When embarking on a path to implement machine safeguard, Titanium is one of the fastest growing materials used in aerospace applications. The prime rationale for designers to choose titanium in their designs is its relative low mass for a given strength level and its relative resistance to high temperature. The metal has long been used in aircraft engine front sections and will continue to be used there for the foreseeable future. In fact, due to its properties, titanium alloys are becoming more prevalent than ever before in structural and landing gear components.
One drawback of these alloys is their poor machinability. Over the past few years, Kennametal has invested heavily in research & development to understand how to better machine titanium.
Machinability of titanium alloys
Machining of titanium alloys is as demanding as the cutting of other high-temperature materials. Titanium components are machined in the forged condition and often require removal of up to 90% of the weight of the workpiece. The high-chemical reactivity of titanium alloys causes the chip to weld to the tool, leading to cratering and premature tool failure. The low thermal conductivity of these materials does not allow the heat generated during machining to dissipate from the tool edge. This causes high tool tip temperatures and excessive tool deformation and wear.
Titanium alloys retain strength at high temperatures and exhibit low thermal conductivity. This distinctive property does not allow heat generated during machining to dissipate from the tool edge, causing high tool tip temperatures and excessive plastic deformation wear — leading to higher cutting forces. The high work-hardening tendency of titanium alloys can also contribute to the high cutting forces and temperatures that may lead to depth-of-cut notching. In addition, the Chip-Tool contact area is relatively small, resulting in large stress concentration due to these higher cutting forces and temperatures resulting in premature failure of the cutting tool.
The low Modulus of Elasticity (Young’s Modulus) of these materials causes greater workpiece spring back and deflection of thin-walled structures resulting in tool vibration, chatter and poor surface finish. Alpha (α) titanium alloys (Ti5Al2.5Sn, Ti8Al1Mo1V, etc) have relatively low tensile strengths (σT) and produce relatively lower cutting forces in comparison to that generated during machining of alpha-beta (α−β) alloys (Ti6Al4V) and even lower as compared to beta (β) alloys (Ti10V2Fe3Al) and near beta (β) alloys (Ti5553).
A generous quantity of coolant with appropriate concentration should be used to minimise high tool tip temperatures and rapid tool wear. Positive-rake sharp tools will reduce cutting forces and temperatures and minimise part deflection.
Efficient coolant delivery
Beyond BLAST™ is a revolutionary insert platform with advanced coolant-application technology that makes cutting more efficient and effective while extending tool life.
We took an entirely different approach to machining high temperature alloys. We determined that the most effective way to deliver coolant would be to channel it through the insert, ensuring that it hits exactly where it does the most good. That means more efficient coolant delivery at a fraction of the cost of high-pressure coolant systems.
Alpha (α) alloys: Pure titanium and titanium alloyed with α stabilizers, such as tin and aluminium (e.g. Ti5Al2.5Sn), are classified as α alloys. They are non-heat treatable and are generally weldable. They have low to medium tensile strength, good notch toughness, and excellent mechanical properties at cryogenic temperatures.
Beta (β) alloys: Beta (β) alloys contain transition metals, such as V, Nb, Ta, and Mo, that stabilize the β phase. Examples of commercial β alloys include Ti11.5Mo6Zr4.5Sn,Ti15V3Cr3Al3Sn, and Ti5553. Beta alloys are readily heat-treatable, generally weldable, and have high strengths. Excellent formability can be expected in the solution treated condition. However, β alloys are prone to ductile-brittle transition and thus are unsuitable for cryogenic applications. Beta alloys have a good combination of properties for sheet, heavy sections, fasteners, and spring applications.
Titanium alloys: Pure titanium (Ti) undergoes a crystallographic transformation, from hexagonal close packed, hcp (alpha, α) to body-centered cubic, bcc (beta, β) structure as its temperature is raised through 1620 ºF / 882 °C. Alloying elements, such as tin (Sn), when dissolved in titanium, do not change the transformation temperature, but elements such as aluminium (Al) and oxygen (O) cause it to increase. Such elements are called “α stabilizers.” Elements that decrease the phase-transformation temperature are called “β stabilizers.” They are generally transition metals. Commercial titanium alloys are thus classified as “α,” “α-β,” and “β.” The α-β alloys may also include “near α” and “near β” alloys depending on their composition.
Alpha-Beta (α-β) alloys: These alloys feature both α and β phases and contain both α and β stabilizers. The simplest and most popular alloy in this group is Ti6Al4V, which is primarily used in the aerospace industry. Alloys in this category are easily formable and exhibit high room-temperature strength and moderate high-temperature strength. The properties of these alloys can be altered through heat treatment.
Characteristics of titanium and titanium alloys
Pure: Ti98.8, Ti99.9
Alloyed: Ti5Al2.5Sn, Ti6Al4V, Ti4Al2Sn4Zr2Mo, Ti3Al8V6Cr4Mo4Zr, Ti10V2Fe 3Al, Ti13V11Cr3Al, Ti5Al5Mo5V3Cr
The following are the material characteristics:
Relatively poor tool life, even at low cutting speeds
High chemical reactivity causes chips to gall and weld to cutting edges
Low thermal conductivity increases cutting temperatures
Usually produces abrasive, tough, and stringy chips
Take precautionary measures when machining a reactive (combustable) metal
Low elastic modulus easily causes deflection of workpiece
Easy work hardening.
The importance of the correct use of coolant
The importance of the correct use of coolants includes achieving the lowest coefficient of friction. A low coefficient of friction is developed by using proper coolant delivery. This results in lower temperature so that the workpiece does not get soft and its tool life is extended. Under pressure and direction, the coolant knocks chips off the cutting edges and provides anti-corrosive benefits for machine tool and work. There is a high correlation between the amount of coolant delivered and the metal removal rate.
