casting

It is a manufacturing process by which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting materials are usually metals or various cold setting materials that cure after mixing two or more components together; examples are epoxy, concrete, plaster and clay. Casting is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods.

Casting is a 6000 years old process. The Egyptians and Chinese used the process in their early history to make statues and jewelry. The investment casting method was largely ignored as an industrial process for the fabrication of parts until the demand for rapidly finished parts during World War II created the need for near net-shape components that could readily be put into their final form.

Types of Casting
Sand Casting
Work flow in typical sand-casting industry
Sand casting uses natural or synthetic sand (lake sand) which is mostly a refractory material called silica (SiO2). The sand grains must be small enough so that it can be packed densely; however, the grains must be large enough to allow gasses formed during the metal pouring to escape through the pores. Larger sized molds use green sand (mixture of sand, clay and some water). Sand can be re-used, and excess metal poured is cut-off and re-used also.

It is used to make large parts (typically Iron, but also Bronze, Brass, Aluminum). Molten metal is poured into a mold cavity formed out of sand (natural or synthetic).

Process
The process cycle for sand casting consists of six main stages, which are explained below.
Mold-making – The first step in the sand casting process is to create the mold for the casting. In an expendable mold process, this step must be performed for each casting. A sand mold is formed by packing sand into each half of the mold. The sand is packed around the pattern, which is a replica of the external shape of the casting. When the pattern is removed, the cavity that will form the casting remains. Any internal features of the casting that cannot be formed by the pattern are formed by separate cores which are made of sand prior to the formation of the mold. Further details on mold-making will be described in the next section. The mold-making time includes positioning the pattern, packing the sand, and removing the pattern. The mold-making time is affected by the size of the part, the number of cores, and the type of sand mold. If the mold type requires heating or baking time, the mold-making time is substantially increased. Also, lubrication is often applied to the surfaces of the mold cavity in order to facilitate removal of the casting. The use of a lubricant also improves the flow the metal and can improve the surface finish of the casting. The lubricant that is used is chosen based upon the sand and molten metal temperature.
Clamping – Once the mold has been made, it must be prepared for the molten metal to be poured. The surface of the mold cavity is first lubricated to facilitate the removal of the casting. Then, the cores are positioned and the mold halves are closed and securely clamped together. It is essential that the mold halves remain securely closed to prevent the loss of any material.
Pouring – The molten metal is maintained at a set temperature in a furnace. After the mold has been clamped, the molten metal can be ladled from its holding container in the furnace and poured into the mold. The pouring can be performed manually or by an automated machine. Enough molten metal must be poured to fill the entire cavity and all channels in the mold. The filling time is very short in order to prevent early solidification of any one part of the metal.
Cooling – The molten metal that is poured into the mold will begin to cool and solidify once it enters the cavity. When the entire cavity is filled and the molten metal solidifies, the final shape of the casting is formed. The mold can not be opened until the cooling time has elapsed. The desired cooling time can be estimated based upon the wall thickness of the casting and the temperature of the metal. Most of the possible defects that can occur are a result of the solidification process. If some of the molten metal cools too quickly, the part may exhibit shrinkage, cracks, or incomplete sections. Preventative measures can be taken in designing both the part and the mold and will be explored in later sections.
Removal – After the predetermined solidification time has passed, the sand mold can simply be broken, and the casting removed. This step, sometimes called shakeout, is typically performed by a vibrating machine that shakes the sand and casting out of the flask. Once removed, the casting will likely have some sand and oxide layers adhered to the surface. Shot blasting is sometimes used to remove any remaining sand, especially from internal surfaces, and reduce the surface roughness
Trimming – During cooling, the material from the channels in the mold solidifies attached to the part. This excess material must be trimmed from the casting either manually via cutting or sawing, or using a trimming press. The time required to trim the excess material can be estimated from the size of the casting’s envolpe. A larger casting will require a longer trimming time. The scrap material that results from this trimming is either discarded or reused in the sand casting process. However, the scrap material may need to be reconditioned to the proper chemical composition before it can be combined with non-recycled metal and reused.

Equipment
Mold
Typical sand mold have the following main parts:
?The mold is made of two parts, the top half is called the cope, and bottom part is the drag.

?The liquid flows into the gap between the two parts, called the mold cavity. The geometry of the cavity is created by the use of a wooden shape, called the pattern. The shape of the patterns is (almost) identical to the shape of the part we need to make.
?A funnel shaped cavity; the top of the funnel is the pouring cup; the pipe-shaped neck of the funnel is the sprue ? the liquid metal is poured into the pouring cup, and flows down the sprue.

