Joining Processes

Joining processes differ from production processes like casting or forging which produces a single component in a single act. Complex configurations can be built by joining different members to yield the desired results.

Welding, brazing, soldering and adhesive bonding are the most frequently used joining processes. (i) Pressure welding. Solid state welding may be carried out at room temperature or at elevated temperature. The joint is formed owing to plastic flow (at the joint) due to applied pressure. In

pressure welding at high temperature, heat is generated due to resistance at the joint as flow of current is high. Other methods of achieving high temperature include friction, induction heating, impact

energy as in the case of explosive welding, seam welding, and projection welding. But welding is well known for resistance welding techniques.

(ii) Fusion (liquid state) welding. In fusion welding, the material around the joint is melted in both the parts to be joined. If necessary, a molten filler metal is added from a filler rod or from the electrode itself. The important zones in fusion welding are: (a) Fusion zone, (b) heat-affected

unmelted zone around the fusion zone, and (c) the unaffected original part. Important factors affecting fusion welding processes are:

1. Characteristics of heat source 2. Nature of weld pool

3. Heat flow from the joint

4. Chemical reactions in the fusion zone

5. Contraction, residual stresses and metallurgical changes.

(iii) Arc welding. Workpiece is positive and rod is negative; usually, 2/3 heat is at the anode due to impingement of high velocity electron. Temperature of arc is about 6000°C (see Fig. 5.31(a)).

(iv) Gas welding. Chemical heat source such as Acetylene (C₂H₂) is used with Oxygen to liberate 1.275 × 106 kJ/kg mole of heat. C₂H₂ in the presence of excess oxygen produces CO₂. In limited oxygen supply, CO is formed, instead of CO₂, and heat = 0.448 × 106 kJ.

(v) Thermit welding. A mixture of aluminium powder and Fe₃O₄ is used; 8Al + 3Fe₃O₄ = 2Fe + 4Al₂O₃ + dH

where dH = 0.24 × 106 kJ/k mol of atomic wt. of O₂. The temperature reached is 3000°C. (vi)

Soldering. The manual soldering method involves using a hard type soldering iron,

made of copper. It is heated to a temperature of 300°C and its tip is dipped in flux and tinned with solder. Then the iron is used for heating the base metal as well as for melting and spreading the solder.

(vii) Brazing. Brazing uses silver, copper etc. as the material. When the base metal is ferrous, copper enters grain boundaries (temperature 450°C), whereas in the case of gold, the

(Ag–Cu) braze metal acts as a filler between sandwich.

Design considerations. As soon as a decision is made to fabricate a product by welding, the next

step is to decide which welding process to use. This decision should be followed by selection of the types of joints, by determining the distribution and locations of the webs, and finally, by making the design of each joint. The following is a brief discussion of the factors to be considered in each design stage.

(i) Selection of the joint type. We have already discussed the various joint designs and realized that the type of joint depends on the thickness of the parts to be welded. In fact, there are other factors that should also affect the process of selecting a particular type of joint. For instance, the magnitude of the load to which the particular joint is going to be subjected during its service life is another important factor. The manner in which that load is applied (i.e. impact—steady or fluctuating) is another factor. While the square butt— simple-V, double-V and simple-U butt joint—is recommended for all loading conditions, the square-T joint is appropriate for carrying longitudinal shear under steady state

conditions. When severe longitudinal or transverse loads are anticipated, other types of joints (e.g. the single-level T the double-level T and the double J) have to be considered. In all cases, it is obvious that cost is a decisive factor, whenever there is a choice between two types of joints that would function equally well.

(ii) Location and distribution of welds. It has been found that the direction of the linear dimensions of the weld with respect to the direction of the applied load does have an effect on the strength of the weld. In fact, it has been experimentally proved that a weld whose linear direction is normal to the direction of the applied load (such as that shown in Fig. 5.31(b)) is 30 per cent stronger than a weld which is parallel to the direction of applied load. A product designer should, therefore, make use of this characteristic when planning the location and distribution of welds.

Fig. 5.31 Design of weld length for strength and stability.

Another important point that must always be considered is the prevention of any tendency of the welded elements to rotate when subjected to mechanical loads. A complete force analysis must be

carried out in order to determine properly the length of each weld. Let us now consider a practical example to examine the cause and the remedy for that tendency to rotate. Figure 5.31(b) indicates an L-angle, which is welded to a plate. Any load applied at the angle would pass through its centre of gravity. Therefore, the resisting forces that act through the welds would not be equal; the force closer to the centre of gravity of the angle would always be larger. Consequently, if any tendency to rotation is to be prevented, the weld which is closer to the centre of gravity must be longer than the other one. It is also recommended that very long welds be avoided. For example, it has been found that two small welds are much more effective than a single long weld.

