure level, and that’s what we will do.”
Beryllium-Aluminum Alloys Come Into Their Own
The manufacturers of a family of beryllium-aluminum alloys has achieved an aerospace first: delivery of the first completed azimuth gimbal housing components made of the alloy, called Beralcast, to Lockhead Martin. The components are part of the Electro Optical Targeting Systems (EOTS) on the F-35 Lightning II, currently under development. The EOTS is the first sensor to combine forward-looking infrared and infrared search-and-track functions to give F-35 pilots situational awareness and air-to-air and air-to-surface targeting from a safe distance.
The main Beralcast alloys are made by IBC Engineered Materials, a wholly owned subsidiary of IBC Advanced Alloys. They’re three times stiffer than aluminum with 22% less weight, and can replace aluminum, magnesium, titanium, and metal matrix composites, said Chris Huskamp, president of IBC Advanced Alloys.
better simulation tolls have played a major part of this alloy’s success. Advanced materials like these have been available for several years. But before, when considering new materials it was all about how to get the price down, said Huskamp. Now, there’s an opportunity to consider advanced materials that can increase performance. “In the last couple of years, the industry has advanced its simulation capability: the quality of analysis tolls even down to CAD programs has gotten pretty spectacular, and the accuracy of fatigue, thermal and finite element analysis modeling has come up,” he said. “So now we can consider how these effects will play into designs using these advanced materials: their analyses can build confidence going into the very first product.”
Beryllium alloys are not direct replacements for steel or aluminum, but niche materials that are effective for many solutions in advanced designs, said Huskamp. Beralcast 363, for example, is a metal matrix composite of 65% beryllium and 35% aluminum by weight, with a slightly higher cost than traditional aluminums. It’s used primarily for high-strength/high-elastic modulus precision cast structural applications where low weight, high stiffness, and thermal performance are critical.
Huskamp said the material is already in use on multiple aerospace platforms. As a result, the cost has been lowered to the point where the company is now able to sell the alloys into some commercial markets.