Titanium machining on a grand scale

 
Austrian machine tool manufacturer, WFL Millturn Technologies says it is the only company worldwide which concentrates exclusively on the manufacture of multifunctional turning, boring, milling centres, focusing on the complete solution from the start. The required software solutions with adapted processing strategies, process design and programming are also provided. According to company applications engineering manager, Reinhard Koll, the aerospace industry represents an important market segment for WFL: “Hard to machine materials are becoming increasingly more prevalent in this sector. Titanium machining in particular is a field in which we score with our accumulated know-how.” Titanium has always placed particular demands on tools and machines during the cutting process. In recent years Ti 6Al4V has become prevalent as a lightweight material with outstanding properties and possessing a combination of strength, corrosion resistance and capacity to withstand stresses. Although it does have good empirical values and cutting data, processing it still remains one of the supreme disciplines in machining. New titanium alloys are constantly being developed for special applications and these are often on the basis of firm customer requirements. This means that Ti 5553 is the desired material for landing gear in the aerospace industry. Ti 5553 stands out due to further improved strength and toughness properties. It's also less sensitive to structural changes on heating. Ti 5553 is at present one of the hardest materials to machine. A cutting speed of 45m/min should not be exceeded when it's being processed as shear stresses of up to 2,780N/mm² can develop at cutting speeds as low as 60m/min. Problems like point heat due to poor heat conduction and associated chemical changes in the material and the formation of built-up edges occur to a greater extent with this material than with other titanium alloys. Therefore it's particularly important that cutting speed, feed rate and penetration depth are matched to one another accurately when working with Ti 5553. The use of suitable cooling lubricants is also just as important. A quick and continuous removal of swarf must be guaranteed; heat dissipation occurs to a much greater extent via the tool. Removal of the forging skin is an additional challenge. The upstream forging process and the resultant thermal and metallurgical influences give this skin a very high level of surface hardness. The low modulus of elasticity means titanium tends to evade the pressure of the tool and to fuse with the cutting edge. Machining should therefore occur at a low cutting speed, but with a relatively high and even feed rate. Vibration free, fixed, sharp tools must be maintained. High speed steels with a high cobalt content, carbide or stellite are used as cutting materials. <This is how it works> For roughing and rough turning, the front rake angle should be between -6° and +6° and between 0° and 15° for final turning. The angle of clearance should always be around 7°. For carbide the angle of inclination should be -4° and for HSS steels it should be 0° to 5°. As titanium tends to fuse with the tool, climb milling is preferred to conventional milling. The wedge shaped swarf is separated at the thinnest point and damage to the milling cutter is reduced. For HSS steels the front rake angle of the milling cutters should be 0° to 10° and for carbide and stellite is should be 0° with an angle of clearance of 12°. It's during grinding that the material properties of titanium become manifest. The relatively high friction coefficients mean that high temperatures occur during grinding which result in chemical reactions between the metal and the abrasive grain and which lead to burning and smearing of the workpiece surface. The abrasive grains become blunt fairly quickly as a result of local overheating and then just slide over the surface. Even if the ground surface isn't visibly burnt, surface tensions may be present which lead to grinding cracks that can affect fatigue strength. Water-based solutions are normally used as cooling lubricants. Aqueous sodium nitride solutions or aqueous solutions of water soluble oil are used here. Sulphurated or chlorinated oil can also be used at temperatures below 200°C (in this case the workpiece must be cleaned following machining). <Experience: the decisive factor> All the above demonstrates that lots of experience in the selection and use of the tools and also the machining strategies is required. A firm WFL project involves the processing of landing gear parts for civil aircraft. In such cases it's customary to demand a concept study from the supplier of the processing machines which includes the essential processing steps. The blanks for the components are usually large forged parts which are therefore very cost-intensive. It's essential the ability to cater for critical aspects of machining during the manufacture is shown as early as the conceptual phase. For example, it's necessary to consider the fact that different material thicknesses in the blank workpiece require modified machining strategies. Heat affected zones must also be considered together with the cutting forces which occur. As a rule, the forging skin cutting forces are around 70-80% higher than those of hardened steel. “Hard to cut materials like titanium have influenced the development of WFL's machines,” states WFL's marketing manager, Dieter Schatzl. “We provide individual solutions for these kinds of demanding applications, which also include topics like cooling and production strategy, as well as the actual machine. We're in a position to adapt the Millturns precisely to the requirements of our customers, such as the emergency retraction for example which retracts the tool from the workpiece immediately if a power outage occurs to prevent workpiece damage.” <Harmonious overall concept> WFL implements customer requirements so that they become an integral part of the overall concept. It's for example possible to respond to the particular requirements of customer projects through a special design for guides and spindles and to adapt the machine components to the respective loads. The topic of cooling is especially important where titanium machining is concerned. As titanium burns at temperatures above 880°C, appropriate fire and explosion protection must be ensured. The advantage of the individual development of spindles is also visible in the coolant feed; the cooling lubricant can be fed directly to the cutting edge via the milling spindle with a pressure of up to 200bar. An additional factor is that it is also possible to measure the workpieces inside the machine when work is in progress on the WFL complete machining centres. “The customer wants a machine where processing is secure,” Schatzl concludes. “This also involves support in the choice of tooling and above all in the machining strategy as just one reject part represents a huge economic loss with workpieces of this size.” www.wfl.at