?The runners are the horizontal hollow channels that connect the bottom of the sprue to the mould cavity. The region where any runner joins with the cavity is called the gate.
?Some extra cavities are made connecting to the top surface of the mold. Excess metal poured into the mould flows into these cavities, called risers. They act as reservoirs; as the metal solidifies inside the cavity, it shrinks, and the extra metal from the risers flows back down to avoid holes in the cast part.
?Vents are narrow holes connecting the cavity to the atmosphere to allow gasses and the air in the cavity to escape.
?Cores: Many cast parts have interior holes (hollow parts), or other cavities in their shape that are not directly accessible from either piece of the mold. Such interior surfaces are generated by inserts called cores. Cores are made by baking sand with some binder so that they can retain their shape when handled. The mold is assembled by placing the core into the cavity of the drag, and then placing the cope on top, and locking the mold. After the casting is done, the sand is shaken off, and the core is pulled away and usually broken off.
Sand
The sand that is used to create the molds is typically silica sand (SiO2) that is mixed with a type of binder to help maintain the shape of the mold cavity. Using sand as the mold material offers several benefits to the casting process. Sand is very inexpensive and is resistant to high temperatures, allowing many metals to be cast that have high melting temperatures. There are different preparations of the sand for the mold, which characterize the following four unique types of sand molds.

?Greensand mold – Greensand molds use a mixture of sand, water, and a clay or binder. Typical composition of the mixture is 90% sand, 3% water, and 7% clay or binder. Greensand molds are the least expensive and most widely used.
?Skin-dried mold – A skin-dried mold begins like a greensand mold, but additional bonding materials are added and the cavity surface is dried by a torch or heating lamp to increase mold strength. Doing so also improves the dimensional accuracy and surface finish, but will lower the collapsibility. Dry skin molds are more expensive and require more time, thus lowering the production rate.
?Dry sand mold – In a dry sand mold, sometimes called a cold box mold, the sand is mixed only with an organic binder. The mold is strengthened by baking it in an oven. The resulting mold has high dimensional accuracy, but is expensive and results in a lower production rate.
?No-bake mold – The sand in a no-bake mold is mixed with a liquid resin and hardens at room temperature.
The quality of the sand that is used also greatly affects the quality of the casting and is usually described by the following five measures:
?Strength – Ability of the sand to maintain its shape.
?Permeability – Ability to allow venting of trapped gases through the sand. A higher permeability can reduce the porosity of the mold, but a lower permeability can result in a better surface finish. Permeability is determined by the size and shape of the sand grains.
?Thermal stability – Ability to resist damage, such as cracking, from the heat of the molten metal.
?Collapsibility – Ability of the sand to collapse, or more accurately compress, during solidification of the casting. If the sand can not compress, then the casting will not be able to shrink freely in the mold and can result in cracking.
?Reusability – Ability of the sand to be reused for future sand molds.

Tooling
The main tooling for sand casting is the pattern that is used to create the mold cavity. The pattern is a full size model of the part that makes an impression in the sand mold. However, some internal surfaces may not be included in the pattern, as they will be created by separate cores. The pattern is actually made to be slightly larger than the part because the casting will shrink inside the mold cavity. Also, several identical patterns may be used to create multiple impressions in the sand mold, thus creating multiple cavities that will produce as many parts in one casting. A pattern for a part can be made many different ways, which are classified into the following four types:
?Solid pattern – A solid pattern is a model of the part as a single piece. It is the easiest to fabricate, but can cause some difficulties in making the mold. The parting line and runner system must be determined separately. Solid patterns are typically used for geometrically simple parts that are produced in low quantities.

?Split pattern – A split pattern models the part as two separate pieces that meet along the parting line of the mold. Using two separate pieces allows the mold cavities in the cope and drag to be made separately and the parting line is already determined. Split patterns are typically used for parts that are geometrically complex and are produced in moderate quantities.

?Match-plate pattern – A match-plate pattern is similar to a split pattern, except that each half of the pattern is attached to opposite sides of a single plate. The plate is usually made from wood or metal. This pattern design ensures proper alignment of the mold cavities in the cope and drag and the runner system can be included on the match plate. Match-plate patterns are used for larger production quantities and are often used when the process is automated.

?Cope and drag pattern – A cope and drag pattern is similar to a match plate pattern, except that each half of the pattern is attached to a separate plate and the mold halves are made independently. Just as with a match plate pattern, the plates ensure proper alignment of the mold cavities in the cope and drag and the runner system can be included on the plates. Cope and drag patterns are often desirable for larger castings, where a match-plate pattern would be too heavy and cumbersome. They are also used for larger production quantities and are often used when the process is automated.

Cost
Material Cost
The material cost for sand casting includes the cost of the metal, melting the metal, the mold sand, and the core sand. The cost of the metal is determined by the weight of the part, calculated from part volume and material density, as well the unit price of the material. The melting cost will also be greater for a larger part weight and is influenced by the material, as some materials are more costly to melt. However, the melting cost in typically insignificant compared to the metal cost. The amount of mold sand that is used, and hence the cost, is also proportional to the weight of the part. Lastly, the cost of the core sand is determined by the quantity and size of the cores used to cast the part.

