Achieving good results with

Adhesives

Many applications require no preparation. For

example, anaerobics needs no pretreatment unless

contamination is excessive. This is because the locking

or jamming mechanism of the coaxial joint plus

adhesion is sufficient to hold components together.

Similarly, cyanoacrylates are almost always used

without surface preparation. On rubber and plastic

they are capable of penetrating surface debris. The

toughened acrylic adhesives are even more hardy,

and some will actually bond through a film of oil (so

too will the heat cured epoxies). Therefore, surface

preparation can safely be omitted, or restricted to

simple degreasing, on a wide range of jobs.

However, where maximum performance is required,

or conditions are particularly severe, it is worth

spending time on surface preparation.

Preparation involves degreasing (to remove oil

grease and contaminants) and abrading or etching

(to increase surface grip).

Degreasing Best done with a chlorinated solvent,

except for sensitive materials.

Abrasion Metals are best abraded by grit blasting.

However, all the following give satisfactory results:

abrasive discs, belts, and cloths; medium-grit emery

paper; and wire brushes. Plastics, when bonded

with cyanoacrylates, generally need no abrading.

When using epoxy-based systems, abrade lightly

with abrasive discs, belts or cloths, or use mediumgrit

emery paper.

Chemical etching Some materials require chemical

treatment to ensure optimum performance.

After chemical etching, wear clean gloves to handle

materials; even a fingerprint can contaminate

the etched surface, and so weaken the bond.

Table 7.5 shows surface preparation for engineering

materials, which needs to be carried out in order to

maximize the performance of any adhesive.

The principles of good adhesive joint design

are shown in Figure 7.66. Note the following in

particular:

1 Do use an adequate overlap, as this gives a

stronger joint.

2 Do choose a rigid adherence where loads are

carried.

3 Do form joints from thick, rigid sections where

possible.

4 Do avoid butt joints.

5 Do refer to the manufacturer’s instructions for

specific adhesives.

Adhesives and the automotive

Industry

Non-structural adhesives

The primary function of non-structural adhesives is

to attach one material to another without carrying

functional loads on the components. Loads are

usually light, particularly for those used inside

vehicles. Some interior applications include fabric

on trim and door panels, headlinings, carpets, and

sound deadening panels. Exterior applications

include moulding, wood grain, striping, and vinyl

roof. The materials used for these bonds include

pressure sensitive tapes, hot melts, epoxy spray,

vinyl plastisol, elastomer solvent, silicon, solventbased

rubber (used for large gap sealing) and butyl

rubber (formerly used to mount windscreens).

Hot melts

The advantages of hot melt adhesives and sealants

are their ability to bond to both pervious and impervious

substrates and the speed with which the ultimate

bond strength is attained, the latter being

particularly valuable in high-rate production. There

are, however, several other significant advantages in

these solvent-free substances. With other types of

adhesive, full bond strength is not achieved until all

the solvent or liquid carrier has evaporated. A hot

Methods of joining 229

Table 7.5Surface preparation materials (Permabond Adhesives Ltd )

Abrasion or chemical

Material Cleaning treatment Procedure

ABS acrylonitrile Degrease with detergent Etch in solution of (parts Immerse for up to 15 min

butadiene solution, except for by weight) Water 30 at room temperature

styrene) plastics cyanoacrylates when Conc. sulphuric acid Wash with clean, cold

cleaning and other (SG 1.84) 10 water, followed by clean,

preparations are Potassium dichromate or hot water

probably unnecessary sodium dichromate 1 Dry with hot air

Add acid to 60% of the

water, stir in sodium

dichromate and add

remaining water

Add acid to water, never

vice versa

Aluminium and alloys Degrease with solvent Etch in a dichromate Heat solution to

solution 68 °C _ 3 °C

Prepare as for ABS Immerse for 10 min

Rinse thoroughly in cold,

running distilled (or

deionized) water

Air dry, oven dry or use

infrared lamps at not over

66 °C for about 10 min

Treated aluminium should

be bonded as soon as

possible and never be

exposed to the atmosphere

of a plating shop. Even

brief exposure reduces

bonding strength

Care should be taken in

handling as the surfaces

are easily damaged

Bonding surfaces should

not be touched (even

with gloves) or wiped with

cloths or paper

Cellulose plastics Degrease with methyl Roughen surface with Repeat degreasing

alcohol or isopropyl fine grit emery paper If using epoxies, heat

alcohol plastics for 1 h at 95 °C

and apply adhesive while

still warm

NB Follow manufacturer’s

instructions to avoid

premature curing of

epoxy adhesives

Ceramics, porcelain Degrease in vapour bath, Use emery paper or Repeat degreasing

and glazed china or dip in solvent sandblast to remove Let solvent evaporate

ceramic glaze completely before

applying adhesive

230Repair of Vehicle Bodies

Table 7.5(Continued)

Abrasion or chemical

Material Cleaning treatment Procedure

Diallylphthalate Degrease with solvent, Abrade with medium grit emery Repeat degreasing

plastics unless using paper

cyanoacrylates

Epoxy plastics Degrease with solvent Abrade with medium Repeat degreasing

grit paper

Expanded plastic Do not use solvent Roughen surface with emery Remove all dust and

(foams, etc.) paper contaminants

Furane plastics Degrease with solvent Abrade with medium grit Repeat degreasing

emery paper

Glass and quartz Degrease with solvent Etch in solution of (parts Immerse for 10–15 min at

(non-optical) by weight) 23 °C _ 1 °C

Distilled water 4 Rinse thoroughly with

Chromium trioxide 1 distilled water

Or, use a silane primer in Dry for 30 min at

accordance with the 98 °C _ 1 °C

manufacturer’s instructions Apply adhesive while glass

or quartz is hot

Glass reinforced Degrease with solvent Abrade with medium grit Repeat degreasing

polyesters (GRP) emery paper

Graphite Degrease with solvent Abrade with fine grit emery Repeat degreasing

paper Allow graphite to stand to

ensure complete

evaporation of solvent

Iron (cast iron) Degrease with solvent Grit blast or abrade with Repeat degreasing

emery paper

Melamine and Degrease with solvent Abrade with medium grit Repeat degreasing

melamine faced emery paper

laminates including

Formica,Warite, etc.

