How to deal with the processing challenges of aviation parts more efficiently

At present, there are about 17,000 passenger aircraft and freighters in operation around the world, and the sky seems to be getting more crowded. Experts predict that by 2025, 25,000 new aircraft will be needed. Despite rapid development, aviation manufacturing industry still faces the problems of rising raw material prices, increasing application of difficult-to-process materials, and limited production capacity. In order to provide new and reliable processing solutions, new processing strategies are needed.

Titanium alloys, Inconel nickel-base alloys and other high-strength heat-resistant alloys and carbon fiber-reinforced composites are replacing traditional aluminum alloys and become the main materials used in aerospace engineering. Aerospace component manufacturers often need to process multiple materials that are composited on the same part because a metal (such as titanium) may be sandwiched between multilayer composites to enhance the stability of the composite structure.

As airlines seek to drastically reduce fuel consumption, lightweight structures are becoming standards and specifications. For example, Lufthansa Germany expects the aircraft to consume 1.25 gallons per person (ie, 1.25 gallons per 100 passengers). However, to achieve this goal, it is an expensive proposition for parts manufacturers, because the processing of lightweight materials requires large-scale investment in new machine tools and cutting programs, which may include large-scale gantry-type machines. The production line (with six or more axes) is converted to machine parts on a single piece of equipment in a machining center to increase the flexibility of the process. However, these costs can be compensated through the use of innovative new machining concepts.

In today's aircraft industry, it is critical to extend tool life and maximize process reliability. In addition to the need to process challenging workpiece materials, it is also necessary to achieve processing rates of up to 90%. In other words, machining machines must spend 90% of their time cutting.

Digging the processing potential in line with economic principles Many aerospace parts still need to be machined from a single piece of rough material. This is justifiable because these parts are not only subject to extreme environmental conditions, but also often have to withstand extremely high mechanical stresses. To eliminate the possibility of fragmentation and other damage to these parts, the stress must be relieved to a large extent, and this can be achieved by machining the part with a complete blank, but the processing of such parts often requires a lot of man-hours. .

So, is there any way to increase productivity? Considering that the cutting speed is closely related to the tool load, the part manufacturer must process within a certain speed limit. They cannot expect a breakthrough in the processing speed of most difficult-to-machine materials. For example, when machining titanium alloys, increasing the cutting speed from 150 sfm to 200 sfm is considered high-speed cutting.

Therefore, for users, the only way to tap new potential is to change the processing strategy. Just a few years ago, the mechanical processing in the aviation industry was mainly based on the use of high-speed steel tools, and its applicable cutting speed was lower than that of cemented carbide tools. However, with the forgiveness of traditional high-speed steel cutting tool materials, solid carbide tools and indexable cutting tools are gaining entry into the aerospace industry with the advantage of increasing productivity. But this requires investing in new machine tools or spending large sums to upgrade existing machines to provide the rigidity needed for more brittle cemented carbide tools and to use higher cutting parameters for carbide tools. This raises a major issue for manufacturers, who have found that it takes too long to realize the return on investment.

Simply switching to more upscale tools may not be enough to solve this problem. As a result, tool manufacturers are working with machine tool manufacturers and end-users to consider the entire manufacturing process from the starting point of the production development process.

To ensure flight safety at a lower cost In the automotive industry, tool makers have been involved in the early stages of product development. In contrast, tool manufacturers' aerospace product processing experts are usually required to intervene after product development is completed, which is largely determined by the industry's inherent tradition.

In the past few decades, airlines have paid little attention to costs, because flight safety is the most important thing that must be ensured at all costs. However, with the beginning of globalization and the rapid growth of cheap air travel, the era of costs has come to an end. The aviation industry is gradually realizing that while flight safety is the most important factor, it should also be achieved at a lower cost that can be affordable. This has brought enormous pricing pressure to aircraft manufacturers and component suppliers. They are eager to learn from the experience of other industries (especially the automotive industry). The biggest challenge is to compete with low-cost Asian manufacturers.

For example, by massively subcontracting manufacturing operations to different suppliers, OEMs in the automotive industry can share risks with their primary and secondary suppliers in development projects. Then, these suppliers deliver the finished parts for the assembly line.

The end user should provide the tool manufacturer with the required data at all stages of the production plan, allowing the tool manufacturer's R&D team to determine the optimal tool cutting edge geometry and match it with the coating through a series of standardized tests. . The focus of these jobs is to increase tool life.

In the manufacture of aircraft, due to the presence of a large number of complex structural parts (such as metal forgings or flat parts characterized by numerous cavities), a large number of pocket milling operations are involved. Therefore, it is very important to find the right cavities efficient machining strategy including chip control and chip evacuation strategies. When using highly efficient tool-milling cavities, an empirical method for producing swarf of a suitable size (thickness 0.15-0.20 mm) is to machine with a depth of cut of 1.5-2.0 mm and a feed of 0.38-0.50 mm per tooth. Determining the correct machining strategy involves not only workpiece materials but also machine tools, tools, and fixtures.

