Manufacturing Processes: Powder Metallurgy, Metal Forming, Bulk Deformation, Sheet Metalworking, and Casting

Comprehensive practice flashcards covering Powder Metallurgy, Metal Forming Fundamentals, Bulk Deformation Processes, Sheet Metalworking, and Metal Casting Processes based on lecture notes and technical figures.

What is powder metallurgy (PM), and what are its two primary production steps?

Powder metallurgy is a metal processing technology where parts are produced from metallic powders. Its conventional sequence consists of: (1) pressing, where powders are compressed in a press using punch-and-die tooling to form a green compact, and (2) sintering, where green compacts are heated below their melting point to bond particles into a hard, rigid mass.

How is mesh count defined in particle size measurement, and how does it relate to particle size?

Mesh count refers to the number of openings per linear inch of screen. Since the mesh is square, a screen with a mesh count of $200$ has $200^2 = 40{,}000$ openings per square inch. A higher mesh count indicates a smaller particle size.

Identify the eight engineering powder particle shapes shown in the diagram.

Top row (left to right): Spherical, Rounded, Cylindrical, Spongey. Bottom row (left to right): Acicular, Flakey, Cubic, Aggregated.

How is interparticle friction evaluated in powder metallurgy, and what powder geometry minimizes it?

Interparticle friction is commonly tested using the angle of repose formed by a pile of powders poured from a narrow funnel, where larger angles indicate greater friction. Spherical shapes exhibit the lowest interparticle friction, whereas deviations from spherical shapes and smaller particle sizes increase friction.

How are packing factor and porosity defined and mathematically related for loose powders?

Packing factor is the bulk density divided by the true density, typically ranging from $0.5$ to $0.7$ for loose powders. Porosity is the ratio of pore volume to bulk volume. Assuming closed particle pores are accounted for, they relate as: $\text{Porosity} + \text{Packing factor} = 1.0$.

What metallic powder production method is depicted in this schematic, and how does it work?

Gas atomization. A high-velocity gas stream flows through an expansion nozzle, siphoning molten metal from a bath and spraying it into tiny droplets that solidify into metal powder within a collection chamber.

Describe the four microscopic stages that take place during the sintering sequence shown below.

(1) Particle bonding is initiated at contact points. (2) Contact points grow into "necks." (3) Pores between particles decrease in size. (4) Grain boundaries develop between particles in place of the necked regions.

What is the key difference between impregnation and infiltration in powder metallurgy secondary processing?

Impregnation permeates oil, fluid, or polymer resin into the pores of a sintered PM part (e.g., self-lubricating bearings). Infiltration fills the open pores with a molten metal having a lower melting point than the PM part via capillary action, producing a nonporous structure with uniform density and higher strength.

According to PM design guidelines for chamfers and corner radii, why must acute punch angles and full outside corner radii be avoided?

Acute angles must be avoided because they compromise punch rigidity (a minimum angle of $45^\circ$ is preferred). Full outside corner radii must be avoided because they cause the punch edge to be fragile; combining an outside radius with a chamfer is preferred.

What two material properties are desirable in metal forming, and how does raising work temperature alter them?

The desirable properties are low yield strength and high ductility. When work temperature is raised, ductility increases and yield strength decreases, reducing the required forming forces and power.

How are cold working, warm working, and hot working differentiated based on absolute melting point ($T_m$)?

Cold working is performed at room temperature or slightly above. Warm working is performed above room temperature up to recrystallization temperature, typically around $0.3\,T_m$. Hot working is conducted above the recrystallization temperature, typically above $0.5\,T_m$.

What are the primary advantages and disadvantages of cold forming?

Advantages include closer tolerances, better surface finish, increased strength/hardness via strain hardening, favorable directional grain properties, and no heating requirement. Disadvantages include higher required forces and power, starting stock that must be clean of scale, and limited deformation due to strain hardening.

What mechanical force difference distinguishes extrusion from wire and bar drawing?

