Materiali e Tecnologie per il Design

Welding of metals

We add heat to cause the physical continuity between part. Permanent joint technique. So welding on metals is a

permanent joining technique that uses heat to melt material in the interface between the parts and create physical

continuity between the parts.

Principle:

- part to be welded (base metal).

- Fading surfaces are the part in contact between the parts that need rot be welted.

- We must heat only the frying surfaces, not all the base metal.

- Filler metal or additional material added to the joint in order to ll.

- Volume of metal liquid in interfaces between parts called melting pool.

- Creation of weld joint. Not created all at once, we must supply gradually heat to the part.

Types of joint:

- Butt joint: two parts connected edge to edge. They also can have an angle

- Lap joint: bars connected face to face, overlapped.

- Fillet joint designed for thick parts:

- Corner joint and tee joint : part connected edge to face, those are l or t shape.

Usually we weld parts that are less than 3 millimeters, but llet joint are for thicker parts.

Between 3 and 6 metal there is a small gap between the two parts going to be lled with ller metal. The gap is

created to allo the heat to reach the whole part. We have to prepare the edges of the part with chamfering. For more

than 6 millimeter we use groove weld with chamfering edges so we creare multiple passes to weld all together.

SHIELDED METAL ARC WELDING OR STICK WELDING

Arc welding means that the heat is created by this electric arc or electrical discharge between the electrode (another

metal) connected to the power and a base metal. This electric discharge creates a lot of heat and this heat brings the

metal to their melting temperature. Mostly applied to thick plate, industrial machinery.

- MIG ( ): ller metal has the same chemical composition of the base material. Inert gas stand for a

METAL INERT GAS

gas that doesn’t react chemically with the metal, important bc we use it to protect the weld form the oxidation of

metal. It's important to do not have rust and spoil the aesthetic appeal of the weld. The electrode wire is the ller

metal and there’s so a roll of this. It should be a constant distant to the base metal. How do we get this? Theres a

speed control that regulates the feeding speed of the wire. Shielding gas is inert gas that doesn’t react to the metal

but goes out and surround the weld protecting the weld against corrosion. Used in high production. It could weld a

wide range of alloy such as stainless steels, aluminum, titanium which are highly oxidable, but we can weld only low

carbon steels. Or carbon steels we have an improved quality of the steel. With others then low carbon steel its not

the best quality. Its relatively fast prices bc we don’t have to interrupt the process. We are not only to weld low

carbon steel but also titanium alloy aluminum alloy but the quality won’t be the same. Wide range of thicknesses

can be welded form 0.5 (bc to low current without running the risk of damaging a very thin material, current could

make holes in thin materials) to 6 mm, not over 6 mm.

- TIG (T I G ): we have a tungsten electron which is a refractory metal that melts a very high

WELDING UNGSTEN NERT AS

temperature, over 300°. The thing is that the base metal and ller metal should melt while the tungsten electrode

should not. A refractory metal remains solid during welding at much higher temperature than the electrode wire of

MIG welding, this is important bc higher temperature lower is the supply of energy (current) TO CREATE THE ARC

and to the base metal. This is important in order to create a very small and uniform in shape. In this weld system we

don’t have a wire but a separate stick of ller metal. We have cooling water to cool the lectured which is heated by

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the electrode. TIG welding is a precise welding process, the welder has to keep the welding gun in one hand and

the ller metal in the other one and the welder must control the arc distance, at a uniform distance from the base

metal. The weld is narrower and more uniform thanks to the low current supplied to the welding gun.

- Same alloys as MIG

- THE PROCESS IS SLOWER then MIG bc of lower current and less supply of energy to the joint

- The quality is optimal in a wide range of material

- Thickness 0.2-3 mm

The rst phase of welding is a tack welds that puts all the parts together, we make only very short welds between parts

to create an assembly stable enough to create a TIG weld which is complete, its slower but we have strenght and

aesthetic requirement.

Arc welding processes we have visible joints, but we don’t want to see the welds. After we have grind and polish to

obtain a nice esthetic result. In mass production we have little time for nishing operations such has grind and polish,

so how are those operations done in mass production?

LASER BEAM WELDING

We have two part of a base metal and a laser source connected to an electric power source. Laser source is an high

complexity device which is able to emit high energy lights focused by lenses which is an optic device. So the light

projected by the laser source is focused in a very precise spot thanks to these optics (a circle of 0.1mm), so we have a

very high energy spot transmitted to the base metal. Of course this is just light but the base metal can capture this

light and converted to heat which is very concentrate and can vaporize the material interface. We need a shielding

gas has well in order to protect the material from the oxidation. We don’t need any ller metal bc we don’t have any

gap or loss of material from the base material.

Advantages or disadvantages:

- High quality bc of the small laser spot, concentrated in a very narrow width.

- Very expensive equipment.

- It’s used when we need a high quality continuous welds, bc it is visible.