Production Cost
The production cost includes a variety of operations used to cast the part, including core-making, mold-making, pouring, and cleaning. The cost of making the cores depends on the volume of the cores and the quantity used to cast the part. The cost of the mold-making is not greatly influenced by the part geometry when automated equipment is being used. However, the inclusion of cores will slightly slow the process and therefore increase the cost. Lastly, the cost of pouring the metal and cleaning the final casting are both driven by the weight of the part. It will take longer to pour and to clean a larger and heavier casting.

Tooling Cost
The tooling cost has two main components – the pattern and the core-boxes. The pattern cost is primarily controlled by the size of the part (both the envelope and the projected area) as well as the part’s complexity. The cost of the core-boxes first depends on their size, a result of the quantity and size of the cores that are used to cast the part. Much like the pattern, the complexity of the cores will affect the time to manufacture this part of the tooling (in addition to the core size), and hence the cost.
The quantity of parts that are cast will also impact the tooling cost. A larger production quantity will require the use of a tooling material, for both the pattern and core-boxes, that will not wear under the required number of cycles. The use or a stronger, more durable, tooling material will significantly increase the cost.

Die Casting
Die casting is a manufacturing process that can produce geometrically complex metal parts through the use of reusable molds, called dies. The die casting process involves the use of a furnace, metal, die casting machine, and die. The metal, typically a non-ferrous alloy such as aluminum or zinc, is melted in the furnace and then injected into the dies in the die casting machine.

This process is similar to permanent mold casting except that the metal is injected into the mold under high pressure of 10-210Mpa (1,450-30,500) psi. This results in a more uniform part, generally good surface finish and good dimensional accuracy, as good as 0.2 % of casting dimension. For many parts, post machining can be totally eliminated, or very light machining may be required to bring dimensions to size.
Die casting can be done by using a cold chamber or hot chamber process.

?In a cold chamber process, the molten metal is ladled into the cold chamber for each shot. There is less time exposure of the melt to the plunger walls or the plunger. This is particularly useful for metals such as Aluminum, and Copper (and its alloys) that alloy easily with Iron at the higher temperatures.

The working machine of a cold chamber process
?In a hot chamber process, the pressure chamber is connected to the die cavity is immersed permanently in the molten metal. The inlet port of the pressurizing cylinder is uncovered as the plunger moves to the open (unpressurized) position. This allows a new charge of molten metal to fill the cavity and thus can fill the cavity faster than the cold chamber process. The hot chamber process is used for metals of low melting point and high fluidity such as tin, zinc, and lead that tend not to alloy easily with steel at their melt temperatures.

The working machine of a hot chamber process
Common Alloys in Die Casting
Aluminum, Zinc and Copper alloys are the materials predominantly used in die-casting. On the other hand, pure Aluminum is rarely cast due to high shrinkage, and susceptibility to hot cracking. It is alloyed with Silicon, which increases melt fluidity, reduces machinability. Copper is another alloying element, which increases hardness, reduces ductility, and reduces corrosion resistance.
Aluminum is cast at a temperature of 650 ?C (1200 ?F). It is alloyed with Silicon 9% and Copper about 3.5% to form the Aluminum Association 380 alloy (UNS A03800). Silicon increases the melt fluidity, reduces machinability, Copper increases hardness and reduces the ductility. By greatly reducing the amount of Copper (less than 0.6%) the chemical resistance is improved; thus, AA 360 (UNS A03600) is formulated for use in marine environments. A high silicon alloy is used in automotive engines for cylinder castings, AA 390 (UNS A03900) with 17% Silicon for high wear resistance.

Zinc can be made to close tolerances and with thinner walls than Aluminum, due to its high melt fluidity. Zinc is alloyed with Aluminum (4%), which adds strength and hardness. The casting is done at a fairly low temperature of 425 ?C (800 ?F) so the part does not have to cool much before it can be ejected from the die. This, in combination with the fact that Zinc can be run using a hot chamber process allows for a fast fill, fast cooling (and ejection) and a short cycle time.
Uses & Properties
Zinc alloys are used in making precision parts such as sprockets, gears, and connector housings. Copper alloys are used in plumbing, electrical and marine applications where corrosion and wear resistance is important.

Minimum wall thicknesses and minimum draft angles for die casting are:
MaterialMin. Thickness
mm (in)Min. Draft Angle (?)
Aluminum alloys0.9 mm
(0.035 in)0.5
Zinc alloys0.6 mm
(0.025 in)0.25
Copper alloys (Brass)1.25 mm
(0.050 in)0.7
Die-castings are typically limited from 20 kg (55 lb) max. for Magnesium, to 35 kg (77 lb) max. for Zinc. Large castings tend to have greater porosity problems, due to entrapped air, and the melt solidifying before it gets to the furthest extremities of the die-cast cavity. The porosity problem can be somewhat overcome by vacuum die casting
From a design point of view, it is best to design parts with uniform wall thicknesses and cores of simple shapes. Heavy sections cause cooling problems, trapped gases causing porosity. All corners should be radiused generously to avoid stress concentration. Draft allowance should be provided to all for releasing the parts-these are typically 0.25? to 0.75? per side depending on the material.

