Formningsteknik - assignment 3 (review+multiple choice quiz)

As particle size increases, interparticle friction (a) decreases, (b) increases, or (c) remains the same?

(a) decreases

What are the three basic steps in the conventional powder metallurgy shaping process?

(1) Blending and mixing of the powders (2) Compaction, in which the powders are pressed into the desired part shape. (3) Sintering, which involves heating to a tempereature below the melting point to cause solid-state bonding of the particles.

What are the three steps in the sintering cycle in PM?

(1) Preheat, in which lubricants and binders are burned off. (2) Sinter (3) Cool down

Which of the following design features would be difficult or impossible to achieve by conventional pressing and sintering (three best answers): (a) outside rounded corners, (b) side holes, (c) threaded holes, (d) vertical stepped holes, and (e) vertical wall thickness of 1/8 inch (3 mm)?

(a) outside rounded corners (b) side holes (c) threaded holes

Which of the following statements is correct in the context of metallic powders (three correct answers): (a) porosity + packing factor = 1.0, (b) packing factor = 1/porosity, (c) packing factor = 1.0 - porosity, (d) packing factor = - porosity, (e) packing factor = bulk density/true density?

(a) porosity + packing factor = 1.0 (c) packing factor = 1.0 - porosity (e) packing factor = bulk density/true density

What are some of the ingredients usually added to the metallic powders during blending and/or mixing?

-Lubricants, such as stearates of zinc and aluminium, in small amounts to reduce friction between particles and at the die wall during compaction. -Binders, which are required in some cases to achieve adequate strength in the pressed but unsintered part. -Deflocculants, which inhibit agglomeration of powders for better flow characteristics during subsequent processing.

What are some of the disadvantages of PM methods?

-Tooling and equipment costs are high -metallic powders are expensive -there are difficulties with storing and handling metal powders (such as degradation of the metal over time and fire hazards with particular metals)

What is the difference between open pores and closed pores in a metallic powders?

Any volume of loose powders will contain pores between the particles. These are called open pores because they are external to the individual particles. Open pores are spaces into which a fluid such as water, oil or molten metal can penetrate. In addition, there are closed pores - internal voids in the structure of an individual particle. The existence of theses internal pores is usually minimal, and their effect when they do exist in minor, but they can influence density measurements.

What is the technical difference between mixing and blending in powder metallurgy?

Blending is when powders of the same chemistry but different particle sizes are intermingled. Mixing refers to powders of different chemistries being combined.

What are the advantages of infiltration in PM?

The resulting structure is relatively nonporus, and the infiltrated part has a more uniform density, as well as improved toughness and strength.

In the screening of powders for sizing, what is meant by the term mesh count?

The term mesh count is used to refer to the number of openings per linear inch of screen. Higher mesh count indicates smaller particle size

What is meant by the term green compact?

The work part after pressing is called a green compact.

Define bulk density and true density for metallic powders.

True density is the density of the true volume of the material. This is the density when the powders are melted into a solid mass. Bulk density is the density of the powders in the loose state after pouring, which includes the effect of pores between particles. because of the pores, bulk density is less than true density.

What are some of the reasons why a controlled atmosphere furnace is desirable in sintering?

- Protection from oxidation - Providing a reducing atmosphere to remove existing oxides - Providing a carburizing atmosphere - Assisting in removing lubricants and binders used in pressing. - Vacuum atmospheres are used for certain metals, such as as stainless steel and tungsten.

In cold isostatic pressing, the mold is most typically made of which one of the following: (a) rubber, (b) sheetmetal, (c) textile, (d) thermosetting polymer, or (e) tool steel?

(a) rubber

The particle size that can pass through a screen is obtained by taking the reciprocal of the mesh count of the screen: (a) true or (b) false?

(b) false

Impregnation refers to which of the following (two best answers): (a) filling the pores of the PM part with a molten metal, (b) putting polymers into the pores of a PM part, (c) soaking oil by capillary action into the pores of a PM part, and (d) something that should not happen in a factory?

(b) putting polymers into the pores of a PM part (c) soaking oil by capillary action into the pores of a PM part

Which of the following powder shapes would tend to have the lowest interparticle friction: (a) acicular, (b) cubic, (c) flakey, (d) spherical, and (e) rounded?

(d) spherical

Name some of the reasons for the commercial importance of powder metallurgy technology.

- PM parts can be mass produced to net or near net shape. - The PM process itself involves very little waste of material (uses about 97%). -Owing to the nature of the starting material in PM, parts having a specified level of porosity. -Certain metals that are difficult to fabricate by other methods can be shaped by powder metallurgy. -Certain metal alloys combinations and cermets can be formed by PM that cannot be produced by other methods. - PM compares favorably with most casting processes in terms of dimensional control of the product. Tolerances of ±0.13 mm are held routinely. - PM production methods can be automated for economical production.

What are the principal methods used to produce metallic powders?

-Atomization -Chemical -Electrolytic

How would one measure the angle of repose for a given amount of metallic powder?

A common measure of interparticle friction is the angle of repose, which is the angle formed by a pile of powders as they are poured from a narrow funnel.

What is meant by the term aspect ratio for a metallic particle?

A simple and useful measure of shape is the aspect ratio - the ratio of maximum dimension to minimum dimension for a given particle.