Machining Titanium Aero-Frames

June 1, 2019

The rise of titanium for aerospace applications has been well documented in recent years. Equally, the challenges associated with the efficient, productive and high-quality machining of this popular material, have also been a topic of debate and scrutiny. Of course, every machine shop wants optimized performance from its cutters when milling titanium, but this can prove less than straightforward without the right technology and know-how in place. Today, however, thanks to a breakthrough in this area, things are beginning to change.

The aerospace segment is expected to demonstrate impressive growth in the coming years as consumers continue to drive demand for new routes and greater flight frequency. Further drivers include global demographics and wealth creation in Asia and the Middle East. In fact, according to Airbus, air traffic is set to double from its current levels over the course of the next fifteen years.

More Aircraft Required

All of these expectations point to one outcome – the need for more aircraft. In turn, the challenge for aircraft OEMs will be to accelerate production.

There is also a clear knock-on effect for the aerospace supply chain. Machine shops around the world serving this sector need to meet increasing requirements for capacity, output, quality, on-time delivery and price. According to a recent survey by Deloitte, the aerospace supply chain is expected to continue its transformation to reduce costs, respond quicker and invest more heavily.

With respect to aircraft structures, the demand for increasing levels of titanium content in aircraft continues to rise, largely as a result of its impressive strength-to-weight performance. However, aeroframes are large components and there is often a considerable amount of material to machine away before the final component emerges ready for subsequent assembly. With this in mind, it is important for cutting-tool suppliers to offer solutions that allow high productivity, and long and predictable tool life. However, more than tooling innovation alone is needed to meet the requirements of modern machine shops faced with the optimised production of titanium aero-frames. Sometimes it takes a whole new strategy.

A New Approach

One such strategy is high-feed side milling, which differs from traditional machining approaches that utilize large radial engagements and obligate to low cutting speeds of 40-60 m/min. Instead, high-feed side milling is characterized by low radial engagement, constant chip thickness, and high feed rate and speed.

At Sandvik Coromant, the company's R&D team set about creating a cutting tool that could leverage the benefits of new milling strategies like high-feed side milling with the support of CAM programming, the thinking being that elevated productivity and extended tool life could be notable benefits. After two years of intensive development work, the result is the CoroMill^® Plura HFS solid-carbide end mill, a unique solution in geometry and grade that has been specifically conceived for titanium alloys and, in particular, to work in combination with high-feed side milling strategies.

Cutting Speeds >100 m/min

Productivity has always been somewhat of a stumbling block for machine shops producing titanium parts. The reason is that machinability is limited by the high chemical reactivity, low thermal conductivity and work hardening properties of titanium. To combat these issues, the latest end mill has been designed to offer reliable performance when machining titanium alloys at cutting speed in excess of 100 m/min, while simultaneously demonstrating long tool life.

The main wear mechanism during the milling of titanium alloys is diffusion wear, which is provoked by a chemical interaction between titanium from the work-piece material and chemical elements from the tool coating and substrate. A higher temperature in the cutting zone intensifies the chemical reaction, so an effective cooling system is especially important for those looking to achieve prolonged tool-life characteristics.

In order to improve heat transfer from the cutting zone and use coolant in a more efficient way, the latest solid-carbide end mill is equipped with a newly developed internal coolant solution. The coolant channels have exits (one per each flute) that are specifically positioned to let the fluid reach the most thermally loaded part of the cutting zone. To increase cooling efficiency even further, the tool is equipped with the exclusive Coolant Booster (patent pending), which features coolant flow grooves on the clearance side that are designed to improve the heat dispersion.

Defining the grade and the geometry

Sharp cutting edges are another prerequisite when milling titanium alloys as these help to decrease cutting forces and minimize the influence of work hardening. On the new tool, sharp edges combine with a new (patentpending) coating that features a TiAlN inner layer and silicon-containing outer layer. The outer layer reacts with titanium alloys and forms a thin sub-micron protective layer on top of the original coating. During the cutting process, the newly formed chips glide on top of the protective layer, preventing fast deterioration of the original coating and prolonging tool life.

The potential results of adopting the new cutter in combination with high-feed side milling strategies have already been demonstrated at a European plant operated by a major aircraft OEM. Here, the company wanted to increase production rates, save time and keep costs under control when machining pockets on aeroframes made from Ti6Al4V using a four-axis horizontal machining centre.

The Results

By replacing the existing high-feed indexable insert cutter with the CoroMill^® Plura HFS ISO S end mill, cutting data could be suitably adjusted to make the required gains. For instance, cutting speed was raised from 50 to 110 m/min, while axial depth of cut was increased from 1 to 30 mm. At the same time, radial depth of cut was reduced. The results proved particularly impressive.

Previously, the machining time for the aero-frame pockets was recorded as 150 minutes, with tool life of 37.5 minutes. Using the CoroMill^® Plura HFS ISO S end mill, machining time was reduced by a factor of three, while tool life was four times longer. As a result, the plant is witnessing annual savings of ⇔ 14,000 for the pocketing operation alone.

This article was written by Corey Schwenke, Product Manager - Solid Round Tools - Americas, Sandvik Coromant (Fair Lawn, NJ). For more information, visit here  .

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