Selecting the Right Metal for Casting
?For any Metal Casting Process, selection of right alloy, size, shape, thickness, tolerance, texture, and weight, is very vital.

?Special requirements such as, magnetism, corrosion, stress distribution also influence the choice of the Metal Casting Process.
?Views of the Tooling Designer; Foundry / Machine House needs, customer’s exact product requirements, and secondary operations like painting, must be taken care of before selecting the appropriate Metal Casting Process.
?Tool cost.
?Economics of machining versus process costs.
?Adequate protection / packaging, shipping constraints, regulations of the final components, weights and shelf life of protective coatings also play their part in the Metal Casting process.
Considerations for Casting
?How do we make the pattern?
Usually craftsmen will carve the part shape by hand and machines to the exact size.

?Why is the pattern not exactly identical to the part shape?
– you only need to make the outer surfaces with the pattern; the inner surfaces are made by the core
– you need to allow for the shrinkage of the casting after the metal solidifies
?If you intersect the plane formed by the mating surfaces of the drag and cope with the cast part, you will get a cross-section of the part. The outer part of the outline of this cross section is called the parting line. The design of the mold is done by first determining the parting line (why ?)
?In order to avoid damaging the surface of the mould when removing the pattern and the wood-pieces for the vents, pouring cup and sprue, risers etc., it is important to incline the vertical surfaces of the part geometry. This (slight) inclination is called a taper. If you know that your part will be made by casting, you should taper the surfaces in the original part design.

?The core is held in position by supporting geometry called core prints (see figure below). If the design is such that there is insufficient support to hold the core in position, then metal supports called chaplets are used. The chaplets will be embedded inside the final part.

?After the casting is obtained, it must be cleaned using air-jet or sand blasting.

?Finally, the extra metal near the gate, risers and vents must be cut off, and critical surfaces are machined to achieve proper surface finish and tolerance.

Advantages ; Disadvantages
Advantages
On Basis of Size of Object to be Manufactured
Size of cast objects vary over large range. An object from 5gm to 200tonn, anything can be cast.

On the basis of complexity
Casting can be effectively used for complex shaped objects. It can work where general machining process can not be used, as in complicated inner and outer shapes of object.

Weight Saving
Component made with casting process is lighter than the component made with other machining processes.

Control Over the Process
Casting provides versatility. Wide range of properties can be attained by adjusting percentage of alloying elements.

Accuracy
Casting can be made with hair like precision provided proper molding and casting technique is employed.

Fibrous Structure
Only casting have this advantage. Casting leaves component with its solid fibrous structure which inherit great compressive strength. So, component subjected to compressive strength are made with casting ex. IC engine cylinder.

Control Over Grain Size
Grain size of cast component can be easily controlled by controlling cooling rate which in turn can be used to modify the properties.

Low Cost
Casing is one of cheapest method for mass production.

Disadvantages
Though casting is cheapest for MASS Production, it becomes non economical in case of JOB production.

Sand casting leaves rough surface which needs machining in most of cases. It adds up the cost in production.

Again in sand casting, poor dimensional accuracy is achieved.

Cast products are superior for compressive loads but they are very poor in tensile or shock loads.(They are brittle).

___________________________________
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen. Formation of an oxide of iron due to oxidation of the iron atoms in solid solution is a well-known example of electrochemical corrosion, commonly known as rusting. This type of damage typically produces oxide and/or salt of the original metal. Corrosion can also refer to other materials than metals, such as ceramics or polymers, although in this context, the term degradation is more common.

In other words, corrosion is the wearing away of metals due to a chemical reaction.

Many structural alloys corrode merely from exposure to moisture in the air, but the process can be strongly affected by exposure to certain substances (see below). Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Because corrosion is a diffusion controlled process, it occurs on exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such as passivation and chromate-conversion, can increase a material’s corrosion resistance. However, some corrosion mechanisms are less visible and less predictable.

Corrosion Resistance
Corrosion takes many different forms. Its initiation and subsequent rate of progress is affected in varying degrees by numerous material and environmental factors. A comprehensive assessment of the exact ‘corrosion resistance’ of a material is therefore difficult. However, corrosion tables covering a vast range of stainless steels and environments are available.

Oxidation
Oxidation is the combination of a metal with oxygen to form the metal oxide which occurs in dry conditions. When this process is ongoing the whole of the metal may be converted. For example, stainless steels are oxidation resistant, even at elevated temperatures. Special heat resisting grades, such as type 310, are operated at temperatures of up to 1100?C.