Nickel Degrease with solvent Immerse for 5 s in conc. nitric Rinse thoroughly in cold

acid solution (SG 1.41) running distilled (or

at 25 °C deionized) water

Dry with hot air

Nylon Degrease with solvent Roughen the surface with Repeat degreasing

medium grit emery paper

Paper laminates Degrease with solvent Abrade with fine grit emery Repeat degreasing

paper

Paper (unwaxed) Do not use solvent Requires no treatment before –

bonding

Phenolic, polyester and Degrease with solvent Abrade with medium grit Repeat degreasing

polyurethane resins emery paper

Polyacetals Degrease with detergent Etch in solution of (parts by Immerse for 5 min at room

solution weight) temperature

Water 33.0 Wash with clean cold

Conc. sulphuric acid water followed by clean

(SG 1.84) 184.0 hot water

Potassium dichromate or Dry with hot air

sodium dichromate 1.43

Methods of joining 231

Table 7.5(Continued)

Abrasion or chemical

Material Cleaning treatment Procedure

Add acid to water, never

vice versa

Polycarbonate, Degrease with methyl Abrade with medium grit Repeat degreasing

polymethylmethacrylate alcohol or isopropyl emery paper alcohol

(acrylic) and polystyrene

Polyester plastics Degrease with solvent Roughen with emery cloth After abrasion, repeat

except when using cloth or etch in sodium degreasing

sensitive materials hydroxide solution After etching wash thoroughly

which require (20% by weight) for in cold running distilled

detergent 2–10 min. at 70–95 °C (or deionized) water

Figure 7.66Adhesive joint design (Permabond Adhesives Ltd )

melt adhesive, on the other hand, relies on mechanical

keying to the surface roughness of the substrate

surfaces, and the adhesive’s chemical affinity with

the substrate. A good hot melt adhesive or sealant

must exhibit six properties: high physical strength;

a degree of flexibility to cater for differential movements

in joints and vibrational stresses; good specific

adhesion; a melt temperature well above maximum

232Repair of Vehicle Bodies

service temperature; a low viscosity at the application

temperature; and a good substrate wetting.

Apart from the polyamide and EVA-based products,

there are a number of other hot melt products

that are useful to the car industry. The thermoplastic

rubber-based materials have many of the properties

of vulcanized rubber systems, yet can be applied as

hot melts; they are especially useful in trim applications

as derived pressure-sensitive adhesives.

Where sealing is most important, such as to prevent

moisture penetration into spot-welded seams, fairly

soft butyl-based materials may be used; these do not

liberate corrosive hydrochloric acid as a decomposition

product when welded through. Another

development is the hot melt systems that cure after

application, either by reaction with moisture or by

the application of heat.

For assembling trim components at the end of an

assembly line, hand or robot guns can be employed.

For continuous rapid assembly, wheel or roller applicators

dipping into reservoirs of the heated adhesive

can be most effective. Machines with one or more

applicator heads, and developed for high rates of

production, are now available. Hot melt adhesives

may even be sprayed. There is also available a hot

melt adhesive in the form of a strip about 0.5 mm

thick, impregnated in a synthetic fabric scrim made

up into a composite tape backed with aluminium

foil. This has been used to join pieces of carpet

butted together. The heat is generated by passing an

electric current through the foil. Light rolling ensures

that wetting is uniform over the whole area.

Toughened structural adhesives

These materials span both the acrylic and epoxy technologies,

where a specific technique is used to prevent

catastrophic crack propagation when joints are

overloaded. Improved performance of the adhesive

film is brought about by the introduction of a rubbery

distortable phase within the load-bearing matrix of

the body of the adhesive. It is this physically separate

but chemically linked zone which absorbs fracture

energy and prevents crack propagation. In this manner

the resistance of the adhesive to catastrophic failure

is enhanced considerably, and such adhesive

compositions show a marked resistance to peel,

cleavage and impact forces (see Figure 7.67).

Since the introduction of high-performance toughened

structural adhesives, many designers and

manufacturers are turning to bonded structures in

order to reap the economic and technical benefits

given by this technology. Materials used for vehicle

manufacture are often chosen for strength,

rigidity or light weight, and in some cases surfaces

will be prefinished to enhance appearance. Bonding

will not only overcome some of the assembly

problems which such materials often present, but

will give a stiffer and stronger structure than

could be obtained from either riveting or welding.

In the correct circumstances, satisfactory bonds

can be made on many metallic and non-metallic

materials.

Adhesives may be used alone; or in a secondary

role to supplement welding, brazing or riveting;

or in a primary role, complemented by welds

or rivets. Weld bonding and rivet bonding are

already standard procedures in many types of

vehicle construction. Intermittent spot welding or

riveting is a useful technique for pinning components

during periods when the adhesive is

uncured or temporarily softened from passage

through a stoving oven.

A number of car manufacturers are already

researching the possibility of a totally bonded car,

especially those who want to use plastic or alloy

bodies. If motor engineers are to build extremely

lightweight vehicles, they will have to consider the

application of adhesive engineering. Motor vehicle

manufacturers are now extensively using highperformance

epoxides (often toughened) to supplement

welded joints in their vehicles. This has

resulted in a major reduction in the number of welds

required and the gauge of metal needed, and at the

same time has increased body stiffness and reduced

corrosion.

Figure 7.67Toughened adhesive: when overloaded,

crack propagation is stopped by the dispersed

rubbery phase (Permabond Adhesives Ltd )

Methods of joining 233

Typical applications of structural adhesives

include the following:

Toughened acrylic Aluminium or steel fitments

bonded to GRP roof sections; patch repairs to metal

or plastic panels; internal steel fixtures bonded to

steel.

Toughened epoxy Aluminium floor sections

bonded to wood; inner and outer door skins bonded

together.

Cyanoacrylates Rubber seals and weather strips.

Adhesives used in vehicle body repair

Weatherstrip adhesive Bonding rubber weatherstripping

to door shuts, boot lid.

