Influence of the thermal treatment on the WC-CO cutting tool-life. Application to the HSM of aerospace aluminium alloys.

Salguero, Jorge ; Batista, Moises ; Gomez, Alvaro 等

1. INTRODUCTION

Machining processes performance can be influenced by a high number of parameters and variables, such as feedrate, cutting speed and depth of cut, toolpath and cooling strategies, or the machine-tool and clamping device characteristics.

When these items are fixed, cutting tools plays a predominant role in that performance. Thus, it is neccessary to search cutting tools life improvement conditions.

Research lines developed for that purpose involve tool wear minimizing and/or process based tool redesign (Sebastian & Faura, 1997). Design changes can be seen from two viewpoints: on the one hand, tool geometrical modifications; on the other hand, modifications of the core and/or covering cutting tool material.

The production requirementes and other factors, such as the initial cost of design modifications to be made by the cutting tool manufacturer, and the results obtained in a previous analysis of the cutting tool wear in the HSM of Al alloys (Cano et Al., 2007), give rise to proposal a cutting tool material modification, executable in the same plant where the machining processes are being carried out.

In this context, this paper shows the benefits of heat treatments on the tool life improvement, for tungsten carbide end milling tools applied in the HSM of one of the most used alloys in the aerospace industry, the UNS A92024-T3.

2. EXPERIMENTAL

The HSM tests were in profile contouring processes. In these tests, sets of large UNS A92024-T3 overlapped sheets, with total thickness under 10 mm, were contour machined by making use of a 3 axis CNC machine-tool, with a spindle speed of 19,800 rpm, feedrate of 1,440 mm/min, and cutting depth of 10 mm, using an MQL cooling system, in order to reduce the environmental impact and to improve by this other way the economic performance (Kelly & Cotterell, 2002).

[FIGURE 1 OMITTED]

Milling tools used were uncoated WC-10%Co with MG10 grain size (K30-K40). Mill geometry has been plotted in the previous Figure 1.

In a first step, tools were tested in 0 to 10 hours machining tests, in order both to verify that the secondary adhesion mechanism, based on the development of Built Up Edge (BUE) and Built Up Layer (BUL), was the predominant wear mechanism (Yousefi & Ichida, 2000), and to evaluate the cutting tool life. After testing, milling tools were analysed by combining Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) techniques, Figure 2.

In a second step, and in order to change the core material properties, the tools were subjected to tempered heat treatments between 550 and 900 K, for intervals between 5 and 15 minutes, making use of the same industrial furnace used to change the thermal state of the aluminium alloys in the factory, having so an immediate access, avoiding the intermediation of the cutting tool manufacturer, as was discussed previously.

After the thermal treatment, the cutting tools were tested in the HSM process until the life end, and after this tests, they were analyzed again with SEM/EDS equipments.

3. RESULTS AND DISCUSSION

The results obtained in the first step have showed that the maximum tool life is about 4,5 hours of machining, from which the cutting-tool degradation (Figure 2) causes the apparition of several burrs. This short tool life is caused by a weakening of the cutting edge, due to a non-homogeneous Cobalt distribution in this zone, possibly due to the negative thermal effects of the grinding process in the cutting-tool manufacturing, and favoured by the formation, development and detachment of the BUE, Fig. 3.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

After the tests developed in the second step, with cutting tools thermal treated, the best results are obtained for treatments at a temperature of 725 K for 15 minutes, reaching levels of improvement in the order of 450%, which involves multiplying the tool life by almost a factor 6, taking the original average life of 4 hours 30 minutes. This fact supposes the overcoming of 24 hours of continuous work on plant, without needing a tool change. This fact is an additional advantage directly associated with the improvement of the economic performance.

Figure 4 includes a SEM image of a tungsten carbide milling tool, treated for 15 minutes at 725 K after more than 20 hours of contour milling operation at 19,800 rpm. As it can be observed, there is not a special damage in the cutting edge, except the appearance of adhered material. This fact has previously been found in other studies developed on this and other aerospace aluminum alloys (Batista et Al., 2009).

Associated EDS profiles included in Figure 4 show that there is not an appreciable loss of Cobalt. Furthermore, the ratio of peak intensities for W/Co evaluated from the EDS spectra remains approximately constant. Thus, it can be concluded that thermal treatment leads to reduce the loss of Cobalt in the milling tool cutting area. On the other hand the edge weakness is avoided.

On the other hand, thermal treatments have showed a high improvement in the tool surface hardness. This can be related to the formation of some complex compounds, such as [W.sub.2]C, [Co.sub.6]W6C or [Co.sub.3][W.sub.3]C, according to (Thakur, et Al., 2008).

[FIGURE 4 OMITTED]

4. CONCLUSIONS

One of the causes of the wear of a WC-Co cutting tool is the weakening of the cutting edge, apparently due to a nonhomogeneous Cobalt distribution. This fact comes favoured by the formation, development and detachment of the BUE in the HSM of aluminium alloys.

This work shows the influence of heat treatment on the improvement of the performance in the HSM process of the UNS A92024-T3 (Al-Cu) alloy, in terms of tool life increase.

The thermal treatments developed have allowed improving the performance of the process, achieving increased tool life. The best conditions have been obtained for 15 minutes tempered thermal treatments at 725 K. This treatment has improved cutting tools life by factor close to 6, being this improvement associated to an homogeneous redistribution of Co in the milling tool. This fact carries on an increase of the tool hardness values, associated to the formation of complex carbides [Co.sub.X][W.sub.Y]C.

5. ACKNOWLEDGEMENTS

This work has been supported by the Spanish Science & Innovation Ministry and by the Andalusian Government.

6. REFERENCES

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