Tham khảo tài liệu 'handbook of materials for product design part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Chapter 2 Mechanical gouging can be done with rotary cutter machines designed for this purpose. Routers developed from woodworking tools are also used to shape aluminum. Aluminum can also be chemically milled usually with sodium hydroxide based or other alkaline solutions. A typical removal rate is in. mm per minute. Metal removal is controlled by masking duration of immersion and composition of the bath. Joining Welding Welding is the process of uniting parts by either heating applying pressure or both. Welding is like the little girl who when she was good was very very good and when she was bad was horrid. Improper welding can be awful while correctly designed and executed welds can solve problems intractable by other means. When heat is used to weld aluminum as is usually the case it reduces the strength of all tempers other than annealed material and this must be taken into account where strength is a consideration. Also welding aluminum is different from welding steel and most steel welding techniques are not transferable to aluminum. Aluminum s affinity for oxygen which quickly forms a thin hard oxide surface film has much to do with the welding process. This oxide is nearly as hard as diamonds attested to by the fact that aluminum oxide grit is often used for grinding. It has a much higher melting point than aluminum itself 3725 F 2050 C versus 1220 F 660 C so trying to weld aluminum without first removing the oxide melts the base metal long before the oxide. The oxide is also chemically stable fluxes to remove it require corrosive substances that can damage the base metal unless they are fully removed after welding. Finally the oxide is an electrical insulator and porous enough to retain moisture. For all these reasons the base metal must be carefully cleaned and wire brushed immediately before welding and the welding process must remove and prevent reformation of the oxide film during welding. The metal in the vicinity of a .
