Aerospace manufacturing Technology

Technorati Tags: Aerospace,Manufacturing,Technology,Aerospace Manufacturing

Even though aerospace manufacturing has improved quite dramatically over the past decade, much more effort is being constantly put into researching a more efficient way of manufacturing. The aerospace manufacturing industry face an uphill task to produce an efficient way of manufacturing with the introduction of more new higher strength materials.

Machining has also been regard as the optimum way to produce a small number of production run components. The versatility of machining is clearly seen where it can be applied to all kind of materials [2].It also has the less effect on the properties of materials compare to other manufacturing tools. Despite these advantages, manufactures are still looking forward to reduce the high cost of consumable tooling and set up time for high volume production to components often requiring several machining operations, thereby making it difficult to effectively control the machine shop and consequently an increase in work in process [2]. Besides that, machining generally produces large amount of scrap metal which is not very cost effective. Continuous effort has been put into

reducing the net shape titanium produced by machining [5]. By further reducing the net shape titanium size, it will enable a more efficient and productive manufacturing process. Nickel and titanium alloys have a fairly low thermal conductivity which causes the cutting temperature to increase up to 1200°C at the tool point/rake face. The high temperature generated would greatly affect the material property which is not desirable (notes). The above problems tend to form the basis in continued research and development activities in this area of manufacturing technology.

Another main concern in developing future machining technologies is that the hardness of many high strength super alloys increases significantly upon heat treatment. The formation of the second phase particles makes the alloy both stronger and more abrasive but extremely difficult to machine [2]. Hence it is an advantage to machine at a softer state. A typical manufacturing process would be to machine the component to a near net shape in a solution treated condition, then age hardened and then finally finish machined to generate the desired surface finish and to eliminate any distortion associated with heat treatment [2]. Positive rake geometry minimizes work hardening of machine surfaces and efficiently shears the chip away from the workpiece. Therefore positive rake geometry is use whenever is possible. Relatively sharp edges should be applied also whenever it is possible because it improves the machining process by preventing material build up. Below shows a table on the practical guide of machining high strength super alloys.

Table 1: Practical guide for machining high temperature alloys [2]

High strength alloys generally have a low thermal conductivity and this would generate lots of heat during the cutting process. A way of solving this problem is to introduce a coolant at the cutting zone to effectively reduce the tool temperature. Cryogenic cooling is an efficient way of maintaining the temperature at the cutting interface well below the softening temperature of the cutting tool material [1]. It works by supplying liquid nitrogen through the nozzles close to the tool tip along the rake face and clearance face of the cutting edge. The figure below shows the schematic setup of cryogenic cooling.

Figure 1: Dual nozzle system for localized LN2 supply [1]

By reducing the temperature of the cutting zone, it allows a higher cutting speed to be used thus saving manufacturing time. The temperature generated at the cutting face is only 829°C which relatively low compare to the softening temperature of CBN material which is around 1500°C [2]. This improves the tool performance and also increases the tool life. It also has less tendency of material smearing. This method however does pose a problem due to the usage of liquid nitrogen. The low temperature of the coolant would tend to shrink the alloy thus creating a geometrical inaccuracy. However this can be solve by incorporating the shrinking factor of the work piece to machine accordingly.

Since a 1940, a lot of research effort has been done to improve the machininibility of high strength aerospace superalloys. A new technique called thermally enhanced machining utilizes an external heat source such as a laser beam to heat and soften the workpiece locally in front of the cutting tool and allows difficult-to-machine materials to be machined with [6]. The figure below shows a schematic diagram describing the technique above.

Figure 2: Schematic diagram describing thermally enhanced machining.

From recent research, it was observed that laser does have a big impact on the cutting performance. The heated area generated by the laser softens the workpiece and indirectly reduces the amplitude of the cutting forces hence, result in a lower friction force between the tool and work piece [6]. However the reduction in cutting forces decline dramatically with increasing cutting speed because of the shorter interaction time between the laser bean and workpiece. Nonetheless the reduction in cutting forces of 26% was still significant at the cutting speed of 93m/min [6].

Figure 3: Graph of reduction of cutting force against surface cutting speed.

To date, laser assited machining of commercially pure Ti₆Al₄V has also been proven to lower the hardness near the machined surface compared with conventional cutting due to the lower shear deformation and it also helps to improve the surface finish due to lower dynamics cutting force. Even though thermally enhanced machining has been able to provide much more advantages then the conventional machining, much more research must still be done to explore the feasibility of laser-assisted milling of these alloys [6].

New technologies and approaches are also required as part of a strategy for taking cost out of manufacturing [5]. By reducing the work content and increasing productivity, it allows us to have a much more simple manufacturing process at a higher rate. This would indirectly reduce the effective manufacturing cost. In order to develop a higher efficiency way of machining in the future, it would rely heavily on intelligent machining system being able to undertake high speed machining with improve surface finish [3]. However the vibration generated by such high speed machining system can severely affect the speed and efficiency of machining. In the near future, the demand for titanium alloys which is increasingly used in aerospace structural applications will outstrip its availability hence increasing procurement lead times and cost. To endure a more sustainable process for the manufacture of titanium alloy aerospace components, a step change in manufacturing process is required [5].

With the right combination of cutting tools, cutting conditions and machine tool that will promote high speed machining without compromising the integrity and tolerance of the machined components the productivity of machining can be significantly improved [1]. This is predominantly important for the economic machining of hard to cut high strength aerospace alloys whose peculiar characteristics generally impair machinability. In the near future, technologies such as laser direct metal deposition will be available to manufacturer to be used for surfacing, repair, hybrid build and most importantly original part build. Laser deposition is far more efficient then any of the machining tool currently available which wastefully subtract additional materials from casting and forging.

Reference

1. E.O. Ezugwu, High speed machining of aero-engine alloys, Journal of Brazilian Society of Mechanical Science and Engineering 26 (1) (2004) 1–11.
2. E.O. Ezugwu, Key improvements in the machining of difficult-to-cut aerospace superalloys, International Journal of Machine Tools & Manufacture 45 (2005) 1353–1367
3. Phil Withers director of University of Manchester Aerospace Research Institute, Advance in aeroengine manufacturing technology, The engine yearbook 2008.
4. Aerospace manufacturing magazine 26, September 2007
5. Aerospace manufacturing magazine 26, July 2007
6. S. Sun & M. Brandt, Laser assisted machining of titanium alloys, Industrial Laser solution.