Fast tack adhesive Repairs to trim fabrics, headlinings

and carpets.

Disc pad adhesive Bonding paper discs to backup

pads.

Tape adhesive Trim applications.

Adhesive/sealant (polyurethane-based compound)

Structural bonding and sealing of replacement

windscreen.

Questions

1 Sketch four types of solid rivet.

2 What is meant by riveting allowance and rivet pitch?

3 Name three types of metals used in making solid

rivets, and state a property which they all have in

common.

4 Sketch and name the type of joint that would be

used in solid riveting.

5 Name, and sketch, the type of rivet required

when fastening a panel to a frame; where the

riveting is only possible from one side.

6 Describe four types of blind rivets and their

placing procedure.

7 Explain the types of materials which are used

in the manufacture of blind rivets.

8 Explain the use of a blind rivet nut.

9 Explain the advantages of the Hemlok system

of structural fasteners.

10 Describe the advantages of the Monobolt system

of structural fasteners.

11 What is meant by a permanent fastening method?

12 Describe the difference between a set screw and

a bolt.

13 Explain what is meant by a self-tapping screw.

14 Explain the advantages of the Taptite screw

system.

15 Describe the use of a screw nail in coachwork.

16 What purpose do nuts and bolts serve in vehicle

body work?

17 List, and describe, four types of self-locking nut.

18 Explain the principle of the Spire speed nut and

state where you would expect to find them on a

vehicle body.

19 Explain the use of captive nuts in vehicle assembly.

20 What is the function of a spring washer when

used as a securing device?

21 Name three types of plastic trim panel fasteners.

22 Sketch a pull-on panel fastener used in trim work.

23 Name the classified groups of adhesives.

24 Explain how an adhesive can be selected for use.

25 Explain what is meant by a non-structural

adhesive.

26 Describe the importance of toughened structural

adhesives.

27 List the applications where toughened adhesives

would be used.

28 Explain how crack propagation is achieved in

structural adhesives.

29 List the types of adhesive that could be used by

the body repairer.

30 What type of adhesive will resist heat, water and

acid, and is used to join metal?

31 What purpose do nuts and bolts serve in vehicle

body repair work?

32 List and describe four types of self-locking nuts.

33 What is the purpose of a spring washer when

used with a nut and bolt?

34 With the aid of a sketch, illustrate and name

a type of self-securing joint.

35 Give the advantages of blind riveting when

compared with solid riveting.

36 With the aid of a sketch, name a type of joint that

would be used when solid riveting.

234Repair of Vehicle Bodies

37 Explain where quick-release fasteners would be

used on a vehicle body.

38 Explain the principle of the Nyloc and Cleveloc

nuts.

39 Name three types of blind riveting tools that could

be used in assembly work.

40 Explain the purpose of using coach screws in

vehicle body assembly.

41 Describe two ways of locating a bonnet stay.

42 How are two bars safely attached to a vehicle

body?

43 How can you ensure that road wheels are fitted

securely?

44 Why are cross-head screws popular?

45 State one advantage of self-tapping screws.

Soft and hard

soldering methods

8.1 Comparison of fusion and non-fusion

jointing processes

The jointing of metals by processes employing

fusion of some kind may be classified as follows:

Total fusion

Temperature range: 1130–1550 °C approximately.

Processes: oxy-acetylene welding, manual metal arc

welding, inert gas metal arc welding.

Skin fusion

Temperature range: 620–950 °C approximately.

Processes: flame brazing, silver soldering, aluminium

brazing, bronze welding.

Surface fusion

Temperature range: 183–310 °C approximately.

Process: soft soldering.

In total fusion the parent metal and, if used, the

filler metal are completely melted during the jointing.

Thin sheet metal edges can be fused together

without additional filler metal being added. Oxyacetylene

welding and manual metal arc welding

were the first processes to employ total fusion. In

recent years they have been supplemented by

methods such as inert-gas arc welding, metal inertgas

(MIG/MAG) and tungsten inert-gas (TIG)

welding, carbon dioxide welding and atomic

hydrogen welding. Welding is normally carried out

at high temperature ranges, the actual temperature

being the melting point of the particular metal

which is being joined. The parent metal is totally

melted throughout its thickness, and in some cases

molten filler metal of the correct composition is

added by means of rods or consumable electrodes

of convenient size. A neat reinforcement weld bead

is usually left protruding above the surface of the

parent metal, as this gives good mechanical properties

in the completed weld. Most metals and alloys

can be welded effectively, but there are certain

exceptions which, because of their physical properties,

are best joined by alternative methods.

In skin fusion the skin or surface grain structure

only of the parent metal is fused to allow the

molten filler metal to form an alloy with the parent

metal. Hard solders are used in this process, and, as

these have greater shear strength than tensile

strength, the tensile strength of the joint must be

increased by increasing the total surface area

between the metals. The simplest method of achieving

this is by using a lapped joint in which the

molten metal flows between the adjoining surfaces.

The strength of the joint will be dependent upon the

wetted area between the parts to be joined. Skin

fusion has several advantages. First, since the filler

metals used in these processes have melting points

lower than the parent metal to which they are being

applied, a lower level of heat is needed than in total

fusion and in consequence distortion is reduced.

Second, dissimilar metals can be joined by applying

the correct amount of heat to each parent metal,

when the skins of both will form an alloy with the

molten hard solder. Third, since only the skin of the

parent metal is fused, a capillary gap is formed in

the lap joint and the molten filler metal is drawn

into the space between the surfaces of the metals.

In surface fusion the depth of penetration of the

molten solder into the surfaces to be joined is so

shallow that it forms an intermetallic layer which

bonds the surfaces together. The process employs

soft solders, which are composed mainly of lead

and tin. As these also have a low resistance to a tensile

pulling force, the joint design must be similar

to that of the skin fusion process, i.e. a lapped joint.

236Repair of Vehicle Bodies

This chapter covers skin and surface fusion

methods; Chapters 9–12 deal with total fusion.

