材料工程基础课件：Chapter 4 The Joining of Metals

1、Chapter Five The Joining of Metals 1. Joining processes There are innumerable instances in which metals have to be joined to one another. In many cases joining is part of the production process, e.g. a component may be too complicated or too big to make in one piece and several simpler or smaller pa

2、rts may have to be joined together. Joining processes may be classified as follows1) Mechanical joining2) Adhesive bonding3) Soldering and brazing4) Welding 2. Mechanical joiningIncluding nuts, bolts, screws and pins, et al.3. Adhesives4. Soldering and brazing In soldering and brazing, although the

3、surfaces to be joined are heated the temperatures developed remain well below the melting point of either of the materials to be joined. However, local dimensional changes may cause distortion of the workpieces and result in residual stresses. If excessive heating is allowed to occur, then structura

4、l changes may take place in the materials.A filler alloy is used which has a much lower melting point than the fusion temperature of the materials being joined. If the filler alloy melts above 500oC, the joining process is called brazing (or hard soldering), while soldering (also called soft solderi

5、ng) processes take place below about 400oC. At these relatively low temperatures only a small amount of atomic diffusion is possible so that joint strengths are rather weak. However, brazed joints are significantly stronger (about 350-450N/mm2) in tension than soldered joints (about 50-60N/mmz) beca

6、use of the greater diffusion, and also because brazing alloys are stronger than solders. 5. Welding processes Welding processes may be divided into two main groups depending on whether or not pressure is used to bring the pieces to be joined together. In fusion welding without pressure, the faces of

7、 the two pieces of metal to be joined are heated and melted and extra molten metal from a filler rod may be added. The filler metal is of the same or similar composition to the metal being joined. In pressure welding, no filler metal is used. The union is achieved by bringing two clean surfaces toge

8、ther with sufficient pressure for a bond to be formed between them. In some pressure welding processes, a small amount of melting of the metal being joined occurs (e.g. in electric resistance welding), but in others no liquid metal is formed. The latter processes are termed solid phase welding. A fe

9、w solid phase welding processes are carried out at room temperature, but most involve some heating of the components to be joined (but not above the melting point). At the higher temperatures involved in fusion welding, there is greater atomic diffusion across the joined faces resulting in a more po

10、sitive bond than in pressure welding, but the risk of forming brittle intermetallic compounds (especially with certain combinations of metals) is greater. The high temperatures developed often cause unwanted structural changes in the regions of the parent metal adjacent to the weld pool-called the h

11、eat affected zone (H.A.Z.). Some basic forms of welded joints are shown (with associated terminology) in fig. 5.1, With increase in metal thickness, edge or joint preparation may be necessary in order to achieve satisfactory welds. Examples of simple edge preparation are also shown in fig. 5.1. Fig.

12、5.1 Type of welded joint and edge preparation 6. Fusion welding Fusion welding processes may be subdivided as shown in fig. 5.2. Fig.5.2 Fusion welding processes In gas welding (fig. 5.3), the required heat is produced by the controlled combustion of a mixture of oxygen and a fuel gas usually acetyl

13、ene (C2H2)-to give a high flame temperature (over 3000oC). The gases are fed into a torch and the relative proportions of oxygen and acetylene at the burner nozzle can be varied to give a flame that is neutral, reducing or oxidising. A neutral flame is the most common flame used in welding and is su

14、itable for joining most ferrous and some non-ferrous alloys; a slightly oxidising flame is used for copper and its alloys in order to prevent pick-up of hydrogen; while a slightly reducing flame is used for aluminium and its alloys to minimise oxidation. Fig 5.3 Gas welding The surfaces to be joined

15、 are brought to melting temperature and the rod melted rapidly to reduce distortion and overheating. The filler rod is held close to the work and the molten metal flows into the prepared joint between the pieces being welded. Since the edges of the work-pieces also melt, a strong continuous joint is

16、 formed. The gas welding flame is able to reduce oxide films present on the surface of low carbon steels, but in the case of other materials reactive fluxes are applied to the parent metal and filler rod before welding. The flux must be removed immediately after welding in order to prevent corrosion

17、 from occurring. In arc welding, the heat required to melt the surfaces to be joined is generated by striking an arc between an electrode and the workpiece. The high current low voltage discharge causes electrons to be transferred from the electrode via an ionized gas (known as plasma) to the workpi

18、ece, or alternatively a nonconsumable electrode may be used together with separate filler rod. Welding may be carried out manually or by the used of automatic machines. Manual arc welding is limited to relatively thin sections, while automatic machines use bigger, heavier electrodes which permit hig

19、her welding speeds so that thicker sections can be joined. In arc welding it is important that the weld pool be protected from the ingress of gases from the atmosphere and this may be achieved either by fluxes or by the use of inert gas. Arc Welding with Fluxes Manual welding using an arc struck bet

20、ween a flux-coated metal electrode and the workpiece (usually called manual metal arc welding) is the most widely used fusion welding process (fig.5.4). The metal electrode carries the current and also acts as a filler rod which deposits molten metal into the joint. Fig5.4 Metal arc welding The func

21、tions of the flux include the following: a) To provide a protective atmosphere to exclude air (the cellulose material burns to give an atmosphere of CO2 around the weld). b) To cover the weld with a protective layer of fusible slag which is easily detached from the finished weld. c) To help in stabi

22、lising the arc. d) To act as a deoxidiser and fluxing agent for impurities in the weld pool.Either a.c. or d.c. power supply may be used for metal arc welding, the choice depending on the metal being welded. Metal arc welding is much faster than gas welding and demands rather less skill from the ope

23、rative. The main limitation of manual metal arc welding is the relatively short length of electrode which can be comfortably handled by the operative. In the execution of long welds the electrode may have to be changed several times. Submerged arc welding (fig. 5.5) is a mechanised version of metal

24、arc welding. The arc is struck between a bare metal filler rod and clean parent metal under a blanket of granulated flux which is fed into the joint area just ahead of the electrode. When cold, the flux is non-conducting but, when molten, it is highly conducting and allows very high welding currents

25、 (more than 1000 A) to be used. The flux near the arc melts and forms a protective coating of slag which is easily detached from the finished weld. The very high current does not present a problem regarding ultra-violet light emission because the arc is not visible during the welding. High-quality w

26、elds of excellent penetration and free from surface ripple can be achieved by this process, which is used for welding pressure vessels, boilers and pipes. The advantages of the process are a high rate of deposition, no visible arc with little fume or spatter and a smooth weld surface for long length

27、sFig 5.5 Submerged arc welding Arc Welding Using Protective AtmospheresThis group of welding processes may subdivided into three types:1) Inert gas (argon) shielded arc process using a consumable electrode (metal inert gas or MIG process).2) Inert gas (argon) shielded arc process using a non-consumable (tungsten) electrode (tungsten inert gas or TIG process).3) CO2 gas shielded arc process using a c