ME 151 capter 15 TWG

Discuss the significance of wear in metal forming:(a)wear on the workpiece. (b) wear on the tooling.

(a) A little wear on a workpiece makes it shiny and stand out. (b) Wear on tooling creates changes in workpiece dimensions and loss in tolerance control resulting in replacement tooling parts.

According to modern friction theory, frictional resistance can be attributed to what two physical phenomenon?

Abrasion and adhesion.

What are some of the secondary effects that may occur when the speed of a metal forming process is varied?

Affects the forces required for deformation, lubricant effectiveness, and the available time for heat transfer.

Why are friction effects in metal working difficult to scale down for laboratory testing or scale up from laboratory conditions to production conditions?

Because of the many variables that determine friction.

If the deformed grains recrystallize during hot working, how can the process impart an oriented or flow structure?

By unfavorably oriented or intersecting surfaces.

What simplifying assumptions are often made regarding friction between the tool and the workpiece?

Friction depends on the contact pressure, contact area, lubricant, speed, and the surface finish and the mechanical properties of the two contacting materials.

What are several ways in which the friction conditions during metal working differ from the friction conditions found in most mechanical equipment?

Friction in mechanical equipment involves two surfaces of similar material and strength, experiencing elastic loads which don't permanently change in shape, wear-in cycles that produce surface compatibility, and operating temperatures in the low-to-moderate-range.

How can hot working be used to improve the grain structure of a metal?

Grain growth, additional deformation and recrystallization, or a drop in temperature that will end diffusion and "freeze in" the recrystallized structure.

How could cold working be used to reduce the cost of a moderate- to high-strength product?

If the quantity for production is very high.

Why is friction such an important parameter in metalworking operations?

It accounts for more than fifty percent of the in put energy which must overcome it. Also, there is rolling metal forming which can not be done unless the right amount of friction exists.

How can the selective placement of the final intermediate anneal be used to establish desired final properties in a cold-formed product?

It can be Judiciously positioned in the deformation cycle.

Is the anisotropy induced by cold working an asset or a liability? What about the residual stresses?

It is both. Residual stresses can be beneficial and quite harmful to a final product.

Why are cold forming processes better suited for large-volume production of precision parts?

It justifies the use of powerful equipment and expensive and exact tools or dies.

Why is lubrication often a major concern in metal forming?

It reduces friction, acts as a coolant, thermal barriers, corrosion inhibitors, and parting compounds.

What are some of the benefits of hydrodynamic lubrication?

May reduce the force and power required by as much as thirty to forty percent, and tool wear becomes almost non-existent.

Why is it often difficult to determine the specific relationships between independent and dependent variables?

Metal forming processes are complex systems composed of many variations of processes amassing to quite a large amount.

Compared to hot-working, what are some of the advantages of cold-working processes?

No required heating, a better surface finish is yielded, superior dimensional control and little to no additional machining. Strength, fatigue, and wear properties are all improved. Contamination is minimized.

What is the primary cause of residual stresses in hot-worked products.

Non-uniform cooling throughout the casting.

What is an acceptable definition of hot working? Is a specific temperature involved?

Plastic deformation of a certain material above its recrystallization temperature. ( for some materials, these temperatures are fairly close to room temperature so "hot working" doesn't imply high temperatures in some cases)

What features have contributed to the expanded use of process modeling?

Process simulations of models on computers which are now quick, inexpensive, and quite accurate.

What are some disadvantages of cold-forming process?

Required higher forces, heavier and more powerful equipment and stronger tooling are required. Less ductility. Metal surfaces must be clean and scale free. Intermediate anneals may be required to compensate for the loss in ductility. Imparted directional properties may be detrimental. Undesirable residual stresses may be produced.

What are Luders bands or stretcher strains, and what causes them to form? How can they be eliminated?

Ridges and valleys formed from deformation causing certain areas to thin and lengthen while others remain thee same. They can be eliminated if the material is first shaped from pre-rolled material.

How might a material's performance vary with changes in the speed of deformation?

Some rate-sensitive materials may shatter or crack if impacted while others are stronger when deformed at higher speeds.

What are some of the common types of metal forming lubricant additives?

Strait oils, water-soluble oils, synthetic lubricants, semi-synthetics, dry-film lubricants, and chemically bonded agents.

What generally restricts the upper temperature to which dies or tooling is heated?

Such high temperatures lead to shorter tool life spans and mold lifes spans.

Lubricants are often selected for properties in addition to their ability to reduce friction, What are some of these additional properties?

The ability to act as a thermal barrier, coolant, and to retard corrosion.

What is plasticity?

The ability to flow as solids without deterioration of their properties.

What are some of the general assets of the metal deformation process? Some general liabilities?

The amount of waste is substantially reduce while on the down side, the forces required are often high, machining and tooling are expensive, and large production quantities may be necessary to justify such an approach.

What are some considerations regarding selection of the starting material for a forming process?

The chemistry, properties and characteristics of the selected material.