8.2 Soft and hard solders

In spite of the growing use of welding, the techniques

of soldering remain one of the most familiar

in the fabrication of sheet metal articles, and the art

of soldering still continues to occupy an important

place in the workshop. While soldering is comparatively

simple, it requires care and skill and can

only be learnt by actual experience.

Soldering and brazing are methods of joining

components by lapping them together and using a

low-melting point alloy so that the parent material is

not melted. Soldering as a means of joining metal

sheets has the advantage of simplicity in apparatus

and manipulation, and with suitable modifications it

can be applied to practically all commercial metals.

8.3 Soft soldering

The mechanical strength of soft soldered sheet metal

joints is usually in the order of 15–30 MN/m2, and

depends largely upon the nature of the solder used;

the temperature at which the soldering is done; the

depth of penetration of the solder, which in turn

depends on capillary attraction, i.e. on the power of

the heated surface to draw liquid metal through itself

(Figure 8.1); the proper use of correctly designed

soldering tools; the use of suitable fluxes; the speed

of soldering; and, especially, workmanship.

metals and its lowest melting point is 183 °C; this

melting point may be raised by varying the percentage

of lead or tin in the alloy (see Table 8.1). A

small quantity of antimony is sometimes used in

soft solder with a view to increasing its tenacity

and improving its appearance by brightening the

colour. The small percentage of antimony both

improves the chemical properties of the solder and

increases its tensile strength, without appreciably

affecting its melting point or working properties.

There is a great variety of solders, e.g. aluminium,

bismuth, cadmium, silver, gold, pewterer’s,

plumber’s, tinman’s; solders are usually named

according to the purpose for which they are

intended. The following solders are the most popular

in use today:

95–100 per cent tin solder, is used for high-quality

electrical work where maximum electrical conductivity

is required, since the conductivity of pure

tin is 20–40 per cent higher than that of the most

commonly used solders.

60/39.5/0.5 (tin/lead/antimony) solder, the eutectic

composition, has the lowest melting point of all

tin–lead solders, and is quick setting. It also has

the maximum bulk strength of all tin–lead solders,

and is used for fine electrical and tinsmith’s work.

50/47.5/2.5 solder, called tinman’s fine, contains

more lead and is therefore cheaper than the 60/40

grade. Its properties in terms of low melting range

and quick setting are still adequate, and hence it is

used for general applications.

45/52.5/2.5 solder, known as tinman’s soft, is

cheaper because of the higher lead content, but has

poorer wetting and mechanical properties. This solder

is widely used for can soldering, where maximum

economy is required, and for any material

which has already been tin plated so that the inferior

wetting properties of the solder are not critical.

30/68.5/1.5 solder, known as plumber’s solder, is

also extensively used by the car body repairer.

Because the material has a wide liquidus–solidus

range (about 80 °C), it remains in a pasty form for

an appreciable time during cooling, and while in

this condition it can be shaped or ‘wiped’ to form a

lead pipe joint, or to the shape required for filling

dents in a car body. Because of its high lead content,

its wetting properties are very inferior and the

surfaces usually have to be tinned with a solder of

higher tin content first.

Figure 8.1Capillary attraction through a soldered

lapped joint

Solders

Soft solder is an alloy of lead and tin, and is used

with the aid of a soldering flux. It is made from

two base metals, tin and lead. Tin has a melting

point of 232 °C and lead 327 °C, but the alloy has

a lower melting point than either of the two base

Soft and hard soldering methods 237

Fluxes

The function of a flux is to remove oxides and

tarnish from the metal to be joined so that the

solder will flow, penetrate and bond to the metal

surface, forming a good strong soldered joint.

The hotter the metal, the more rapidly the oxide

film forms. Without the chemical action of the

flux on the metal the solder would not tin the surface,

and the joint would be weak and unreliable.

As well as cleaning the metal, flux also ensures

that no further oxidation from the atmosphere

which could be harmful to the joint takes place

during soldering, as this would restrict the flow

of soldering.

Generally, soft soldering fluxes (see Table 8.1)

are divided into two main classes: corrosive fluxes

and non-corrosive fluxes.

Corrosive fluxes

These are usually based on an acid preparation,

which gives the fluxes their corrosive effect. They

are very effective in joining most metals. If the

flux is not completely removed after use, corrosion

is set up in and around the joint, and the risk of this

happening prevents the use of these fluxes in electrical

trades and food industries.

The following substances are corrosive fluxes:

Zinc chloride (killed spirits) This is made by dissolving

pure zinc in hydrochloric acid until no more

zinc will dissolve in the acid. This changes the acid

into zinc chloride – hence the name killed spirits.

As an all-round flux for soft soldering zinc chloride

is without equal, but it has one disadvantage for

some purposes – its corrosive action if the joint is

not afterwards cleaned thoroughly with water.

Hydrochloric acid Although hydrochloric acid is

not a good substitute for zinc chloride, it is nevertheless

used with excellent results on zinc and galvanized

iron. It can be used neat, but it is better to

dilute it with at least 50 per cent zinc chloride.

Ammonium chloride (salammoniac) This may be

used as a solution in water in much the same way

as zinc chloride, but is not quite so effective for

cleaning the metal.

Phosphoric acid This is effective as a flux for

stainless steel, copper and brass, and does not have

the corrosive effect of other acid types of flux.

Non-corrosive fluxes

These prevent oxidation on a clean or bright metallic

surface during soldering. In general non-corrosive

fluxes are not so active in cleaning the metal and

Table 8.1Soft solders, fluxes, and their method of application for different sheet-metals

Solder constituents (%)

Sheet metal Flux Tin Lead Other Used with

Aluminium Stearin 85 – Zn8, Al7 Soldering bit or blowpipe

Brass Zinc chloride or resin 65 34 Sbl Soldering bit

Copper Zine chloride, ammonium chloride or resin 65 34 Sbl Soldering bit

Galvanized steel Dilute hydrochloric acid 50 50 – Soldering bit

Lead Tallow or resin 40 60 – Soldering bit or blowpipe

Monel Zinc chloride 66 34 – Soldering bit or blowpipe

Nickel Zinc chloride 67 33 – Soldering bit

Pewter Olive oil, resin or tallow 25 25 Bi5o Soldering bit or blowpipe

Silver Zinc chloride 70 30 – Soldering bit

Stainless steel Phosphoric acid _ zinc chloride 66 34 – Soldering bit

Terne steel Zinc chloride 50 50 – Soldering bit

Tin plate Zinc chloride 60 40 – Soldering bit

Tinned steel Zinc chloride 60 40 – Soldering bit

Zinc Zinc chloride or hydrochloric acid 50 50 – Soldering bit

Iron Zinc chloride or ammonium chloride 50 50 – Soldering bit

Steel Zinc chloride or ammonium chloride 60 40 – Soldering bit

238Repair of Vehicle Bodies

serve chiefly as a measure of protection against further

oxidation, when the material is hot. Therefore

these fluxes should only be used when the metal has

been cleaned prior to soldering.

