(slovní zásoba v sekci slovíčka)

Metal forming processes

Metal forming is a chipless process, which means that energy and money for removing and depreciating wasted material is saved. Forming is now used more than machining. In this process a force is applied to a workpiece to modify its geometry rather than remove material. the workpiece is plastically deformed without reducing its structural cohesion and physical properties of metal are improved. It is used mainly in mass production.

Metal materials can be formed in cold or hot conditions.¨

Hot forming processes:

Forging – the process wherein a workpiece is plastically deformed by applying compressive force. Such process produces high-strength components. It can be performed in different ways (pressing or hammering) and temperatures (hot). Forging temperature is a temperature at which a metal becomes soft or its shape can be changed by applying a relatively small force without creating cracks in metal.

There are two types of forging: open die forging and closed die forging.

Open die forging – compressive forming performed between plain dies with no restriction at the end. During deformation the workpiece is squeezed between flat dies or dies of simple shape while it is hammered or pressed. It is usually used for making heavy products (over 150 tons).

Closed die forging – the workpiece is plastically deformed in a die. The workpiece is confined in a die which is a two-part form attached to a hammer anvil or press. The cavity of a die has an inverse pre-formed profile of the desires part and it must be resistant to thermal and mechanical fatigues. The workpiese is heated before placing in a die and then is pressed or hammered until it flow and fill the die cavity. The forging is done under great pressure and while the two halves of a die are brought together a portion of the metal flows beyond the die cavity to form a flash. The flash is excess metal and after cooling it is trimmed out to chieve the end product geometry. It is used to produc difficult geometry with high accuracy.

Cold forming processes: for soft materials (otherwise the workpiece would crack). The force is much higher than in hot forming. It is cheap high speed manufacturing technique. It includes: blanking, bending, extrusion, rolling etc.

Bending – material is plastically deformed into various bend angles. The act of bending results in both tension and compression in the sheet metal. The outside fibres are subjected to tensile strain and they extend while the inside fibres are subjected to compressive strain and they shrink. The compressive stress changes into tensile stress at neutral axis where neither compressive nor tensile strain occurs.  For bend stabilization the bend axis should be perpendicular to fibre direction.

When the bending pressure is removed at the end of the deformation operation, the material has a tendency to partially return to its original shape. Due to this elastic recovery called spring-back, it is necessary to overbend the sheet to achieve the desired bend angle.

Drawing – uses tensile forces to stretch metal into desired shape. The workpiece is stretched around a plug and then pulled through a die. Common shapes for drawn products include cylinders for aluminium cans, kitchen sinks etc.

Extrusion – process by which long straight metal parts with various cross-sections can be produced. The cross-sections vary from round, rectangular, L shapes, T shapes, tubes etc. It is done by squeezing metal in a closed cavity and then pushed through an extrusion of the desired cross-section. It is usually used for lead, tin, aluminium alloys, copper, titanium, steel etc.

Metal joining processes

Joining of metal materials is required when the desired component cannot be made by means of metal forming or machining. the essential metal joining operations are: welding, soldering, adhesive bonding, riveting, bolting.

Welding – the most common way of joining two workpieces to produce a single piece of metal. The coalescence of metal is done by applying heat, pressure or both and the weld is made when the edges of workpieces are melted so that they fuse permanently. This is done with or without the use of a filler material. There are several types of welding: gas welding, arc welding, laser welding, electron beam welding, resistance welding etc.

Soldering – two workpiece are joined together with a molten metal – a solder. This material is characterized by a lower melting point than the joining metal. The surface is cleaned mechanically and then by applying a melting flux. Then the workpiece is heated with a soldering iron above the melting point of the solder and the solder is drawn into the joint where it is smoothly spread to form a bond. It is appropriate for dissimilar materials.

Adhesive bonding -  used to fasten two workpieces together through an adhesive which must be highly adhesive and cohesive. The best choice of adhesive depends on the materials to be bonded. The bond is strong, corrosion resistant, clean-looking and often has a good heat, sound and electrical insulation. Adhesive application is also critical for bond quality – so the way of adhesive distribution and the depth of distributed layer are very important. It is often used for different materials with dissimilar thickness.

Riveting – two workpieces are connected with rivets. A rivet is a permanent metal fastener in a shape of a pin with a head on one end and a plain tail, a shank, on the other. The rivet is placed in a punched hole passing through two connecting pieces. Then, the part of shank protruding from the hole is hammered or pressed to form a second head to the rivet and to connect the workpiece together. The inserted rivets may be hot or cold.

Bolting – a temporary method of joining metal materials. The workpiece is connected by a threaded fastener, bolt. It comprises of a hexagonal head and a circular shank which may be fully or partly threaded. The bolt is screwed in a threaded hole passing through two workpieces and then fully tightened. The bolted joint can withstand huge amount of pressure. The advantage is that the bolted joins can be easily disassembled by removing bolts.