Materials Chapter 13 Review Questions

Continuous casting is usually employed in the solidification of basic shapes that become the feedstock for deformation processes such as rolling and forging.

How can continuous casting be used in the direct production of products?

The mold is turned upside down and positioned above a sealed, airtight chamber that contains a crucible of molten metal. A small pressure difference then causes the molten metal to flow upward into the die cavity.

How does low-pressure permanent-mold casting differ from the traditional gravity-pour process?

Dies need to be properly vented with wide, thin vents positioned along the parting line.

How does the air in the mold cavity escape in the die-casting process?

Usually slight cracks between mold halves or by very small vent holes that permit the escape of trapped air but not the passage of molten metal.

How is venting provided in the permanent-mold process?

Impregnation, where porosity that is or connected to free surfaces can be filled with resinous material. If the pores are filled with a lower-melting point metal, the process becomes infiltration.

How might defective castings be repaired to permit successful use in their intended applications?

They can dry molds, coat cores, vent molds, and clean or lubricate dies. They can tend stationary, cyclic equipment. They can be equipped with plasma cutters or torches to remove sprues, gates, and runner. They can perform grinding and blasting operations.

What are some of the ways that industrial robots can be employed in metal-casting operations?

In a hot-blast cupola, the stack gases are put through a heat exchanger to preheat the incoming air. Oxygen-enriched blasts can also be used to increase the temperature and accelerate the rate of melting. Plasma torches can be employed to melt the iron scrap.

What are some of the ways that the melting rate of a cupola can be increased?

Limited to low melting temperature metals and alloys. Has a high initial cost and shape size and complexity are limited.

What are some possible limitations of multiple-use molds?

Semi centrifugal casting mold shape is more complex and cores can be placed inside the mold to further increase complexity. Centrifuging, or centrifuge centrifugal casting, uses centrifugal action to force metal from a central pouring reservoir or sprue, through spoke-type runners, into separate mold cavities that are offset from the axis of rotation.

What are the key differences between true centrifugal casting, semicentrifugal casting, and centrifuging?

To maintain the metal at the proper temperature for pouring and to ensure that only high-quality metal is introduced into the molds.

What are the primary functions of a pouring operation?

Alloy being cast, mold material, pouring temperature, mold temperature, mold configuration.

What features affect the life of a permanent mold?

A material that can be handled like a solid but flows like a liquid. Because the material is already partially solid, the lower injection temperatures and reduced solidification time act to extend tool life.

What is a thixotropic material? How does it provide an attractive alternative to squeeze casting or rheocasting?

Materials that aren't suitable for hot-chamber such as aluminum, magnesium, and copper as well as high-aluminum zinc.

What metals are routinely cast with cold-chamber die-casting machines?

A significant amount of gray, nodular, and white cast iron.

What types of metals are commonly melted in cupolas?

Candlesticks, lamp bases, and statuary from the low-melting-temperature metals

What types of products would be possible candidates for manufacture by slush casting?

Low quantities can be justified if extensive secondary machining or surface finishing can be eliminated.

When might low quantities be justified for the die-casting process?

It retains its heat better due to the secondary coil of molten metal where all the liquid metal is free to circulate through the loop and gain heat.

Why are channel induction furnaces attractive for metalholding applications where molten metal must be held at a specified temperature for long periods of time?

Prevents the formation of hot tears that may form if the product is restrained during the shrinkage that occurs during cool down to room temperature.

Why are permanent-mold castings generally removed from the mold immediately after solidification has been completed?

High injection pressure can cause turbulence and air entrapment, a lower injection pressure that gradually increases over time would reduce porosity and inclusion content.

Why might it be advantageous to vary the pressure on the molten metal during the die-casting cycle?

High initial mold cost requires a large production to be profitable.

Why might low production runs be unattractive for permanent-mold casting?

Risers are not used in the die-casting process since the high injection pressures ensure the continuous feed of molten metal from the gating system into the casting. Sand cores cannot be used in die casting because the high pressures and flow rates cause the cores to either disintegrate or have excessive metal penetration.

Are risers employed in die casting? Can sand cores be used?

If the rate of temperature change is the dominant feature, the problem is called heat checking. If the number of cycles is the primary cause, the problem is called thermal fatigue.

By what mechanisms do die-casting dies typically fail?