In extrusion (c), the work billet is pushed through the die opening by a compressive ram force ($F$). In wire and bar drawing (d), the work metal is pulled through the die opening by a tensile pulling force ($F$).

What basic bulk deformation process is depicted below, and what dual action do the rolls perform?

Flat rolling. The opposing rotating rolls pull the work into the gap between them via friction while simultaneously applying compressive forces that squeeze the work to reduce its cross-sectional thickness.

What are the three intermediate rolled forms of steel, and what final products are derived from each?

Blooms produce structural shapes and rails. Slabs produce coils, plates, and sheets. Billets produce bars and rods.

What deformation process is illustrated below, and why does it produce stronger threads than machining?

Thread rolling. It forms threads by cold rolling a cylindrical blank between reciprocating flat dies. It produces stronger threads and better fatigue resistance because of favorable grain flow and strain hardening, while offering higher production rates and zero material waste.

Explain the ring rolling operation shown in the diagram and identify its typical manufactured components.

A thick-walled ring of smaller diameter is compressed between a main drive roll and an idler roll, elongating the metal to enlarge the ring's diameter into a thin-walled section while edging rolls constrain its height. Typical products include ball and roller bearing races, steel tires for railroad wheels, and machinery rings.

What distinguishes flashless forging from impression-die forging, as demonstrated in the diagram?

In flashless forging, the workpart is completely constrained within a closed punch-and-die cavity so that no excess metal escapes as flash. Because there is no flash allowance, starting work volume must equal die cavity volume within very close tolerances.

What upset forging operation is represented here, and what types of hardware products does it produce?

Heading (upset forging). Wire or bar stock is fed against a stop, gripped in dies, and axially compressed by a punch to form heads on nails, bolts, and screws.

What tooling component is incorporated in direct extrusion to produce the hollow cross sections shown below?

A mandrel. Attached to the ram, the mandrel projects through the billet and into the die opening, forcing the metal to flow through the annular gap between the die and mandrel to create hollow or semi-hollow tubing.

Identify the equipment shown below and describe its operation.

A hydraulically operated bar draw bench. Starting bar stock from an entry table is pulled through a stationary die stand by a carriage driven along an exit rack by a hydraulic cylinder, reducing the bar cross section.

How do wire drawing machines differ from bar drawing benches?

Bar drawing uses straight-pull draw benches for large-diameter stock in discrete lengths. Wire drawing continuous coiled stock down to sizes as small as $0.03\,\text{mm}$ ($0.001\,\text{in}$) using continuous multi-die machines ($4$ to $12$ dies) separated by power-driven accumulating capstan drums.

What thickness threshold separates sheet metal from plate stock?

Sheet metal ranges from $0.4\,\text{mm}$ ($1/64\,\text{in}$) to $6\,\text{mm}$ ($1/4\,\text{in}$) in thickness. Commercial stock with a thickness exceeding $6\,\text{mm}$ is defined as plate stock.

Identify the four sequential events occurring during sheet metal cutting as shown in the diagram.

(1) Just before punch contacts work. (2) Punch pushes into work, causing plastic deformation. (3) Punch penetrates into work, causing a smooth cut surface. (4) Fracture is initiated at opposing cutting edges to separate the sheet.

What is the operational distinction between blanking and punching (piercing)?

In blanking, the piece sheared out from the surrounding sheet stock is the desired part (called a blank). In punching (piercing), the piece sheared out is scrap (called a slug), and the remaining sheet is the desired part.

What mechanical advantage is achieved by using the inclined upper cutting blade on the power shears shown below?

An inclined upper cutting blade shears the sheet incrementally along its width rather than all at once, significantly reducing the maximum cutting force and tonnage required by the machine.

In sheet metal bending, what stress states develop across the cross section relative to the neutral plane?

Metal on the inside of the neutral plane is placed in compression, while metal on the outside of the neutral plane is stretched under tensile stress.

What sheet metal operation is illustrated below, and what is its primary effect on a drawn cup?

Ironing. It forces a pre-drawn cup through a clearance die narrower than the original wall, causing thinning and vertical elongation of the cup wall to achieve uniform thickness.