The following substances are non-corrosive

fluxes:

Resin and linseed oil Resin, finely powdered and

dissolved in linseed oil, forms a good flux where

non-corrosion is important. It is necessary that the

parts to be soldered should be quite clean before

applying oil and resin as a flux.

Tallow and palm oil Tallow and palm oil is often

used as a flux for soldering lead. The surface of the

lead must be first scraped clean before the flux is

applied and the joint soldered. Tallow is a popular

flux with plumbers for the purpose of wiping joints

in lead pipes.

Olive oil A thin oil such as olive oil is generally

used as a flux for soldering pewter. The soldering

temperature for pewter is rather lower than for

most soldering operations, hence the use of a thinner

oil as a flux. As a flux, olive oil is sometimes

called Gallipoli oil.

Soldering tools

Soldering irons are made in different sizes and

shapes. The head is nearly always made of copper,

although for soldering aluminium a nickel bit is

necessary. Electrolytic copper is the best as it gives

longer life and holds the solder well on the working

faces, and forged soldering bits are far better

heat retainers than cast copper bits as they are less

liable to crack at the tips. A small soldering tool is

not suitable for soldering any heavy or comparatively

large sheet metal articles, because the heat

loss by conduction is too fast to allow an even temperature

to result which will allow the solder to

flow freely and sweat into the seams or joints.

Soldering tools heated by gas, electricity and oxyacetylene

are now available to speed up the process

of soldering in mass production work.

Figure 8.2 shows a selection of soldering bits.

Soft soldering process

Soldering is a process of joining two lapped metal

surfaces together by fusing another metal or alloy

of a lower melting point in between them in such a

way that the melted metal bonds firmly to the other Figure 8.2Soldering bits

Soft and hard soldering methods 239

two. A soldering iron or copper bit is usually used

to apply the solder, although sometimes a blowpipe

is used to sweat the solder into the joint. A liquid

flux such as zinc chloride or resin or linseed oil is

generally used to assist the solder to flow and run

smoothly into the joint (Figure 8.3).

recleaning the bit, and hence unnecessary wear

and a shorter service life. When the flame turns

green around the soldering bit it is at the correct

temperature; if the colour is allowed to change

into a bright green the copper bit will begin to

become red hot. With an electric soldering iron the

heat is automatically controlled at the right temperature

because the tool has a built-in thermostat

for controlling the temperature.

Tin the soldering iron Dip the heated copper bit

into flux to obtain a complete coating of flux on

the surface faces of the bit, then rub the fluxed portion

of the bit on a piece of solder to obtain a film

of solder over the copper surface or working faces

of the soldering iron. This operation is referred to

as tinning the bit, and makes it easier for the bit to

pick up solder and then discharge it on the workpiece.

Tinning also protects the bit against further

oxidation, thus increasing its life.

Clean the surface of the workpiece All metals

have a covering of oxide on their surface, although

it may not be visible to the naked eye, and this

oxide film will prevent the solder from bonding to

the metal to be soldered and therefore create a

weak joint. First clean the surface to a bright finish

with coarse emery paper or steel wool, then

immediately apply the flux. The flux helps to

clean the surface chemically so that the molten

solder can flow and penetrate into the metal, forming

a strong joint. Also it prevents oxides reforming

on the surface of the work as the soldering is

carried out.

Reheat the bit until a green flame forms around

it, again taking care not to overheat it and destroy

the tinning on its surface. Dip the bit into flux,

then hold the tinned face of the bit against the solder

stick until all the face that is tinned is covered

with molten solder. The soldering iron is now

ready for use.

Apply the soldering iron loaded with solder to

the face of the workpiece or joint which has previously

been smeared with flux. The metal surrounding

the iron is heated to the melting point of

the solder by conduction of heat from the soldering

iron, and the solder will start to flow. Draw

the iron slowly along the face of the joint, allowing

solder from the bit to flow into the joint. A

good joint has only a very thin film of solder, as

too much solder weakens the joint. The length of

joint that can be soldered before the bit needs

Figure 8.3Soldering process

The soldering process comprises the following

steps:

Choose the right materials The choice of the

soldering iron and its shape is governed by the size

and accessibility of the material to be soldered; in

general always use as large a soldering iron as is

practicable. Some means of heating the soldering

iron is necessary, together with a quantity of selected

solder, file, emery cloth and a tin of flux, either

corrosive or non-corrosive to suit the work in hand.

Clean the soldering iron Solder will not adhere

to or bond to a dirty or greasy soldering iron, and

whether the iron is new or old it must be clean and

bright on its working surfaces, i.e. approximately

20 mm up from the point on each face. An emery

cloth could be used for this purpose, but generally

a file is preferred. It is important to see that just

sufficient copper is removed to get rid of the pitting

or scale and leave a clean bright surface.

Heat the soldering iron A clean flame such as

gas is best for this purpose. Care must be taken

not to allow the bit to become red hot, as overheating

of the bit causes heavy scaling of the surface

due to oxidation; this will mean refiling and

240Repair of Vehicle Bodies

recharging with solder depends upon the size of

the bit, its temperature and the size of the job to

be soldered.

The most important points in soft soldering are:

1 A perfectly cleaned joint.

2 A soldering iron that has been tinned and

heated to the correct temperature.