What material feature is considered to be the driving force for isothermal forming?

The close tolerances, low residual stresses, and fairly uniform metal flow and product quality.

What is a dependent variable in a metal forming process?

The consequences of the independent variable selection.

Why is isothermal forming considerably more expensive than conventional hot forming.

The constant temperature of the workpiece, dies, and tooling, resulting in shorter lifespans for all.

Why might large production quantities be necessary to justify metal deformation as a means of manufacturing?

The cost and force needed for manufacturing is great, and is too expensive if it is only for a small yield when cheaper means are available that better suit low production quantity.

What is the significance of tool and die geometry in designing a successful metal forming process?

The detail from the mold, bend radius, and the control of metal flow as the material goes from starting shape to finished product.

What features may limit the accuracy of a mathematical model?

The differing microstructures of the same material that yield from different prior processes.

Why might a rolled thread offer improved strength and fracture resistance compared to a machined thread?

The flow lines of the rolled thread are parallel to the external surfaces at all high-stress locations while for the machined thread, the flow lines run along the external surface allowing the easier formation of cracks and fractures.

What is a constitutive relation for an engineering material?

The mathematical descriptions of material behavior as a function of the process conditions.

What type of information about a material being deformed may be particularly significant to a metal forming engineer?

The strength or resistance of a material to deformation at the relevant conditions of temperature, speed of deformation, and amount of prior straining? What are the formability limits and conditions of anticipated fracture? What is the affect of temperature changes? To what extent does the material strain-harden? What are the re-crystallization kinetics? Will the material react in certain environments or to certain lubricants? What is the speed of deformation and their affects?

What is tribology?

The study of friction and lubrication.

Why are heated dies or tools often employed in hot working processes?

The temperature difference would otherwise damage the molds and tools at a faster rate.

How can the tensile test properties of a metal be used to assess its suitability for cold forming?

The use of stress strain diagrams to gage the region of forces needed to deform the metal.

Why is there no standard test to assess or qualify formability?

There is no universal formability scale and any material may excel in on field of forming and then fail miserably in another.

What is an independent variable in a metal forming process?

They are aspects of a process over which the engineer or operator has direct control, and they are generally selected or specified during setup.

Why is it important to control the final properties of a deformation product?

They are the properties that the customers are looking for.

What are some of the negative aspects of hot working?

Un-desirable reactions between the metal and its surroundings. Poorer tolerances and warping or distortion can occur.

Why is it important to control not only the external shape of a formed product, but also its internal flow?

While the objective of an operation is to attain a desired shape and possesses thee right set of companion properties, with out any surface or internal defects.

Why is it important to able to predict the forces or powers required to perform specific forming processes?

Without a reasonable estimate, we are unable to specify the equipment for the process, select appropriate tool or die materials, compare various die designs or deformation methods, or ultimately optimize the process.

What are some of the attractive manufacturing and metallurgical features of hot-working processes?

Deformation can be used to alter the shape of a metal without fear of fracture and without excessively high forces.

What engineering properties are likely to decline during the cold working of a metal?

Ductility, elongation, reduction in area, electrical conductivity, and corrosion resistance will all decline.

If the temperature of a material is increased, what changes in properties might occur that would promote the ease of deformation?

A decrease in strength and rate of strain-hardening, and an increase in ductility.

Define the various regimes of cold working, warm working, and hot working in terms of the melting point of the material being formed?

-Cold- plastic deformation of metals below the re-crystallization temperature. -Warm- deformation at temperatures intermediate to hot and cold forming. -Hot- plastic deformation above the re-crystallization temperature.

How might friction be mathematically described under conditions of light, elastic loading? Under conditions of heavy loads, sufficient to induce plastic deformation?

-Light and Elastic- friction is directly proportional to the applied pressure with the proportionality constant. -At high pressures- friction becomes independent of contact pressure and is more closely related to the strength of the weaker material.

Why is it important to know and control the thermal history of a metal as it undergoes deformation?

Changes in the temperature can lead to the breakdown of lubricants and the alterations of properties

What are some of the advantages of warm forming compared to cold forming? Compared to hot forming?

Compared to cold, it has reduced loads on the tooling and equipment, increased material ductility, and the possible reduction in the number of anneals. Compared to hot, lower temperatures produce less scaling, decarburization, enabling the production of products with better dimensional precision and smooth surfaces.

What factors generally set the upper temperature limit for hot working?

Excess oxidation, grain growth, or undesirable phase transformations.

What are the three distinct ways of determining the inter-relation of independent and dependent variables?

Experience, Experiment, and Process modeling.

What are some uses or applications of process models?

Experiments and experience revealing unnoticed features that can be used to eliminate defects, optimize performance, or extend a process into a previously unknown field.

What features limit the value of laboratory experiments in modeling metal forming processes?

Experiments are time consuming and costly.

Why is elastic springback an important consideration in cold-forming process?

it is the region or amount of deformation that the product can endure while still being able to revert back to its original dimensions.