Forms of Corrosion
There are different forms of corrosion ?
General Corrosion is the uniform overall attack of a component across its whole surface. It is avoided by correct grade selection.

Pitting Corrosion is the highly localised attack seen as small spots across a surface occurring mainly at sites of metallurgical heterogeneity. Particularly prevalent in chloride environments, especially if Oxyqen is in plentiful supply, it can lead to perforation. Higher Chromium, Nickel and Molybdenum contents improve pitting
resistance, thus type 316 is extensively used in such situations. Laboratory tests can be carried out to confirm pitting resistance such as ASTM 48-03.

Atmospheric Corrosion occurs due to the attack from oxygen, water and the pollutants therein such as chlorides, sulphur compounds and solids. The problem is particularly prevalent in coastal, industrial and highly polluted areas where necessitates the use of type 316 is recommended for outdoor applications in such environments.

Acid Corrosion occurs due to aggressive attack by acids which may be accelerated by the presence of other chemicals. A large number of acid environments are resisted by stainless steels – resistance to oxidising solutions is very good, provided the correct grade is used.

Galvanic Corrosion covers situations where attack is caused by a potential difference. This potential difference can be set up in a number of ways including contact between dissimilar metals in an aqueous or conducting solution, differential aeration (variation in oxygen concentration) and local variations in concentration of the solution. Correct material selection and good design can eliminate this.

Waterline Corrosion is a form of galvanic corrosion taking place at the surface of a liquid.

Crevice Corrosion occurs in crevices such as joints, cavities, holes, corners, grooves, slots, gaskets and gaps between components. It is caused by the breakdown on the protective oxide layer, normally in reducing environments and can be seen as another types of galvanic corrosion. Good design should be used to eliminate such crevices and thus avoid this phenomenon.

Stress Corrosion Cracking (SCC) can occur in austenitic stainless steels when they are operated under tensional stress in chloride environments at temperatures in excess of about 60?C. The stress could arise through inservice loading, pressurisation of pipework and vessels or as residual stress from cold working. Nickel content assists resistance to SCC and thus grade 316 is more resistant to this form of attack than 304, whilst the duplex grades such as 2205 perform very well. Ferritic stainless steels are immune to this form of attack.

Sulphide Stress Corrosion (SSC) is another form of Stress Corrosion Cracking that can occur in environments containing both chlorides and Hydrogen Sulphide (H2S). It is of particular concern in the off-shore oil and gas industry.

Bacterial Corrosion occurs due to the presence and activity of certain types of
bacteria and tends to be localised, for example in crevices. It is overcome by good design, continuous flow and regular cleaning.

Inter-Granular Corrosion in austenitic stainless steels is a rapid and localized
phenomenon. Called sensitization, it is caused by Chromium Carbide precipitation at grain boundaries which depletes the surrounding area of chromium thus reducing its corrosion resistance. This may be caused by incorrect heat treatment, heat input during welding, service in the temperature range 450 to 850?C or service at a higher temperature and slow cooling through this range. Modern steelmaking, correct grade selection and post weld heat treatment prevents this occurring.

Weld Decay is a form of intergranular corrosion occurring in the heat-affected zone of the parent metal parallel to the weld. Susceptibility to this attack is assessed using
one of these standard tests:
?Test as given in BS1449/BS1501 using boiling copper sulphate/sulphuric acid.

?Test as given in ASTM A262 Practice C using boiling nitric acid. Low carbon grades perform better in the more severe (latter) test.

Fretting Corrosion which can also be called corrosion-abrasion is caused by continuous removal of corrosion product due to surfaces rubbing together, which leads to progressive wasting of material.

Corrosion Protection Methods
Applied Coatings
Plating, painting, and the application of enamel are the most common anti-corrosion treatments. They work by providing a barrier of corrosion-resistant material between the damaging environment and the (often cheaper, tougher, and/or easier-to-process) structural material. Aside from cosmetic and manufacturing issues, there are tradeoffs in mechanical flexibility versus resistance to abrasion and high temperature. Platings usually fail only in small sections, and if the plating is more noble than the substrate (for example, chromium on steel), a galvanic couple will cause any exposed area to corrode much more rapidly than an unplated surface would. For this reason, it is often wise to plate with a more active metal such as zinc or cadmium.

Reactive Coatings
If the environment is controlled (especially in recirculating systems), corrosion inhibitors can often be added to it. These form an electrically insulating and/or chemically impermeable coating on exposed metal surfaces, to suppress electrochemical reactions. Such methods obviously make the system less sensitive to scratches or defects in the coating, since extra inhibitors can be made available wherever metal becomes exposed. Chemicals that inhibit corrosion include some of the salts in hard water (Roman water systems are famous for their mineral deposits), chromates, phosphates, polyaniline, other conducting polymers and a wide range of specially-designed chemicals that resemble surfactants (i.e. long-chain organic molecules with ionic end groups).