3 The correct flux for the particular job in hand.

4 A good quality tin–lead solder.

5 Allow the heat from the soldering iron to penetrate

into the metal before moving the iron

along the joint; this will give the solder a

chance to flow into the joint.

6 The correct type of joint must be used. Soft

soldering can only be used on metals that are

lapped one over the other to form the joint.

This allows for capillary attraction of the solder

(Figure 8.4).

8.4 Hard soldering

Brazing

Brazing is used extensively throughout the panel

beating trade as a quick means of joining sheet

metal panels and other automobile parts. Although

a brazed joint is not as strong as a fusion weld, it

has many advantages which make it useful to the

panel beater. Brazing is not classed as a fusion

process, and therefore cannot be called welding,

because the parent metals are not melted to form

the joint but rely on a filler material of a different

metal of low melting point which is drawn through

the joint. The parent metals can be similar or dissimilar

as long as the alloy rod has a lower melting

point than either of them. The most commonly

used alloy is of copper and zinc, which is, of

course, brass. Brazing is accomplished by heating

the pieces to be joined to a temperature higher than

the melting point of the brazing alloy (brass). With

the aid of flux, the melted alloy flows between the

parts to be joined due to capillary attraction, and

actually diffuses into the surface of the metal, so

that a strong joint is produced when the alloy

cools. Brazing, or hard soldering to give it its

proper name, is in fact part fusion and is classed as

a skin fusion process.

Brazing is carried out at a much higher temperature

than that required for the soft soldering

process. A borax type of powder flux is used,

which fuses to allow brazing to take place between

750 and 900 °C. There are a wide variety of alloys

in use as brazing rods; the most popular compositions

contain copper in the ranges 46–50 and

58.5–61.5 per cent, the remaining percentage being

zinc (Table 8.2).

The brazing process comprises the following

steps:

1 Thoroughly clean the metal to be joined.

2 Using a welding torch, heat the metals to a

temperature below their own critical or melting

temperature. In the case of steel the metal is

heated to a dull cherry red.

3 Apply borax flux either to the rod or to the

work as the brazing proceeds, to reduce oxidation

and to float the oxides to the surface.

4 Use the oxy-acetylene torch with a neutral

flame, as this will give good results under normal

conditions. An oxidizing flame used for

Figure 8.4Types of soft soldered joints

The strength of a soft soldered joint is not governed

by the amount of solder between the plates.

The more solder, the weaker the joint; therefore a

good joint has only a very thin film of solder

between the metal plates, which forms a surface

alloy by using the tin in the solder.

Soft and hard soldering methods 241

Table 8.2Copper-phosphorus brazing alloys

BS Nominal composition (wt%) Melting Tensile

1845 range strength Elongation Hardness

ref. Cu Zn Mn Ni P Bi Si Sn Others (°C) (N/mm2) (%) (HV) Notes

Ag

CP1 Balance – – 14–15 4.3–5.0 – – – 645–800 670–700 10 187 Fluxless brazing

For details High strength,

of impurities good ductility

CP2 Balance – – 1.8–2.2 6.1–6.9 – – – see British 645–825 490–560 5 195 Good strength

CP3 Balance – – – 7.0–7.8 – – – Standards 710–810 490–550 7 192 Good strength

CP4 Balance – – 4.5–5.5 5.7–6.3 – – – 645–815 490–530 7 192 Good strength

CP5 Balance – – – 5.6–6.4 – – – 690–825

CP6 Balance – – – 5.9–6.5 – – – 710–890 _

242Repair of Vehicle Bodies

materials having a high percentage of brass

content will produce a rough-looking brazed

joint, which nevertheless is slightly stronger

than if brazed with a neutral flame.

5 Use only a small amount of brazing rod; if too

much is used this weakens the joint.

6 The two pieces of material to be brazed must

be either lapped or carefully butted after edge

preparation and must fit tightly together during

the brazing operation (Figure 8.5). Iron, steel,

copper and brass are readily brazed, and metals

of a dissimilar nature can also be joined.

Typical examples are as follows:

Copper to brass

Copper to steel

Brass to steel

Cast iron to mild steel

Stainless steel to mild steel.

Also, coated materials like zinc-plated mild

steel can be better brazed than welded.

7 Carefully select the types of metal to be joined.

Although dissimilar metals can be joined by

hard or soft soldering, corrosion may occur due

to the electrolytic action between the two dissimilar

metals in the presence of moisture. This

action is an electrochemical action similar to

that of an electric cell, and results in one or

other of the two metals being corroded away.

The main advantages of brazing are:

1 The relatively low temperature (750–900 °C)

necessary for a successful brazing job reduces

the risk of distortion.

2 The joint can be made quickly and neatly,

requiring very little cleaning up.

3 Brazing makes possible the joining of two dissimilar

metals; for example, brass can be joined

to steel.

4 It can be used to repair parts that have to be

rechromed. For instance, a chromed trim

moulding which has been deeply scratched can

be readily filled with brazing and then filed up

ready for chroming.

5 Brazing is very useful for joining steels which

have a high carbon content, or broken cast iron

castings where the correct filler rod is not

available.

Silver soldering

Silver solder probably originated in the manufacture

and repair of silverware and jewellery for the

purpose of securing adequate strength and the

desired colour of the joint, but the technique of

joining sheet metal products and components with

silver solder has come into wide usage in the automobile

industry. The term ‘soft soldering’ has been

widely adopted when referring to the older process

to avoid confusion with the newer hard soldering

process, known generally as either silver soldering

or silver brazing. The use of silver solder on metals

and alloys other than silver has grown largely

because of the perfection by manufacturers of

these solders which makes them easily applicable

to many metals and alloys by means of the oxyacetylene

welding torch. This process is employed

Figure 8.5Joint design for brazing, showing the brazing equivalents to welding

Soft and hard soldering methods 243

for joining metal parts when greater strength is

required than can be obtained by soft soldering,

when the parts have to withstand a temperature that

would cause soft solder to melt, and in cases where

the high temperature developed by welding would

seriously distort the metal parts. Vehicle parts

which are manufactured from light-gauge sheet

brass, stainless steel, very thin mild steel, sheet

products or components fabricated from nickel,

bronze or copper, can be very effectively joined by

silver soldering.