What sheet metal process is illustrated below, and how does it create continuous profiles?

Roll forming. It is a continuous bending process in which opposing matched pairs of shaped rolls progressively deform strip or coil stock into complex longitudinal contours like channels and structural sections.

What is the Guerin process in sheet metal forming, and what is its main advantage?

The Guerin process uses a thick rubber pad mounted in a press head to force sheet metal over an underlying form block. Its primary advantage is low tooling cost, as form blocks can be made of easily worked materials like wood or plastic, and a single rubber pad serves multiple block shapes.

What are the core trade-offs between expendable mold and permanent mold casting processes?

Expendable mold processes sacrifice the mold to retrieve each casting; they can produce more complex part shapes, but production rates are limited by mold preparation time. Permanent mold processes reuse metallic molds, enabling higher production rates and closer tolerances, but geometry is restricted by the necessity to open the mold.

Identify the four types of sand casting patterns displayed in the image.

(a) Solid pattern (b) Split pattern (c) Matchplate pattern (d) Cope and drag pattern

What is the composition of typical foundry sand used in greensand molding?

A standard greensand mix consists of $90\%$ silica sand ($\text{SiO}_2$), $7\%$ bonding clay, and $3\%$ water.

What formula calculates the net buoyancy force ($F_b$) acting on a sand core during casting pouring?

$F_b = W_m - W_c$, where $W_m$ is the weight of the molten metal displaced by the core and $W_c$ is the physical weight of the core.

What step in the shell molding process is shown below, and what physical change does it produce?

Step (2): The dump box is inverted so that the sand and thermosetting resin mixture falls onto a preheated metal pattern, causing the resin to partially cure and form a hard sand shell over the pattern surface.

Explain how the expanded polystyrene (lost-foam) casting process operates during the pouring stage shown in step (2).

A refractory-coated polystyrene foam pattern is packed with sand in a mold box. When molten metal is poured, the heat vaporizes the polystyrene foam ahead of the advancing liquid metal front, allowing the metal to fill the cavity without requiring the mold to be split or the pattern pulled.

What initial assembly stage of investment casting is illustrated in the diagram?

(1) Production of individual wax patterns and (2) attachment of multiple wax patterns onto a central wax sprue to form a pattern tree.

How does hot-chamber die casting operate, and what metals are suitable for this machine?

In hot-chamber die casting, a plunger and gooseneck are submerged directly within a molten metal pot, forcing liquid metal under high pressure into the closed die. It is restricted to low-melting-point alloys that do not attack the plunger, such as zinc, tin, lead, and magnesium.

How does cold-chamber die casting differ in metal introduction from the hot-chamber process?

Molten metal is transferred from an external furnace by a ladle into an unheated shot chamber, after which a horizontal hydraulic ram forces it under high pressure into the die. This isolates the injection components from constant contact with liquid metal, allowing the casting of higher-melting-point alloys such as aluminum and brass.

What centrifugal casting variant is shown below, and how does it fill the individual part cavities?

Centrifuge casting. Mold cavities are situated away from the central rotational axis, and rotation forces molten metal poured into the central sprue outwards through radial runners to fill each cavity under centrifugal pressure.

What casting defect is illustrated in this diagram, and what causes it?

Cold shots. They form when metal splatters during pouring, creating premature solid globules that become entrapped within the advancing molten stream without fully fusing.

What sand casting defect is shown below, and how does it form?

Pin holes. They are many tiny gas cavities formed at or slightly beneath the casting surface due to the release and entrapment of mold gases during pouring.

What standard draft angle guidelines and machining allowances are recommended for casting parts?

Draft angles are typically $1^\circ$ for expendable sand molds to ease pattern removal, and $2^\circ\text{ to }3^\circ$ for permanent molds to eject the finished part. Machining allowances for sand castings typically range from $1.5\,\text{mm}\text{ to }3\,\text{mm}$ ($1/16\,\text{in}\text{ to }1/4\,\text{in}$).