Dmp Exam #3

Wrought Structure: During hot rolling, cast structure is converted to a wrought structure, which has finer grains and enhanced ductility.

Bloom or slab: A bloom or a slab is product of first hot-rolling operation. A bloom has a square cross-section, and a slab is rectangular in cross-section. Blooms are processed further by shape rolling into structural shape, such as I-beams and railroad rails. Slabs are rolled into plates and sheets.

Billets are usually square shaped with a cross-sectional less than blooms. Billets are further rolled with shaped rolls into various shapes, such as round rods and bars.

Wavy edges: Due to roll bending, the strip is thinner along its edges than at its center. Therefore, the edges elongates more than the center consequently the edges buckle because of constrained of the central region creating the wavy edges.

Alligatoring is a complex phenomenon and is typically caused by nonuniform bulk deformation of the billet during rolling or by the presence of defects in the original cast material.

The usual method of avoiding crowning is to grind the rolls in such a way that their diameter at the center is slightly larger than at their edges (camber) Thermal camber is produced due to the heat generated due to the work in plastic deformation. The diameter of the roll is slightly increased in the center giving it a barrel shaped geometry. It can be corrected with proper cooling.

Flattening: rolls force tending to flatten the rolls elastically. Spreading: With smaller width-to-thickness ratios, the widths of plates and sheets increase significantly as it passes through the rolls. This increase in width is called spreading.

The 4 standard rolling configurations are two-high, three-high, four-high, and tandem. Cluster milling lowers roll forces and power requirements and reduces spreading. Moreover, when worn or broken, small rolls in cluster mills can be replaced at lower cost than can large ones.

Open die forging is when the work piece is compressed between two flat dies. Also called upsetting or flat-die forging. Barreling can occur due to friction between the die face and the work piece. This can be reduced by lubrication. Barreling can also be caused by using hot work pieces and cold dies. This can be eliminated by using heated dies. Cogging is another type of open die forging. Where the thickness of a bar will be reduced by successive open die forging steps at specific intervals along the length of the bar.

In impression die forging and closed die forging the work piece is compressed between two shaped dies. In impression die forging, material flows outward and forms a flash. In closed die forging a flash is not formed. For this reason it is also called flash-less forging.

In order to enhance the ductility of the metals and lower the forces required.

High melting point metals, brittle metals, (and mercury). For very high melting point metals, it is difficult to find furnaces that can withstand extreme temperatures during a forging operation. Brittle materials, such as cast iron will crack if forged with hammers and presses. Also, mercury which is a liquid at room temperature cannot be forged with hammers and presses.

Forged parts when subjected to a tension, will exhibit greater ductility and are generally tougher than cast parts. Depending on the type of processes and heat treatments used, forged parts will have smaller grain size. Contrarily, cast parts will be more isotropic than forged parts. For large quantities, forging is more economical. Sand casting is the more economical process for fewer than about 20,000 pieces

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As forged products obtain a wrought structure, as opposed to as-cast products. The cast structure results in a coarse, equi-axed structure of medium to large grains if suitably inoculated. Otherwise, they have a mixture of equi-axed, columnar and chilled zones. The wrought structure is better processed, with finer grain size along preferred directions and enhanced ductility. The wrought structures are stronger as a result, even prior to heat treatment.

Smaller rolls have a smaller contact area with the strip. This results in lower friction and hence lower forces and power consumed. Also, lower rolls are cheaper to replace, if they are damaged during the process due to wear and tear. Four-high Mills and Cluster milling lowers roll forces and power requirements and reduces spreading. Moreover, when worn or broken, small rolls in cluster mills can be replaced at lower cost than can large ones

Lubrication cannot be applied because fluids or solids used in lubrication displace volume in the forged product. This is because these lubricants are not compressible.

Due to roll bending, the rolled strip tends to be thicker at its center than at its edges. This is a crown on the strip. The usual method of avoiding crowning is to grind the rolls in such a way that their diameter at the center is slightly larger than at their edges. This is to say that the rolls are given a camber.

Due to the heat generated by plastic deformation during rolling rolls can become slightly barrelshaped. Thus, they are said to have acquired a thermal camber.

Flattening: rolls forces tend to flatten the rolls elastically. Spreading or side-spread: With smaller width-to-thickness ratios, the widths of plates and sheets increase significantly as it passes through the rolls. This increase in width is called spreading.