(iii) Joint design. In addition to the procedures and rules adopted in the common design practice, there are some guidelines that apply to point design:

1. Try to ensure accessibility to the locations where welds are to be applied. 2. Try to avoid overhead welding.

3. Consider the heating effect on the base metal during welding operation. Balance the

welds to minimize distortion. Use short intermittent welds. Figure 5.32(a) indicates distortion caused by an unbalanced weld, whereas Figs. 5.32(b) and (c) provide the methods for reducing that

distortion.

Fig. 5.32 Avoiding distortion of welded component through balanced welding.

4. Avoid crevices around welds in tanks as well as grooves (and the like) that would allow dirt to accumulate. Failure to do so may result in corrosion in the welded joint.

5. Do not employ welding to joint steels with high hardenability.

6. Do not weld low-carbon steels and alloy steels by the conventional fusion-welding methods because they have different critical cooling rates and hence cannot be successfully welded.

7. When employing automatic welding (e.g. submerged arc), the conventional joint design of manual welding should be changed. Wider vees (for butt joints) are used, and singlepass welds replace multi-pass welds.

Design of brazed joints. For the proper design of brazed joints, two main factors have to be taken

into consideration: The first factor involves the mechanics of the process, i.e., the fact that the brazing filler metal flows through the joint by capillary action. The second factor is that the strength of the filler metal is poorer than that of the base metals. The product designer should take the following steps:

1. The filler metal is placed on one side of the joint and allocating a space for locating the filler metal before (or during) the process.

2. The joint clearance is so adjusted that it ensures optimum conditions during brazing. The clearance is dependent on the filler metal used and normally takes a value less than 0.005 in (0.125 mm), except

for aluminium, where it can go up to 0.025 in. (0.625 mm).

3. The distance to be travelled by the filler metal is shorter than the limit distance, as demanded/required by the physics of capillarity.

4. Enough filler metal is provided.

5. The arc of the joint is increased since the filler metal is weaker than the base metal.

There are actually three types of joint-area geometries, butt, skirt, and lap. The butt joint is the

weakest, while the lap is the strongest. Nevertheless, when designing lap joints, one should make sure that the joint overlap is more than 3t, where t is the thickness of the thinner parent metal. Examples of some good and poor practices in the design of brazed joint are shown in Fig. 5.33, as guidelines in product design.

Fig. 5.33 Design of brazed joints.

Adhesive bonding has gained popularity due to developments in polymer chemistry and is widely used now. Structural adhesives include one or more polymers. In the unhardened state, these

adhesives can take the form of viscous liquids or solids with softening temperatures of about 212°F (100°C). The unhardened adhesive agents are often soluble in ketones, esters and higher alcohols as well as in aromatic and chlorine hydrocarbons. Nevertheless, the hardened adhesives resist nearly all solvents. The following is a brief description of the adhesives which are commonly used in industry.

Epoxies. These are thermosetting polymers (see Chapter 7) which require the additions of a hardener

or the application of heat so that (hey can be cured. They are considered to be the best sticking agents because of their versatility, resistance to solvents, and ability to develop strong and reliable joints.

Phenolics. These are characterized by their low cost and heat resistance (up to about 930°F

(500°C)). They can be cured by a hardener, by heating, or used in solvents that evaporate, thus

enabling setting to occur. Like epoxies, phenolics are thermosetting polymers with good strength, but they generally suffer from brittleness.

Polymides. The polymide group of polymers is characterized by its oil and water resistance.

solvents in which they have been dissolved. Polymides are normally used as seam sealants and for other similar purposes. They are used as hot-melt for shoes.

Silicones. These adhesives can perform well at elevated temperatures. However, cost and strength

are the major limitations. Therefore, silicones are normally used as high-temperature sealants. Other adhesives that find industrial applications, in bonding two nonmetallic workpiece, include

cyanacrylates, acrylics and polyurethanes.

Joint preparation. The surfaces to be bonded must be clean and degreased, since most adhesives do

not amalgamate with fats, oils or wax. Joint preparation involves grinding with sandpaper, grinding, filing, sand blasting and pickling and degreasing with tri-chlorethylene. Oxide films, electroplating coats and varnish films need not be removed (as long as they are fixed to the surface). Roughening of the surface is advantageous if it is not overdone.

Joint design for adhesive joining. There are basically three types of adhesive-bonded joints. These

are shown in Fig. 5.34 and include tension, shear and peel joints. Most of the adhesives are weaker in peel and tension than in shear. Therefore, when selecting an adhesive, one should always keep in mind the types of stresses to which the joint is going to be subjected. It is also recommended that one avoid using tension and peel joints and change the design to replace these by shear joints, whenever possible.

Fig. 5.34 Design of adhesive joints and their loading. 5.3 WELDING PROCESSES

Welding processes are classified as shown in Fig. 5.35.