Anodization
Aluminium alloys often undergo a surface treatment. Electrochemical conditions in the bath are carefully adjusted so that uniform pores several nanometers wide appear in the metal’s oxide film. These pores allow the oxide to grow much thicker than passivating conditions would allow. At the end of the treatment, the pores are allowed to seal, forming a harder-than-usual surface layer. If this coating is scratched, normal passivation processes take over to protect the damaged area.

Controlled Permeability Formwork
Controlled permeability formwork (CPF) is a method of preventing the corrosion of reinforcement by naturally enhancing the durability of the cover during concrete placement. CPF has been used in environments to combat the effects of carbonation, chlorides, frost and abrasion.

Cathodic Reaction
Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell.

It is a method used to protect metal structures from corrosion. Cathodic protection systems are most commonly used to protect steel, water, and fuel pipelines and tanks; steel pier piles, ships, and offshore oil platforms.

Examples of ALWC (accelerated low water corrosion) are in abundance in the north sea gas field off the Scottish coast. The steel structures used for oil and gas exploration and transport freely corrode when there is no CP ( Cathodic protection ) systems in place.

Sacrificial Anode Protection
For effective CP, the potential of the steel surface is polarized (pushed) more negative until the metal surface has a uniform potential. With a uniform potential, the driving force for the corrosion reaction is halted. For galvanic CP systems, the anode material corrodes under the influence of the steel, and eventually it must be replaced. The polarization is caused by the current flow from the anode to the cathode, driven by the difference in electrochemical potential between the anode and the cathode.

Impressed Current Cathodic Protection
For larger structures, galvanic anodes cannot economically deliver enough current to provide complete protection. Impressed Current Cathodic Protection (ICCP) systems use anodes connected to a DC power source (a cathodic protection rectifier). Anodes for ICCP systems are tubular and solid rod shapes of various specialized materials. These include high silicon cast iron, graphite, mixed metal oxide or platinum coated titanium or niobium coated rod and wires.

___________________________________


Forging
Forging is the process by which metal is heated and is shaped by plastic deformation by suitably applying compressive force. Usually the compressive force is in the form of hammer blows using a power hammer or a press.
Forging refines the grain structure and improves physical properties of the metal. With proper design, the grain flow can be oriented in the direction of principal stresses encountered in actual use. Grain flow is the direction of the pattern that the crystals take during plastic deformation. Physical properties (such as strength, ductility and toughness) are much better in a forging than in the base metal, which has, crystals randomly oriented.
Forgings are consistent from piece to piece, without any of the porosity, voids, inclusions and other defects. Thus, finishing operations such as machining do not expose voids, because there aren’t any. Also coating operations such as plating or painting are straightforward due to a good surface, which needs very little preparation.
Forgings yield parts that have high strength to weight ratio-thus are often used in the design of aircraft frame members.
A Forged metal can result in the following ?
?Increase length, decrease cross-section, called drawing out the metal.

?Decrease length, increase cross-section, called upsetting the metal.

?Change length, change cross-section, by squeezing in closed impression dies. This results in favorable grain flow for strong parts
Common Forging Processes
The metal can be forged hot (above recrystallization temperatures) or cold.
Open Die Forgings / Hand Forgings: Open die forgings or hand forgings are made with repeated blows in an open die, where the operator manipulates the workpiece in the die. The finished product is a rough approximation of the die. This is what a traditional blacksmith does, and is an old manufacturing process.
Impression Die Forgings / Precision Forgings: Impression die forgings and precision forgings are further refinements of the blocker forgings. The finished part more closely resembles the die impression.
Design Consideration:
?Parting surface should be along a single plane if possible, else follow the contour of the part. The parting surface should be through the center of the part, not near the upper or lower edges. If the parting line cannot be on a single plane, then it is good practice to use symmetry of the design to minimize the side thrust forces. Any point on the parting surface should be less than 75? from the principal parting plane.

?As in most forming processes, use of undercuts should be avoided, as these will make the removal of the part difficult, if not impossible.

?Recommended draft angles are described in the following table.
MaterialDraft Angle (?)
Aluminum0 – 2
Copper Alloys (Brass)0 – 3
Steel5 – 7
Stainless Steel5 – 8
?Generous fillets and radius should be provided to aid in material flow during the forging process. Sharp corners are stress-risers in the forgings, as well as make the dies weak in service. Recommended minimum radiuses are described in the following table.
Height of Protrusion
mm
(in)Min. Corner Radius
mm
(in)Min. Fillet Radius
mm
(in)
12.5
(0.5)1.5
(0.06)5
(0.2)
25
(1.0)3
(0.12)6.25
(0.25)
50
(2.0)5
(0.2)10
(0.4)
100
(4.0)6.25
(0.25)10
(0.4)
400
(16)22
(0.875)50
(2.0)
?Ribs should be not be high or narrow, this makes it difficult for the material to flow.