Three types of nonferrous casting alloys are Aluminum, Magnesium and Copper.

A solid, cylindrical specimen is upset between flat dies, and the reduction in height at which cracks on the barreled surface begin to develop is measure. The greater the deformation prior to cracking, the better the forgeability of the metal.

A round specimen is twisted continuously in the same direction until it fails. This test is performed on a number of specimens and at different temperatures, the number of complete turns that each specimen undergoes before failure at each temperature is then plotted. The temperature at which the maximum number of turns occurs becomes the forging temperature for maximum forgeability. The test has been found to be useful particularly for steels.

-Strength and toughness, especially at elevated temperatures.

-Hardenability and ability to harden uniformly.

-Resistance to mechanical and thermal shock.

-Wear resistance, particularly resistance to abrasive wear, because of the presence of scale in hot forging.

-Advantages-

• Simple and inexpensive dies

• Wide range of part sizes

• Good strength

-Limitations-

• Limited to simple shapes

• Difficult to hold close tolerances

• Machining necessary for final shape

• Low production rate

• Poor utilization of material

• High Degree of skill required

-Advantages-

• Good material utilization

• Better properties than open-die forgings

• Good dimensional accuracy

• High production rates

• Good reproducibility

-Limitations-

• High die cost

• Not economical for small quantities

• Machining often necessary

-Advantages-

• Close dimensional tolerances

• Very thin webs and flanges possible

• Machining generally not necessary

• Very good material utilization

-Limitations-

• High forging forces

• Intricate dies

• Provision for removing forging from dies.

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The investment casting also called as lost-wax process is the process of making a wax pattern, loading with refractory slurry and melted out the wax and then used to make castings.

It is an expendable mold type process as the mold is broken to remove casting. Tolerances achieved are better (tighter control) than sand casting. It is an inexpensive process compared to die casting (set up/equipment cost of dies is very expensive).

- Hot-chamber die casting

- Cold-chamber die casting

Hot-chamber die casting requires lower pressure than in cold-chamber die casting because materials at lower temperatures need higher pressure to be forced into (and fill) the mold cavity. Hot-chamber die casting pressure ranges up to 35 MPa (5000 psi), while cold-chamber die casting pressure ranges from 70 MPa (10,000 psi) to 150 MPa (20,000 psi).

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Examples of differences are that permanent-mold castings generally possess a better surface finish, closer dimensional tolerances, more uniform mechanical properties, and sounder thin-walled sections than sand castings. However, sand castings generally will be of more intricate shapes, larger overall sizes, and are lower in equipment cost than permanent-mold casting.

Size and shape of the casting, the dimensional accuracy and the quantity of castings required all should be considered. Strength and durability of the material selected for a pattern must reflect the number of castings that the mold will produce. Wear in critical regions should be reduced

Cores are utilized for castings with internal cavities or passages. Cores are placed in the mold cavity to form the interior surfaces of the casting and are removed from the finished part during shakeout and further processing. Cores must possess strength, permeability, the ability to withstand heat, and collapsibility.

In die casting, molten metal is forced into the die cavity at higher pressure than is commonly in pressure casting. In pressure casting, the molten metal is forced upward by gas pressure into a graphite or metal mold or by a vacuum.

Because of the high thermal conductivity the metal dies exhibit, there is a limiting thickness below which the molten metal will solidify prematurely before completely filling the mold cavity.

True centrifugal casting.

A riser supplies additional molten metal to the casting as it shrinks during solidification. A vent is placed in molds to carry off gases produced when the molten metal comes into contact with the sand in the mold and the core, it also exhausts air from the mold cavity as the molten metal flows into the mold.

Equipment costs, particularly the cost of dies, are somewhat high, but labor costs are generally low, because the process is semi- or fully automated.

Material, Equipment, Labor: Powering, solidification, extraction and finishing.

Die casting is one further example of permanent-mold casting, while in investment casting process, the mold will be broken. In die casting, molten metal is forced into the cavities at high pressure, while in investment casting, the molten metal is just poured into the mold cavity through gravity. More intricate shapes can be produced from investment casting than from die casting.

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-Smaller rolls

-Reduce friction through lubrication

-Reduce thickness reduction in a single pass

Whenever the width is greater than the thickness, there is no spreading. When width is less than the thickness or nearby, there is spreading.

The pattern is made of wax, or a plastic

-Wash

-Shrinkage

-Porosity