For example, one method is to drill a starting hole at the corner of the cavity first, and then use a square shoulder milling cutter to remove the workpiece material layer by layer through two-axis milling. According to the machine torque and horsepower, the appropriate depth of cut for each layer is 3.8-5.1mm.

Alternatively, the user can cut the pyramid-shaped blank material through the entire depth of cut with a small cutting width. In this way, the radial cutting amount and the axial cutting amount of the tool are in every pass. It is a change. This type of machining is often used when workpiece materials such as stainless steel tend to score cutters or blades. Because deep cuts are axial and are distributed at different locations along the cutting edge, this machining technique can separate deep cuts and extend them along the cutting edge. Plunge milling is usually a correct machining strategy when the tool has a long overhang.

Optimized coating design Although precise cutting edge geometries are precisely designed, there are still technical limitations on the cutting speed when processing aerospace materials. The only way to significantly increase productivity is to use multi-tooth tools and extend tool life. This depends on choosing the right tool coating. The optimized coating not only reduces wear but also protects the cutting edge from the effects of cutting heat. For example, PVD alumina coating is particularly suitable for processing aerospace materials. Its main advantage is excellent high temperature performance, which can effectively avoid the formation of built-up edge on the cutting edge and the generation of micro chipping.

When cutting titanium alloys, the heat generated at the cutting edge is a big problem. Due to the poor thermal conductivity of the workpiece material, most of the heat is transferred to the cutting edge. However, when coated on the cutting edge with a heat-resistant coating, heat is forced to flow in other directions (eg, incoming chips). When machining titanium alloys, the thermal barrier provided by the PVD alumina coating can increase tool life by 50%, and in many cases, machining of a workpiece topography (eg, pockets) can be completed without the need for tool change. Machine tool auxiliary processing time.

Another example is the improved Al2O3 coating. When the coating is deposited on the tool, the surface is usually rough. To this end, Walter Corporation applied a layer of TiCN coating on top of its Al2O3 coating, and then removed the top layer of the rake face by shot blasting. In this way, a smooth coated surface is provided, which reduces friction when cutting chips hit the cutting edge, thus increasing tool life. With this method, a two-colored blade that looks different is also produced, and it is easy to recognize the wear of the rear face.

New processing methods In the manufacture of aerospace parts, in order to increase productivity, on the one hand, high-efficiency tools adapted to the specific requirements of the machine tools used can be used, and on the other hand, new processing methods can be used. Using a modern machining center can significantly shorten the processing cycle. For example, the machining cycle can be shortened by changing the processing method of landing gear parts. The part is up to 3m in size and is made of 10-2-3 or 5553 titanium alloy (depending on the model). In the past, spiral mills were used to reciprocate back and forth on a three-axis machine tool to machine the workpiece to the required dimensions. With the new five-axis car/milling machining center, the workpiece itself can be rotated to meet the machining requirements of turning tools, contour milling cutters, and square shoulder milling cutters. Turn milling machining can be used for areas that cannot be achieved by turning machining. Another alternative machining method is to use a contour cutter to continuously reciprocate the Z-axis feed plane, removing one layer of material at a time. These processing methods, which are considered quite traditional in the tool and mold manufacturing industry, are now finding a place in aviation manufacturing.

Another example of a revolutionary processing method is the manufacture of aircraft engines. An aircraft consists of four main parts: airframes, power systems (engines, generators), landing gear and flight control systems. Each part needs to be made of different materials and different processing methods. Manufacturing engines requires processing of materials such as titanium alloys, Inconel nickel base alloys, and the like that can withstand high dynamic and thermal loads. In order to make the rear parts of the engine (such as high pressure/low pressure turbines), Inconel alloys need to be machined. This type of machining can be done using standard large-feed milling cutters. To meet the machining needs of low-power machine tools, some tool manufacturers have made improvements to the tool.

Taking into account the rapid development of the aircraft manufacturing industry, the number of machine tools could not fully meet the increase in demand. Therefore, it will inevitably lead to an extension of the delivery period. This status quo must be fundamentally changed because competitors from Russia and China are putting pressure on incumbent aircraft manufacturers not only in pricing but also in innovation. Some aerospace parts (such as engine hoods) that are difficult to process are already manufactured in these countries. Manufacturers in these countries will sooner or later acquire equipment and know-how to produce all the complex parts.

At present, the great challenge facing the aviation industry is the ability to combine high quality with high productivity. In order to meet this challenge, new processing strategies will be produced in the coming years, and it may even provide advanced processing modes for other industries.