Tolerances:
?Dimension tolerances are usually positive and are approximately 0.3 % of the dimension, rounded off to the next higher 0.5 mm (0.020 in).

?Die wear tolerances are lateral tolerances (parallel to the parting plane) and are roughly +0.2 % for Copper alloys to +0.5 % for Aluminum and Steel.

?Die closure tolerances are in the direction of opening and closing, and range from 1 mm (0.040 inch) for small forgings, die projection area < 150 cm2 (23 in2), to 6.25 mm (0.25 inch) for large forgings, die projection area > 6500 cm2 (100 in2).

?Die match tolerances are to allow for shift in the upper die with respect to the lower die. This is weight based and is shown in the the following table.

MaterialFinished Forging Weight
Trimmed kg (lb)
< 10
(< 22)< 50
(< 110)> 500
(> 1100)
Die Match Tolerance
mm (in)
Aluminum, Copper Alloys, Steel0.75
(0.030)1.75
(0.070)5
(0.200)
Stainless Steel, Titanium1.25
(0.050)2.5
(0.100)6.5
(0.260)
?Flash tolerance is the amount of acceptable flash after the trimming operation. This is weight based and is shown in the following table.

MaterialFinished Forging Weight
Trimmed kg (lb)
< 10
(< 22)< 50
(< 110)> 500
(> 1100)
Flash Tolerance
mm (in)
Aluminum, Copper Alloys, Steel0.8
(0.032)3.25
(0.125)10
(0.4)
Stainless Steel, Titanium1.6
(0.064)5
(0.2)12.5
(0.5)
A proper lubricant is necessary for making good forgings. The lubricant is useful in preventing sticking of the workpiece to the die, and also acts as a thermal insulator to help reduce die wear.
Press Forgings: Press forging use a slow squeezing action of a press, to transfer a great amount of compressive force to the workpiece. Unlike an open-die forging where multiple blows transfer the compressive energy to the outside of the product, press forging transfers the force uniformly to the bulk of the material. This results in uniform material properties and is necessary for large weight forgings. Parts made with this process can be quite large as much as 125 kg (260 lb) and 3m (10 feet) long.
Upset Forgings: Upset forging increases cross-section by compressing the length, this is used in making heads on bolts and fasteners, valves and other similar parts.
Roll Forgings: In roll forging, a bar stock, round or flat is placed between die rollers which reduces the cross-section and increases the length to form parts such as axles, leaf springs etc. This is essentially a form of draw forging.
Swaging: Swaging – a tube or rod is forced inside a die and the diameter is reduced as the cylindrical object is fed. The die hammers the diameter and causes the metal to flow inward causing the outer diameter of the tube or the rod to take the shape of the die.
Net Shape / Near-Net Shape Forging: In net shape or near-net shape forging, forging results in wastage of material in the form of material flash and subsequent machining operations. This wastage can be as high as 70 % for gear blanks, and even 90+ % in the case of aircraft structural parts. Net-shape and near-net-shape processes minimize the waste by making precision dies, producing parts with very little draft angle (less than 1?). These types of processes often eliminate or reduce machining. The processes are quite expensive in terms of tooling and the capital expenditure required. Thus, these processes can be only justified for current processes that are very wasteful where the material savings will pay for the significant increase in tooling costs.
Tips for selecting the right Forging Material
?Technology to remain competitive must come out with cost effective alternatives. That is the reason computer aided techniques like CAD, CAM, CAE and Finite Element Analysis (FEA) based computer simulation, are used to selecting the right forging process.

?Understanding the forged material’s flow behaviour under processing conditions.
?Knowledge of the die geometry and materials.
?Environmental considerations.
?Evaluating the mechanics of deformation process-stress and strain.
?Friction and Lubricating process.
?Nature of the Forging equipment.
Applications of Forging Process
Automobile Industry: Wheel spindles, kingpins, axle beams and shafts, torsion bars, ball studs, idler arms and steering arm.

Agro-Industries: Engine and transmission components, levers, gears, shafts and spindles to tie-rod ends, spike harrow teeth and cultivator shafts.

Aerospace: Bulkheads, hinges, wing roots, engine mounts, brackets, beams, shafts, landing gear cylinders and struts, wheels, brake carriers and discs and arresting hooks, blades, buckets couplings etc.

Hand Tools: Sledges, pliers, hammers, wrenches and garden tools, as well as wire-rope clips, sockets, hooks, turnbuckles and eye bolts are common examples.

Industrial Equipment: Connecting rods, blanks, blocks, cylinders, discs, elbows, rings, T’s, shafts and sleeves.

___________________________________
Welding
Welding is the process of permanently joining two or more metal parts, by melting both materials. The molten materials quickly cool, and the two metals are permanently bonded. Spot welding and seam welding are two very popular methods used for sheet metal parts.

Spot welding is primarily used for joining parts that normally upto 3 mm (0.125 in) thickness.

Spot-weld diameters range from 3 mm to 12.5 mm (0.125 to 0.5 in) in diameter.

