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Causes of failure
Published in William Bolton, R.A. Higgins, Materials for Engineers and Technicians, 2020
Metallic coating can be applied in a number of different ways: Hot dipping can be used to coat the surface of iron and steel components with both tin and zinc. Tinplate is still manufactured in South Wales, where the industry was established some 300 years ago. Clean mild-steel sheets are passed through a bath of molten tin and then through squeeze rolls, which remove the surplus tin. Galvanising is a similar process, whereby articles are coated with zinc. In the past, buckets, dustbins, wheelbarrows, cold-water tanks and barbed wire have all been coated by immersion in molten zinc. Many of these galvanised items are now largely superseded by plastics materials products, though barbed wire festoons the British countryside in increasing amounts as it replaces hedgerows grubbed-out or systematically killed off by the mechanical flail.Spraying can be employed to coat surfaces with a wide range of molten metals, though zinc is most often used. In the Schoop process, an arc is struck between two zinc wires within the spray gun, and the molten metal so produced is carried forward in an air blast. This type of process is useful for coating structures in situ, and was employed in the protection of the Forth Road Bridge.Sherardising is a ‘cementation’ process, similar in principle to carburising. Steel components are heated in a rotating drum containing zinc powder at about 370°C. A very thin, but uniform, layer of zinc is deposited on the surface of the components. It is an ideal method for treating nuts and bolts, the threads of which would become clogged during ordinary hot-dip galvanising.Electroplating is used to deposit a wide variety of metals onto both metallic and non-metallic surfaces. Gold, silver, nickel, chromium, copper, cadmium, tin, zinc and some alloys can be deposited in this way. Electroplating is a relatively expensive process, but provides a very uniform surface layer of very high quality, since accurate control of the process is possible at all stages. Moreover, there is no heating of the component being coated, so any mechanical properties developed by previous heat treatment are unaffected.Cladding is applicable mainly to the manufacture of ‘clad’ sheet. The basis metal is sandwiched between sheets of the coating metal and the sandwich is then rolled to the required thickness. During the process, the coating film welds onto the base metal. Alclad, which is duralumin coated with pure aluminium, is the best known of these products.
Characteristics and strength of hybrid friction stir welding and adhesive bonding lap joints for AA2024-T3 aluminium alloy
Published in The Journal of Adhesion, 2022
Yue Wang, Peng Chai, He Ma, Yanhua Zhang
The lap interface of the AS in the SZ is completely stirred by the welding tool, and the material of the upper and lower sheets is welded completely. This part of the lap region is considered as completely FSW zone (CWZ), which includes the distance from the hook tip to the tip of the unconnected lap interface in the RS. The tips of the unconnected lap interfaces in the RS for the conventional and hybrid joints present the tip of the cold lap interface (Figure 8a) and the tip of the adhesive layer (Figure 8b), respectively. During the welding process, alclad of the sheet surfaces in the CWZ is broken, and the broken alclad moves with the rotation of the welding tool forming the arc-shaped alclad flow trace. This flow trace is distributed in the upper and lower regions of the welded lap interface. For the hybrid joint, the adhesive and SZ material are stirred and then mixed in the CWZ. However, no black adhesive is observed in this region (Figure 6a), and this region is analyzed in detailed in the following. Through cross-sectional morphology analysis (Figures 5 and 6), the amount of the broken alclad for the hybrid joint is rather less, and the arched-shaped flow trace below the lap interface is almost non-exist compared with the conventional joint. Based on the above analysis, it can be inferred that the addition of the adhesive influences the flow of the material in the SZ of the hybrid joint.
Identifying combination of friction stir welding parameters to maximize strength of lap joints of AA2014-T6 aluminium alloy
Published in Australian Journal of Mechanical Engineering, 2019
C. Rajendran, K. Srinivasan, V. Balasubramanian, H. Balaji, P. Selvaraj
Salari et al. (2016) investigated the effect of FSLW parameters on microstructural characteristics and mechanical properties of AA5456 aluminium alloy plates with various thickness and temper conditions. Using four different types of pin tools (conical threaded, cylindrical-conical threaded, stepped conical thread pin and flared tri-flute pin), it was found that stepped conical thread pin produced homogeneous microstructure with finer grains than other tools. However, the effect of tool rotational speed and welding speed on the macrostructure and defect formation in FSLW joints of AA5456 aluminium alloy was investigated. Lower welding speed produced less hook height and higher welding speed produced kissing bond defect, while increased tool rotational speed results in increasing hooking region (Shirazi, Kheirandish, and Safarkhanian 2015). Zhengwei et al. (2016) analysed the effect of external stationary shoulder on joint appearance and lap shear strength of FSLW alclad 2024 aluminium alloy. Stationary shoulder was found to be more beneficial to the joint formation and avoids arc corrugation, flash and voids.