Aluminium and its alloys are increasingly used in modern industry in particular for its characteristics of light weight and resistance.
Aluminium is used in almost every industry, thanks to its specific technological qualities
, among which the lightness and thermal and electrical conductivity. Few elements in nature can constitute such a large number of alloys as aluminum.
Aluminium alloys and their characteristics
Aluminium alloys have common characteristics, such as low melting temperature, low mass density, high electrical and thermal conductivity and an Al content of greater than 95%. In addition to these properties, others vary significantly with alloying elements. Copper, silicon, magnesium, zinc and manganese are the binders used to form the alloys but also other elements can be added, said corrective, which improve some performance aspects, i.e. Nickel, Titanium, Zirconium, Chromium, Iron.
There are more than 1,000 different alloys of Al, produced mainly with the aim to improve the characteristics of the base metal, even if mass production focuses on the 15 ÷ 20 in greater use.
Aluminium and chip removal
Aluminum alloys are well suited to machining by chip removal even if, to make economic processing necessary to adopt suitable techniques, both as regards the machine and the tool, taking into account that different alloys can have very different behaviors during processing. First of all, it should be emphasized that the processing for removal of the aluminum takes place by “cutting” and not, as in the case of steel, for deformation. The processing is strongly influenced by the degree of hardness of the alloy, which involves the formation of tenacious and flowing chip or fragile and rigid one: a function of hardness will be identified the appropriate cutting geometry of the tool so as to avoid the formation of keystrokes and obstructions that may affect the tool life and part finish.
The achievement of the performance
Performing tool machining of the alloys of Al machines requires fast and powerful machines, able to reduce production times. High cutting speeds are possible because the chip does not heat the tool so much to lower the hardness even if the temperature reached is sufficient to reduce the resistance to deformation of the material. It can be profitable to look for the most advantageous cutting speed, consistent with the geometry of the pieces and the type of machine tool: a cutting speed of 400 to 500 m/min leads to economic and production advantages, in addition to the improvement of the surface finish. The machine that allows for a high number of revolutions to the spindle must be rigid if it is necessary a good finish, but the higher the rpm, the more the machine is delicate and requires careful handling by the operator. In order to reduce process times, also a good tool magazine is essential, in order to allow more machining and contain the dead times. Aluminium is a “delicate” material that requires a series of precautions: if a material is easy to work, however, is critical to block because of its tendency to warp. It may happen that, due to a locking too effective, it incurs deformations that need to be re-worked, in order to return the artifact under the conditions provided. Working by turning long bars, due to the low modulus of elasticity, can be employed more constraint elements in order to reduce the deformations caused by shear stresses. The aluminum alloys tend to be stained by oils, in particular by hydraulic oils that may be present in the coolant, which is appropriate for a large cleaning. Besides extremely efficient washing systems of the cabin able to keep very clean the working area and the guides, also the cutting fluid must be clean and efficient, and then suitably filtered, according to traditional methods or more sophisticated and evolved, able to reach an advanced filtering. The alloys can be also dry- machined, as long as they are not particularly hard, the tools are in good condition and cutting is not exasperated. In general it is preferred, however, the use of emulsions which have excellent stability and cleaning ability, capable of preventing staining and anti-foaming, with a pressure of use on machining centers around 40 bar. The cutting fluid should be used with abundance on both the tool tip that on the whole piece so as to contain the temperature, without creating zones of thermal shock, in which the Al alloys are particularly sensitive.
Machining of aluminum alloys typically requires a dedicated tooling. Common feature to the tools are sharp edges and acute cutting angles strongly positive, in order to make the tool more “cutting”, in particular when working alloys with low silicon content. In the case of abrasive alloys, with silicon content exceeding 5%, coated tools are recommended, characterized by excellent wear resistance and therefore able to allow, even with the difficult alloys, high machining speeds. In the case of deep hole drilling, with a depth of 20 to 40 D, the tool tends to be deviated due to the molecular structure. The geometry of the cutting edges of the tip is typically optimized to obtain chips easily evacuated .. Increasingly widespread is the technology which involves the use of tools with internal coolant.
Also machining of aluminium is sensitive to the occurrence of chatter, namely those vibrations induced by increased spindle speed and advancement which lead to instability of the cutting process, with problems of surface finishing and also tool breakage. The chatter, which manifests itself as the achievement of “critical”, is closely related to the dynamic behavior of the system machine-piece: working below the critical threshold is a common practice which, while ensuring stability in the process, limits productivity, sometimes also in an important way. Since the effect of the chatter is limiting, high performance can be achieved only in the face of appropriate cutting parameters and of a careful analysis of the dynamic properties of the system constituted by the machine, tool and fixing system. The diagram lobe, obtained by experimental measurements, it is a tool that allows today to predict the onset of chatter.
Al-Li alloys: the new generation
Due to the increase in the cost of fuel, in the 70’s were developed aluminum alloys based on Lithium, which could replace, in aeronautical field, the alloys in use, without any major design changes, while ensuring that a much lower weight. Despite initial enthusiasm their development was in principle blocked by technological difficulties inherent in the production, due to the explosive reaction that these alloys in the liquid state have with water. The alloys of the first generation, while presenting interesting features, also showed several limitations including high cost, high anisotropy of the mechanical characteristics, a low fracture toughness. Today was putting the new generation of Al-Li alloys, higher performance, which limits the content of lithium at less than ‘1.8% and sees the presence of other alloy elements, such as Al-Li-Zr, where the Zr is used for grain refinement, Al-Li-Mg-Cu or Al-Si-Li. The latest generation of innovative alloys have better balanced mechanical properties, better formability and workability for chip removal and excellent corrosion resistance.