Low pressure operates around 3 to 15 psi while die casting is around several thousand psi.

Contrast the feeding pressures on the molten metal in low pressure permanent molding and die casting.

Iron is not included in die casting like it is in gravity-pour permanent-mold casting because it can't withstand the high pressures.

Contrast the materials used to make dies for gravity-pour permanent-mold casting and die casting.Why is there a notable difference?

Strong dense exterior with all of the lighter impurities collecting on the inner surface of the casting.

Contrast the structure and properties of the outer and inner surfaces of a centrifugal casting.

Cost of patterns, molds, dies, melting and pouring equipment, scrap material, cleaning, inspection, and all related labour. Sand casting are a lot cheaper due to the lower cost of sand compared to die casting.

Describe some of the features that affect the cost of a cast product. Why might the cost vary significantly with the quantity to be produced?

Mold is reusable. A good surface finish is obtained if the mold is in good condition. Near-net shapes can be produced that require little finish machining. Dimensions are also consistent from part to part.

Describe some of the process advantages of permanent-mold casting.

In the squeeze casting process, molten metal is introduced into the die cavity of a metal mold, using large gate areas and slow metal velocities to avoid turbulence. When the cavity has filled, high pressure (20 to 175 MPa, or 3000 to 25,000 psi) is then applied and maintained during the subsequent solidification.

Describe the squeeze casting process.

Zinc, Tin, and Lead based alloys due to their lower melting points.

For what types of materials would a hot-chamber die-casting machine be appropriate?

Removing cores. Removing gates and risers. Removing fins, flash, and rough spots from the surface. Cleaning the surface. Repairing any defects.

What are some of the typical cleaning and finishing operations that are performed on castings?

Mechanical properties are typically about 5% better than those of conventional permanent-mold castings. Cycle times are somewhat longer, however, than those of conventional permanent molding.

What are some additional advantages of vacuum permanent mold casting over the low-pressure process?

Gray cast iron, alloy cast iron, steel, bronze, graphite, etc. Most frequently cast are aluminum, magnesium, zinc, lead, and copper based alloys along with gray cast iron.

What are some common mold materials for permanent-mold casting? What are some of the metals more commonly cast?

Sand cores can usually be removed by mechanical shaking. Sometimes they need to be removed by chemically dissolving the core binder. Runners, gates, and risers can sometimes be knocked off. They can also be but with an abrasive cutoff wheel.

What are some common ways to remove cores from castings? To remove runners, gates, and risers?

Small castings are often tumbled in barrels to remove fins, flash, and sand that may have adhered to the surface.

What are some of the alternative methods of cleaning and finishing castings?

Rapid melting rates. Ability to hold the molten metal for any desired period of time. Greater ease of incorporating pollution control equipment.

What are some of the attractive features of arc furnaces in foundry applications?

Attractive aspects include smooth surfaces and excellent dimensional accuracy. Thinner sections can be cast than with either sand or permanent-mold casting.

What are some of the attractive features of die casting compared to alternative casting methods?

In all of the semisolid casting processes, the absence of turbulent flow during the casting operation minimizes gas pickup and entrapment.

What are some of the attractive features of semisolid casting?

This metal is exceptionally clean, and product quality is further enhanced by the nonturbulent mold filling, which helps to minimize gas porosity and dross formation.

What are some of the attractive features of the low-pressure permanent-mold process?

The temperature needed to melt and superheat the metal. The alloy being melted and the form of available charge material. The desired quality of the metal. The availability and cost of various fuels. The variety of metals or alloys to be melted. Whether melting is to be batch or continuous. The required level of emission control. The various capital and operating costs.

What are some of the factors that influence the selection of a furnace type or melt procedure in a casting operation?

Economic, technical, and management considerations.

What are some of the key factors that should be considered when selecting a casting process?

In each of the expendable-mold processes a separate mold had to be created for each pour. This results in rather low production rates.

What are some of the major disadvantages of the expendable-mold casting processes?

Mix of pure primary metal and commercial scrap, along with recycled gates, runners, sprues, and risers as well as defective cases.

What are some of the possible feedstock materials that may be put in foundry melt furnaces?

Stirring action, temperature control, and chemistry control are often poor, and furnace size and melting rate are limited. Nevertheless, these furnaces do offer low capital and operating cost.

What are some of the pros and cons of indirect fuel-fired furnaces?