Solders and fluxes

Silver solders are more malleable and ductile than

brazing rods, and hence joints made with silver

solder have a greater resistance to bending stresses,

shocks and vibration than those made with ordinary

brazing alloys. Silver solders are made in

strip, wire (rod) or granular form and in a number

of different grades of fusibility, the melting points

varying between 630 and 800 °C according to the

percentages of silver, copper, zinc and cadmium

they contain (Table 8.3).

As in all non-fusion processes the important factor

is that the joint to be soldered must be perfectly

clean. Hence special care must be taken in preparing

the metal surfaces to be joined with silver solder.

Although fluxes will dissolve films of oxide

during the soldering operation, sheet metal that is

clean is much more likely to make a stronger,

sounder joint than when impurities are present.

The joints should fit closely and the parts must be

held together firmly while being silver soldered,

because silver solders in the molten state are

remarkably fluid and can penetrate into minute

spaces between the metals to be joined. In order to

protect the metal surface against oxidation and to

increase the flowing properties of the solder, a suitable

flux such as borax or boric acid is used.

Silver soldering process

In silver soldering the size of the welding tip used

and the adjustment of the flame are very important

to avoid overheating, as prolonged heating promotes

oxide films which weaken both the base

metal and the joint material. This should be

guarded against by keeping the luminous cone of

the flame well back from the point being heated.

When the joint has been heated just above the temperature

at which the silver solder flows, the flame

should be moved away and the solder applied to the

joint, usually in rod form. The flame should then be

played over the joint so that the solder and flux

flow freely through the joint by capillary attraction.

The finished silver soldered joint should be smooth,

regular in shape and require no dressing up apart

from the removal of the flux by washing in water.

When making a silver solder joint between dissimilar

metals, concentrate the application of heat

on the metal which has the higher heat capacity.

This depends on the thickness and the thermal conductivity

of the metals. The aim is to heat both

members of the joint evenly so that they reach the

soldering temperature at the same time.

The most important points during silver soldering

are:

1 Cleanness of the joint surfaces

2 Use of the correct flux

3 The avoidance of overheating.

Aluminium brazing

There is a distinction between the brazing of aluminium

and the brazing of other metals. For aluminium,

the brazing alloy is one of the aluminium

alloys having a melting point below that of the parent

metal. For other metals, the brazing alloys are

often based on copper-zinc alloys (brasses – hence

the term brazing) and are necessarily dissimilar in

composition to the parent metal.

Wetting and fluxing

When a surface is wetted by a liquid, a continuous

film of the liquid remains on the surface after

draining. This condition, essential for brazing,

arises when there is mutual attraction between the

liquid flux and solid metal due to a form of chemical

affinity. Having accomplished its primary duty

of removing the oxide film, the cleansing action of

the flux restores the free affinities at the surface of

the joint faces, promoting wetting by reducing the

contact angle developed between the molten brazing

alloy and parent metal. This action assists

spreading and the feeding of brazing alloy to the

capillary spaces, leading to the production of well

filled joints. An important feature of the brazing

process is that the brazing alloy is drawn into the

joint area by capillary attraction: the smaller the

gap is between the two metal faces to be joined,

the deeper is the capillary penetration.

244Repair of Vehicle Bodies

Table 8.3Silver solders

BS Nominal composition (wt%) Melting Tensile

1845 range strength

ref. Ag Cu Zn Mn Ni Cd Sn (°C) (N/mm2) Notes

Cadmium-containing alloys

AG1 49–51 14–16 14–18 – – 18–20 – 620–640 470 Low melting point, very fluid,

high strength, general purpose

AG2 41–43 16–18 14–18 – – 24–26 – 610–620 470 Lowest melting point, very fluid.

General purposes, especially

small components

AG3 37–39 19–21 20–24 – – 19–21 – 605–650 – Very fluid and strong

AG9 49–50 14.5–16.5 13.5–17.5 – 2.5–3.5 15–17 – 635–655 480 Limited flow, useful for

fillet joints

AG11 33–35 24–26 18–22 – – 20–22 – 610–670 –

AG12 29–31 27–29 19–23 – – 20–22 – 600–690 485

Cadmium-free alloys

AG5 42–44 36–38 18–22 – – 0.025 – 690–770 400 Cadmium-free for food

equipment, etc.

AG7 71–73 27–29 – – – 0.025 – 780 – Fluxless brazing of copper

AG7V 71–73 27–29 – – – 0.025 – 780 – High-purity alloy for vacuum

assemblies

AG8 99.99 – – – – 0.25 – 960 – Pure silver

AG13 59–61 25–27 12–16 – – 0.025 – 730–695 – –

AG14 54–56 20–22 21–23 – – 0.025 1.7–2.3 660–630 450 –

AG18 48–50 15–17 21–25 6.5–8.5 4–5 0.025 – 705–680 360 For carbide brazing

AG19 84–86 – – 14–16 – 0.025 – 960–970 – –

AG20 39–41 29–31 – – – 0.025 1.7–2.3 710–650 – –

AG21 29–31 35–37 – – – 0.025 1.7–2.3 755–665 – –

Soft and hard soldering methods 245

The various grades of pure aluminium and certain

alloys are amenable to brazing. Aluminiummagnesium

alloys containing more than 2 per cent

magnesium are difficult to braze, as the oxide film

is tenacious and hard to remove with ordinary

brazing fluxes. Other alloys cannot be brazed

because they start to melt at temperatures below

that of any available brazing alloy. Aluminium-silicon

alloys of nominal 5 per cent, 7.5 per cent or 10

per cent silicon content (Table 8.4) are used for

brazing aluminium and the alloy of aluminium and

1.5 per cent manganese.

The properties required for an effective flux for

brazing aluminium and its alloys are as follows:

1 The flux must remove the oxide coating present

on the surfaces to be joined. It is always important

that the flux be suitable for the parent

metal, but especially so in the joining of aluminium-

magnesium alloys.

2 It must thoroughly wet the surfaces to be joined

so that the filler metal may spread evenly and

continuously.

