Sheet Metal Forming and Surface Treatment

Flashcards covering the definitions, processes, and technical characteristics of sheet metal forming methods and surface treatment technologies as described in the lecture.

Foils

Sheet metal workpieces with a thickness of less than $0.2\,mm$.

Sheets

Metal workpieces with a thickness between $0.2\,mm$ and $6\,mm$.

Plates

Metal workpieces with a thickness higher than $6\,mm$.

Sheet Making

The industrial process used to produce sheet metal, primarily utilizing hot rolling followed by cold rolling.

Sheet Forming

The manufacturing process where an existing sheet metal piece is shaped into a desired geometry through plastic deformation or cutting.

Cold Working

Processes performed at room temperature that increase dislocation density, thereby strengthening the product through strain hardening.

Applied Stress Rule (Deformation)

The requirement that the applied stress must be at least larger than the yield stress to achieve plastic deformation.

Applied Stress Rule (Cutting)

The requirement that the applied stress must be higher than the ultimate shear stress to successfully cut the sheet metal.

Spring Back

The elastic recovery of a plastically deformed sheet metal piece that occurs when the forming force is withdrawn, causing it to bounce back toward its original shape.

Anisotropy

A characteristic of sheet metal where mechanical properties are directionally dependent, caused by the elongated grains produced during the cold rolling process.

Isotropic

Materials that exhibit the same resistance and strength properties regardless of the direction from which they are squeezed or pulled.

Blanking

A shearing process where the removed piece (the blank) is the desired useful part, and the remaining material is scrap.

Punching

A shearing process where the piece removed by the punch is unwanted scrap, and the remaining part of the sheet is the useful component.

Ultimate Shear Strength of Steel

Typically quantified as $0.7$ times the ultimate tensile strength ($UTS$).

Shear Force Formula

$F = 0.7 \times UTS \times t \times L$, where $t$ is thickness and $L$ is the total length of the sheared edge.

Burnish Depth

The section of a sheared edge that appears smooth and shiny because it was directly rubbed by the punch.

Fracture Depth

The rough section of a sheared edge formed where the metal cracks and propagates until complete separation.

Shearing Clearance

The distance between the die and the punch, calculated as $c = a \times t$ where $a$ is an allowance value related to the metal type.

Fine Blanking

A high-precision shearing operation that uses cushions at the bottom and a plate at the top to restrain the sheet, combined with very small clearances to produce a smooth edge.

Press Brake

The standard mechanical or hydraulic machine used for sheet metal bending operations.

Bend Allowance (BA)

The length of the neutral axis corresponding to the bend, used to determine the necessary original length of the sheet metal.

Bend Allowance Formula

$BA = \alpha \times (R + Kt)$, where $\alpha$ is the bend angle, $R$ is the bend radius, and $K$ is a constant ($0.33$ if $R < 2t$ or $0.5$ if $R \ge 2t$).

Minimum Bend Radius

The specific radius at which a crack first appears on the stretched tensile area of a bent metal piece.

Stretch Forming

A process used for aerospace and marine skins where sheet metal is simultaneously stretched and bent against a contour die.

Deep Drawing

A process using a punch to force sheet metal into a die cavity to create deep features, commonly used for beverage cans and cooking pots.

Ironing

A specific step in deep drawing that thins the container walls while increasing its height using a series of progressively smaller rings.

Draw Beads

Small resistance-adding components between the die and blank holder used in deep drawing to control metal flow and minimize wrinkling.

Earring

A deep drawing defect characterized by a wavy top edge caused by the anisotropy (elongated grains) of the sheet metal.

Stamping

A high-production volume process using male and female dies in heavy presses to form complex 3D shapes from metal sheets.

Multipoint Forming

An advanced method where custom dies are replaced by a matrix of punch rods with hemispheric ends whose individual heights can be adjusted to form various contours.

Incremental Sheet Forming (ISF)

A flexible, point-by-point manufacturing process where a CNC-controlled single tool induces localized plastic deformation on a sheet, ideal for prototypes.

Mechanical Surface Treatment

A process of applying force to plastically deform the surface of a component specifically to induce residual compressive stress.

Shot Peening

A mechanical treatment where the surface is bombarded with spherical media (steel, glass, or ceramic) to improve fatigue life and resistance to stress corrosion cracking.

Residual Compressive Stress

Beneficial surface stress that counteracts applied tensile stress to delay crack initiation and propagation.

Effective Case Depth

The depth from the surface to the point where the hardness of a hardened part drops to $50\,HRC$.

Austenite

A phase of steel formed when it is heated over approximately $800^{\circ}\text{C}$.

Martensite

A hard, needle-like microstructure in steel formed by quenching austenite rapidly, provided the carbon content is higher than $0.3\%$.

Thermal Treatment (DHT)

A surface hardening method for steels with carbon content $> 0.3\%$ using heat sources like flames, induction, or lasers followed by quenching.

Carburizing

A thermochemical treatment for low-carbon steels ($< 0.3\%$ carbon) where the metal is heated in a carbon-rich environment to diffuse carbon into the surface before quenching.

Electroplating

An electrochemical process where a workpiece acts as the cathode to receive a metal coating from ions reduced in an electrolyte solution.

Anodizing

A process for metals like aluminum or titanium where the workpiece acts as the anode to build a dense, protective, and often colorful oxide layer.

Galvanizing

The application of a zinc coating to steel, creating a sacrificial anode that corrodes to protect the underlying material.

Thermal Spray

A coating process where molten or semi-molten material is sprayed onto a substrate at high velocity ($50$ to $1,000\,m/s$) to form thick, robust layers.

Physical Vapor Deposition (PVD)

A vacuum chamber process where material is vaporized and condensed onto a substrate to form highly accurate, thin coatings.

Ion Implantation

A high-energy process where ions are accelerated and embedded into the lattice of a target material to modify its physical or electrical properties.

Line of Sight Process

A coating characteristic of methods like cold spray, PVD, and ion implantation where coverage is limited to areas directly accessible by the material source.