Welding Processes
Arc Welding
These processes use a welding power supply to create and maintain an electric arc between an electrode and the base material to melt metals at the welding point. They can use either direct (DC) or alternating (AC) current, and consumable or non-consumable electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas, and filler material is sometimes used as well. Types of arc welding ?
Atomic hydrogen welding, Bare metal arc welding, Carbon arc welding, Electrogas welding, Electroslag welding, Flux cored arc welding, Gas metal arc welding, Gas tungsten arc welding, Plasma arc welding, Shielded metal arc welding, Stud arc welding & Submerged arc welding.

Oxyfuel Gas Welding
The most common gas welding process is oxyfuel welding, also known as oxyacetylene welding. It is one of the oldest and most versatile welding processes. The equipment used is relatively inexpensive and simple, generally employing the combustion of acetylene in oxygen to produce a welding flame temperature of about 3100 ?C. The flame, since it is less concentrated than an electric arc, causes slower weld cooling, which can lead to greater residual stresses and weld distortion, though it eases the welding of high alloy steels. A similar process, generally called oxyfuel cutting, is used to cut metals. Other gas welding methods, such as air acetylene welding, oxygen hydrogen welding, and pressure gas welding are quite similar, generally differing only in the type of gases used. A water torch is sometimes used for precision welding of small items such as jewelry. Gas welding is also used in plastic welding, though the heated substance is air, and the temperatures are much lower. Types of oxyfuel gas welding are ?
Air acetylene welding, Oxyacetylene gas welding, Oxyhydrogen welding & Pressure gas welding.

Resistance Welding
Resistance welding involves the generation of heat by passing current through the resistance caused by the contact between two or more metal surfaces. Small pools of molten metal are formed at the weld area as high current is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are somewhat limited and the equipment cost can be high. Spot welding is a popular resistance welding method used to join overlapping metal sheets of up to 3 mm thick. Two electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. Weld strength is significantly lower than with other welding methods, making the process suitable for only certain applications. It is used extensively in the automotive industry ? ordinary cars can have several thousand spot welds made by industrial robots. A specialized process, called shot welding, can be used to spot weld stainless steel.
Like spot welding, seam welding relies on two electrodes to apply pressure and current to join metal sheets. However, instead of pointed electrodes, wheel-shaped electrodes roll along and often feed the workpiece, making it possible to make long continuous welds. In the past, this process was used in the manufacture of beverage cans, but now its uses are more limited. Other resistance welding methods include flash welding, projection welding, and upset welding.

Solid-State Welding
Like the first welding process, forge welding, some modern welding methods do not involve the melting of the materials being joined. One of the most popular, ultrasonic welding, is used to connect thin sheets or wires made of metal or thermoplastic by vibrating them at high frequency and under high pressure. The equipment and methods involved are similar to that of resistance welding, but instead of electric current, vibration provides energy input. Welding metals with this process does not involve melting the materials; instead, the weld is formed by introducing mechanical vibrations horizontally under pressure. When welding plastics, the materials should have similar melting temperatures, and the vibrations are introduced vertically. Ultrasonic welding is commonly used for making electrical connections out of aluminum or copper, and it is also a very common polymer welding process. Other solid-state welding processes include co-extrusion welding, cold welding, diffusion welding, exothermic welding, friction welding (including friction stir welding), high frequency welding, hot pressure welding, induction welding, and roll welding.

Energy Beam
Energy beam welding methods, namely laser beam welding and electron beam welding, are relatively new processes that have become quite popular in high production applications. The two processes are quite similar, differing most notably in their source of power. Laser beam welding employs a highly focused laser beam, while electron beam welding is done in a vacuum and uses an electron beam. Both have a very high energy density, making deep weld penetration possible and minimizing the size of the weld area. Both processes are extremely fast, and are easily automated, making them highly productive. The primary disadvantages are their very high equipment costs (though these are decreasing) and a susceptibility to thermal cracking. Developments in this area include laser-hybrid welding, which uses principles from both laser beam welding and arc welding for even better weld properties, and X-ray welding.

Materials to be used
?Low carbon steel is most suitable for spot welding. Higher carbon content or alloy steels tend to form hard welds that are brittle and could crack. This tendency can be reduced by tempering.
?Austenitic Stainless steels in the 300 series can be spot welded as also the Ferritic stainless steels. Martensitic stainless steels are not suitable since they are very hard.
?Aluminums can be welded using high power and very clean oxide free surfaces. Cleaning the surface to be oxide-free, adds extra costs (that can be avoided with low carbon steel).

?Dissimilar materials cannot be spot welded due to different melt properties and thermal conductivities. Plated steel welding takes on the characteristics of the coating. Nickel and chrome plated steels are relatively easy to spot weld, whereas aluminum, tin and zinc need special preparation inherent to the coating metals.

___________________________________
___________________________________