3 It must flow freely at a temperature just below

the melting point of the filler metal.

4 Its density, when molten, must be lower than

that of the brazing alloy.

5 It must not attack the parent surfaces dangerously

in the time between its application and removal.

6 It must be easy to remove from the brazed

assembly.

Many types of proprietary fluxes are available for

brazing aluminium. These are generally of the

alkali halide type, which are basically mixtures of

the alkali metal chlorides and fluorides. Fluxes and

their residues are highly corrosive and therefore

must be completely removed after brazing by

washing with hot water.

Brazing method

When the cleaned parts have been assembled, brazing

flux is applied evenly over the joint surface of

both parts to be brazed and the filler rod (brazing

alloy). The flame is then played uniformly over the

joint until the flux has dried and become first powdery,

then molten and transparent. (At the powdery

stage care is needed to avoid dislodging the flux,

and it is often preferable to apply flux with the filler

rod.) When the flux is molten the brazing alloy is

applied, preferably from above, so that gravity

assists in the flow of metal. In good practice the

brazing alloy is melted by the heat of the assembly

rather than directly by the torch flame. Periodically

the filler rod is lifted and the flame is used to sweep

the liquid metal along the joint; but if the metal is

run too quickly in this way it may begin to solidify

before it properly diffuses into the mating surfaces.

Trial will show whether more than one feed point

for the brazing alloy is necessary, but with proper

fluxing, giving an unbroken path of flux over the

whole joint width, a single feed is usually sufficient.

Bronze welding

Bronze welding is carried out much as in fusion

welding except that the base metal is not melted.

The base metal is simply brought up to tinning

temperature (dull red colour) and a bead is

deposited over the seam with a bronze filler rod.

Although the base metal is never actually melted,

the unique characteristics of the bond formed by

the bronze rod are such that the results are often

Table 8.4Aluminium filler alloys for brazing

Nominal composition (wt%) (balance aluminium)

Melting

BS 1845 ref. Si Cu (max.) Mn (max.) Zn (max.) Ti (max.) Mg (max.) range (°C)

4004 9.0/10.5 0.25 0.10 0.20 – 1.0/2.0 555–590

4043A 4.5/6.0 0.30 0.15 0.10 0.15 0.20 575–630

4045 9.0/11.0 0.30 0.05 0.10 0.20 0.05 575–590

4047A 11.0/13.0 0.30 0.15 0.20 0.15 0.10 575–585

4104 9.0/10.5 0.25 0.10 0.20 – 1.0/2.0 555–590

4145A 9.0/11.0 3.0/5.0 0.15 0.20 0.15 0.10 520–585

4343 6.8/8.2 0.25 0.10 0.20 – – 575–615

246Repair of Vehicle Bodies

comparable with those secured through fusion

welding. Bronze welding resembles brazing, but

only up to a point. The application of brazing is

generally limited to joints where a close fit or

mechanical fastening serves to consolidate the

assembly and the joint is merely strengthened or

protected by the brazing material. In bronze welding

the filler metal alone provides the joint

strength, and it is applied by the manipulation of a

heating flame in the same manner as in gas fusion

welding. The heating flame is made to serve the

dual purpose of melting off the bronze rod and

simultaneously heating the surface to be joined.

The operator in this manner controls the work:

hence the term ‘bronze welding’.

Welding rods and fluxes

Almost any copper-zinc alloy or copper-tin alloy

or copper-phosphorus alloy (see Table 8.2) can be

used as a medium for such welding, but the consideration

of costs, flowing qualities, strength and

ductility of the deposit have led to the adoption of

one general purpose 60–40 copper-zinc alloy with

minor constituents incorporated to prevent zinc

oxide forming and to improve fluidity and

strength. Silicon is the most important of these

minor constituents, and its usefulness is apparent

in three directions. First, in the manner with

which it readily unites with oxygen to form silica,

silicon provides a covering for the molten metal

which prevents zinc volatilization and serves to

maintain the balance of the constituents of the

alloy; this permits the original high strength of

the alloy to be carried through to the deposit.

Second, this coating of silica combines with the

flux used in bronze welding to form a very fusible

slag, and this materially assists the tinning operation,

which is an essential feature of any bronze

welding process. Third, by its capacity for retaining

gases in solution during solidification of the

alloy, silicon prevents the formation of gas holes

and porosity in the deposited metal, which would

naturally reflect unfavourably upon its strength as

a weld.

It is essential to use an efficient and correct flux.

The objects of a flux are: first, to remove oxide

from the edges of the metal, giving a chemically

clean surface on to which the bronze will flow, and

to protect the heated edges from the oxygen in the

atmosphere; second, to float oxide and impurities

introduced into the molten pool to the surface,

where they can do no harm. Although general-purpose

fluxes are available, it is always desirable to

use the fluxes recommended by the manufacturer

of the particular rod being employed.

Bronze welding procedure

1 An essential factor for bronze welding is a clean

metal surface. If the bronze is to provide a

strong bond, it must flow smoothly and evenly

over the entire weld area. Clean the surfaces

thoroughly with a stiff wire brush. Remove all

scale, dirt or grease, otherwise the bronze will

not adhere. If a surface has oil or grease on it,

remove these substances by heating the area to a

bright red colour and thus burning them off.

2 On thick sections, especially in repairing castings,

bevel the edges to form a 90° V-groove.

This can be done by chipping, machining, filing

or grinding.

3 Adjust the torch to obtain a slightly oxidizing

flame. Then heat the surfaces of the weld area.

4 Heat the bronzing rod and dip it in the flux.

(This step is not necessary if the rods have been

prefluxed.) In heating the rod, do not apply the

inner cone of the flame directly to the rod.

5 Concentrate the flame on the starting end until

the metal begins to turn red. Melt a little

bronze rod on to the surface and allow it to

spread along the entire seam. The flow of this

thin film of bronze is known as the tinning

operation. Unless the surfaces are tinned properly

the bronzing procedure to follow cannot

be carried out successfully. If the base metal

is too hot, the bronze will tend to bubble or

run around like drops of water on a warm

stove. If the bronze forms into balls which

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