- Extremely good result without any polishing.

- Another application is to create small sized spot welds, ex. Razor.

- Could also be used to weld dissimilar materials.

RESISTANCE SPOT WELDING

Spot weld means welding just one point at the time, while resistance refers to the electrical resistance bc this welding

process exploits the electrical resistance of the material, so materials with high electrical resistance can be welded

much better by this process.

We have an electric power source connected to two copper electrodes. The weld creates a lap joint: we have two

sheet metal parts between the copper electrodes that are able to apply a contact pressure to the two parts of base

material. The current supply forces a very high energy to the parts creating the joul effect: a physical phenomenon, an

electrical current that ows in a conductor for example a metal, develops an amount of heat that is proportional to the

electrical resistance of the conductor, which is our base metal. So the current that ows to the thickness of the base

metal heats the base metal where the electrical resistance is at its maximum, so in this case at the surfaces between

the parts. So this Joule effect brings some heat to the interface between the two parts, this heat melts the material at

the interface, the material solidi es and create the spot weld.

We wont see the joint bc is inside the two parts, we will need more than one spot weld more precisely we will need

one spot every 20/30 time the thickness of the parts, but the exact number of spots is decided by the strenght of the

joint that we require trough a structural analysis made by an engineer.

Advantages or disadvantages:

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- Its very fast so we have 2/3 seconds, advantageous in car applications. It is the fastest.

- We can weld all materials but is most used in steels: carbon, stainless etc. much less on aluminum or

titanium. Aluminum is less used bc its a good electrical conductor, so the electrical resistance is lower

and so we will need an higher amount of current with the risk of damaging the parts around the welds.

- This weld can join up to 3-4 layers of this base metal, so a thickness of 15/20 mm.

- A variation of this welding is a which is a continuous weld (saldature a rulli)

RESISTANCE SEAM WELDING

where the two copper electrodes are replaced by two disks that drive the parts by friction and as long

the parts translate some current is forced to the thickness of the parts. So we will have many spot welds

that overlap and create a continuous weld. This could be advantageous if we want a water or air thighs

weld for example for kitchen sinks.

- It could be done in a machine with the parts brought to the machine manually, but in mass production

we need to increase speed and avoid any loss of time due to the manual handling of the material, so

we have a robotic welding line: the spot welder device is mounted on a robotic arm controlled by a pc.

BRAZING

In brazing the base metal does not weld, so the joint is created by addition of the two parts of the base metal to a

common layer of material placed between metal that in this case function as a ller metal of course. So if we have two

base metal parts that need to be welded together in this case do not melt, but we will have a foil of ller metal putted

between the parts of the base metal. Of course, since the ller metal is the only part that melts in this process needs

to have a lower melting temperature than the base metal.

Since we need some heat we have an Oxyfuel Torch which is a manual tool used to heat all the ller metal, but how

does it works: we have a fuel gas that is burned by the contact with fuel, so Oxyfuel means that we develop a heat in

form of a ame and this ame is created by the fuel gas burned by the Oxygen. When the ller metal (brace alloy)

solidi es it creates a sort of metal glue able to stick the two parts together. We also need some Flux which is a powder

or paste applied in order to prevent the oxidation or corrosion of metal.

The ller metal could be in form of foil but also of paste usually mixed with the ux. The ller metal could also be a

wire and this is useful and practical when we have complex surfaces and complex joints. In this case we will have a

metal wire that when melts it is attracted in the thin gap between the parts (less then 0.1mm). So the spreading of ller

metal over the whole surface create the joints. This happened due to capillarity: a physical phenomenon.

Advantages or disadvantages:

- We could have a torch brazing (manual process) and for mass production we have furnace brazing: a

furnace that keeps the parts at a constant temperature and a sort of belts transporter that drives the

parts in the furnace, bringing them outside. We could also have furnaces in the common type so with a

door. Of course the temperature inside the furnace should be between the melting temperature of the

base metal and ller metal.

- Materials are steels, copper alloys, aluminum alloys or dissimilar material. Melting temperature are

important because close in connection with the braze alloy: steel and copper are welded with a copper

and silver alloy .

- We can weld complex parts and create watertight joints bc the gap within the parts is lled very

precisely. Used also in jewelry.

THE DESIGN OF WELD JOINT:

We want to avoid defects as well as reduce costs.

Defects are:

SHRINKAGE DEFECTS

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The contraction of the material when it cools from melting temperature to normal one. In welding we have a thermal

gradient which is a difference of temperature between the weld part and the base metal. The weld will shrink much

more then the base metal, so the weld is forced to have a length that is longer then its natural one, so the weld is

subjected to residual stress of tension because there is a force that stretches the weld compared to its natural one. Of

course the base metal its compress to equilibrium of this forces. The consequence of residual stress is warpage that

is a deformation in weld and base metal and if the residual stress overcome the strenght of the material we could also