For instance, the company drills are high-performance, solid carbide tools. To optimise their performance, they must be adequately cooled. With the proper coolant flow, tool life and higher maximum effective cutting speeds can be reached. In milling and turning processes, applying coolant using our newest technology — coolant delivered at the cutting edge, through-the-tool coolant, or coolant nozzles to each insert — is an optimal way to increase tool life and maximise productivity. Coolant nozzles direct a concentrated stream of coolant to the cutting edge, providing multiple benefits. First, the cutting edge and workpiece are kept as cool as possible. Second, the cutting edge and workpiece are also lubricated for a minimum coefficient of friction.
Finally, the coolant stream effectively forces the cut chips away from the cutting edge, thereby, eliminating the possibility of recut chips. It is important to provide a generous ‘volume’ of coolant when machining titanium, and when applying drills and mills in a vertical application to improve chip evacuation and increase tool life. We must also use a high coolant concentration to provide lubricity, which will aid in tool life, chip evacuation, and finer surface finishes. High-pressure coolant, either through the tool or through a line adjacent and parallel to the tool, should always be considered for increased tool life and production. One should not use multi-coolant lines. Instead, we need to make use of one line with 100% of the flow capacity to evacuate the chips from the work area.
It is also important to use synthetic or semi-synthetic at proper volume, pressure, and concentration. A 10% to 12% coolant concentration is mandatory. Through-coolant for spindle and tool can extend the tool life by four times. An inducer ring is an option for through-spindle flow.
We can also maximise flow to the cutting edges for best results. At least 3 gal/min (13 liter/min) is recommended, and at least 500 psi (35 bar) is recommended for through-tool-flow.
Keep it steady
In order to achieve rigidity, the following must be considered:
Use gravity to your advantage
Horizontal spindles enable chips to fall from your work
Horizontal fixturing necessitates use of angle plates
Keeping this in mind, we must consider the following:
Keeping work closest to strongest points of fixture
Keeping work as close as possible to spindle/quill
High-pressure, high-volume, through-spindle coolant delivery will increase tool life tremendously (>4x)
Knowing the power curve of your machine
Ensuring sufficient axis drive motors for power cuts
Finding the weak link of every set-up
Finally, rigidity will make or break your objectives. Hence, look for weak parts of machine structure and avoid moves that may compromise the rigidity.
Fixturing the workpiece
If vertical spindles are employed, your fixturing is still an important aspect. In either case, there may be directions of work movement that are not secured. Rigidity is paramount. Try to keep work close to the strongest points of the fixture to help avoid the effects of harmonics. Hence, keep work low and secure and keep work as close as possible to the spindle/quill.
The productivity factor between typically used cutting tools can easily be 4-to-1 in many cases. Older tools can be replaced by today’s tools if the entire system is modified where needed and accounted for where it is unalterable. Tool life can be increased by the same factor simply by changing from flood to through-tool coolant delivery and utilising our newest technology, coolant delivered directly at the cutting edge. It is suggested to not ask more of your machine than it can deliver. Most machines cannot constantly cut at a rate of 30 cubic inches (492cc) per minute. There are many usual failures or weak points in every system. They include but are not limited to drive axis motors, adapter interface, a weak joint, torque available to the spindle, machine frame in one or more axes, or compound angles relevant to machine stability and system dampening.
Importance of a strong spindle connection
In the construction of today’s modern aircraft, many component materials are switching to high-strength lighter materials like titanium to increase fuel efficiencies. To save time and money with this tougher-to-machine material, machinists are challenged to maximise metal removal rates at low cutting speeds and considerably higher cutting forces. Machine tool builders must also provide greater stiffness and damping in their spindles to minimise undesirable vibrations that deteriorate tool life and part quality.
Although all these advances add to greater productivity, the weakest point is often the spindle connection itself — needing high torque and overcoming high-bending applications. Kennametal’s response to this traditionally weak point has been with the KM™ system.
Overview of existing spindle connection
To fulfill the increasing demand for high productivity, an important element to be considered is the tool-spindle connection. The interface must withstand high loads and yet maintain its rigidity. In most cases, it will determine how much material can be removed on a given operation until the tool deflection is too high or the onset of chatter.
High-performance machining can be accomplished with the use of high feeds and depths of cut. With the advances in cutting tools, there is a need for a spindle connection that makes possible the best utilisation of the available power.
Several different types of spindle connection have been developed or optimised over the last few decades. The 7/24 ISO taper became one of the most popular systems in the market. It has been successfully used in many applications but its accuracy and high-speed limitations prevent it from growing further. The recent combination of face contact with 7/24 solid taper provides higher accuracy in the Z-axis direction, but this also presents some disadvantages, namely the loss in stiffness at higher speeds or high side loads. Most of these tools in the market are solid and the spindles have relatively low clamping force.
Making the right choice
When machining tough materials like titanium, cutting speeds are relatively low due to thermal effects on cutting tools. In response, machine tool builders have improved stiffness and damping on spindles and machine structures over the years. Spindles have been designed with abundant torque at low rotational speeds. Nevertheless, the spindle connection remains the weak link in the system.
The spindle connection must provide torque and bending capacity compatible with the machine tool specifications and the requirements for higher productivity. It becomes obvious that in end-milling applications where the projection lengths are typically greater, the limiting factor is bending capacity of the spindle interface.
With more materials that are tougher to machine and require considerably higher cutting forces from the machine tool, choosing wisely on the spindle interface to maximise cutting edge performance is the key to success.
The article is reproduced with courtesy to Kennametal