Warping - is, which often happens when machining parts from a thermally hardenable aluminum alloys. The cause buckling of these parts - residual stresses, which were formed during their hardening during thermal hardening.
by the way, famous aluminum one-piece body iPhone devices 5 и iPhone 6 Apple's made it so. The whole aluminum extruded billet is subjected to numerous milling operations on a sophisticated computer controlled machine with constant powerful cooling., to avoid her warping.
(Cm. more details: Aluminum alloys in smartphones and laptops).
Solution heat treatment
The operation of hardening aluminum alloys consists of two stages:
- The first stage is heating to the hardening temperature: from 450 ºC for alloys 7xxx series to 520 ° C for the 6xxx series alloys.
- After a certain soaking rapid cooling should be at a temperature of quenching - different for different alloys.
Required for complete quenching cooling rate greatly differs for different aluminum alloys, eg:
- immersion in cold or warm water for high-alloy 7075,
- Cooling jets of water to alloy 6061,
- cooling fan or compressed air for the alloy 6060.
The purpose of quenching - keep the aluminum in solid solution is dissolved alloying elements - different for different alloys. Quenching is followed by an aging stage - natural or artificial (at elevated temperatures). During aging by precipitation of solid-solution strengthening components, containing alloying elements, is an increase in strength aluminum alloy.
Warping during machining
This unpleasant phenomenon - warping - particularly "unpleasant" topics, It is making it difficult or impossible to maintain strict dimensional tolerances aluminum products, which are obtained by machining, for example, milling. This is especially true of large and / or complex shaped products. Therefore, a high level of rejection milled aluminum parts from heat-hardenable alloys, because of deviations from given dimensions due to warpage is commonplace.
The figure below shows warpage, which arose in the sheet alloy 7075 in condition T6 (hardening and artificial aging) thickness 25 mm and length 250 mm after cutting for the electro-erosive machine along the central plane into two halves. The maximum value of the resultant gap reached 4,3 mm.
Figure 1 - Warping sheet of alloy 7075-T6
after cutting along the center plane
Residual stresses in aluminum
The level of residual stresses in the hardened product aluminum, for example in the same thick sheet, increases with its thickness, till, finally, It does not reach the yield strength of aluminum alloy in the hardened condition.
Famous - Aerospace – alloy 7075 has a significantly higher yield strength in the quenched condition compared with other alloys, including, with such popular in the industry, as an alloy 6061. Therefore, an alloy 7075 It is the most prone to the formation of residual stresses and, Consequently, warping during machining. Residual stresses in quenched alloy sheets 7075 thickness of more than 33 mm can reach 230 MPa. The hardened sheets of alloy 6061 thickness over 15 mm residual stresses can reach "only" 90 MPa.
Why do residual stresses occur?
Residual stresses arise due to nonuniform cooling articles when hardened and the associated reduction of its dimensions inhomogeneous, which causes warping. When a relatively thick product is immersed in a water bath quenching, the surface layers of the product are cooled first, and so reduced in size much faster, than its core. At the same time at the beginning of cooling hot core has a low resistance to reduce the size of the surface layers - a soft core is plastically deformed under the influence of compression of the outer layers. Later in the course of hardening, However, core too cool and too "wants" to reduce its size. However, this reduction in the barriers already cold and relatively strong outer layers of the product.
Thus, products inside the tensile residual stresses produced because, stuff here wants to reduce its size, but can not do so because of the resistance of the outer layers. These tensile stresses in the inner layers of product are balanced compressive stresses near its surface.
The residual stress in the aluminum sheets
The figure 2 shows the distribution of balanced residual stresses over the thickness of the hardened sheet. The maximum compressive (negative) stresses are achieved on both outer surfaces of the sheet, and the maximum residual tensile stresses occur within the sheet – at the center plane. This distribution of residual stresses is typical for the simple case of a semi-infinite sheet (slab). For details of less regular shape distribution of residual stresses will be much more complicated.
Figure 2 – EXAMPLE balanced residual stresses
thick aluminum sheet
Peak values of compressive and tensile residual stresses in the hardened aluminum sheet depends on the quenching intensity, sheet thickness and yield strength of the alloy in the hardened state level.
Usually residual stresses:
- are not very high in relatively thin sheets;
- increase with thickness and reach the quenched yield strength in very thick sheets.
With increasing thickness, the difference between the temperature on the surface of the sheet and the temperature in its core increases.. This leads to high residual stresses.
If during subsequent machining of hardened and aged sheet material is removed asymmetrically with respect to the distribution of residual stresses, then a warping of the sheet in the form of its spontaneous deflection.
How to avoid warpage?
To reduce the level of residual stress in aluminum products and prevent buckling parts when machined can be applied in various degree four approaches.
- Tempering is not normal, and in warm water. It reduces the level of residual stresses, which are formed during hardening. However, this technology can prevent the achievement of full aluminum alloy hardening and reduce the strength of a subsequent artificial aging.
- The mechanical reduction of residual stresses, for example, by stretching, in the hardened state, before aging. It reduces the level of residual stress, without prejudice to the strength properties. However, it is difficult to implement for complex shapes.
- Thermal reduction of residual stresses (during normal artificial aging or less than normal aging). With artificial aging (T6), a relatively insignificant decrease in residual stresses occurs. When aging in overaging mode (for example, T73), a more significant reduction in residual stresses is achieved, but due to the undesirable reduction in strength properties.
- Cold plastic deformation (after aging). This operation does not reduce the magnitude of residual stresses, but can change their distribution. However, cold working may cause additional warping during subsequent machining.