Bulk Deformation Forming - Forging Chapter 2 Bulk Deformation Forming - Forging

Forging Process Application of compressive force applied through various mechanisms The forming of workpieces through a succession of tools and dies One of the oldest metalworking operations Initially just a hammer on an anvil (jewelry, horse shoes, sword making) Used to improved properties as well as form a shape Produces discrete parts

Forging Process History Molds of stone helped initial forming efforts Now forces are – Mechanical (hammer presses) – Hydraulic Dies are tool steel Near net shape forming

Forging Practice Prepare raw material including cleaning Heat workpiece (for hot forging) Descale if necessary Preheat and lubricate dies (hot forging) Forge in appropriate dies and in correct sequence Remove excess material (flashing) Clean Check dimensions Straighten if necessary Machine to final dimensions Heat treat if necessary Inspect

Forging Process Capabilities Tolerances of 0.5% to 1% can be achieved Material properties can be tailored by appropriate die design – Directed material flow

Forging Processes Advantages – Metal flow and grain structure can be controlled – Results in good strength and toughness – Near net shape – Parts of reasonable complexity can be created • Landing gear • Connecting rods • Complex shafts Disadvantages – Dies are expensive, particularly for hot forging – Highly skilled labor required

Forging Process Categories

Open Die Forging and Cogging Simplest and cheapest Also called upsetting or flat-die forging Advantages – Cheap – Can form a wide variety of simple shapes with the same dies • Squares, cylindrical – Useful for preparing material for other forms of forging or machining – Can handle large items (35 tons) Disadvantages – Barreling of shape due to high friction

Open Die Forging and Cogging

Open Die Forging Force F = Yf p r2 (1 + 2mr/3h) where Yf is the flow stress of the material m is the coefficient of friction r is the radius h is the height of the workpiece Examples – Stainless steel workpiece, 150 mm diameter, 100 mm high reduced with flat dies to 50% of original height. Coefficient of friction is 0.2 – Force is 5000 tons

Impression and Closed Die Forging Use dies with the approximate end shape Usually requires more than one die to complete process Fullering and Edging dies prepare material to take up die shape – Fullering moves material away from center – Edging moves material away from edges Flashing produced from excess material Often used to ensure good die filling

Stages in Impression Die Forging

Load in impression-die forging

Stages in the forging of a con-rod

Terminology of Impression Forging

Impression and Closed Die Forging Advantages – Produces near net shape – Material properties tailored to application Disadvantages – High die costs – Highly skilled labor required

Precision Forging A further development of closed die forging Close calculation of material required to fill die minimizes scrap and flashing Dies have more detail minimizing subsequent shaping operations Advantages – Little subsequent shaping – Good to excellent properties Disadvantages – Expensive – Difficult to control

Closed Die Forging Force F = k Yf A where Yf is the flow stress A is the area and k is a factor given below Shapes k Simple, no flashing 3-5 simple, with flashing 5-8 Complex, with flashing 8-12

Related Processes Coining – Similar to precision forging but much older – Die cavity completely closed – Very high pressures involved – Used in coin making Heading – Used mostly for bolts

Related Processes Piercing – Exactly as it sounds – Makes holes – Used in conjunction with closed die forging Hubbing – Like piercing but for making cavities, not complete penetrations larger areas Roll Forging – Uses rolls to shape parts – Similar to shape rolling but makes discrete parts – (cross-rolling) operation. Tapered leaf springs and knives can be made by this process with specially designed rolls.

Skew rolling Production of steel balls for bearings by the skew rolling process.

Orbital Forging – Forms the part incrementally – Small forging forces because the die contact is – concentrated on a small part of the workpiece at anyone time – Applicable to mostly cylindrical shapes Incremental forging – Blank formed in several small steps like orbital – non-rotational parts can be made

Isothermal forging – Dies at same temperature of workpiece – No workpiece cooling – Low flow stresses – Better material flow – More close tolerances and finer details can be achieved Swaging – Cylindrical parts subjected to radial impact forces by reciprocating dies – Used to reduce tube diameter and introduce rifling into gun barrels

Die Design Requires knowledge of – Material strength – Sensitivity of these to deformation rate and temperature – Friction and its control – Shape and complexity of workpiece – How the metal will flow to fill the die cavity – Great skill and expertise – Multiple dies to move the material in the right direction

Forgeability Defined as the capability of a material to undergo deformation without cracking Common test is the upset test – Upset cylindrical specimen to fixed, large deformation – Examine barrel surfaces for cracks Another is the hot torsion test – Twist long cylindrical specimen around its axis – No of twists to failure is forgeability – Also used for rolling and extrusion deformation capabilities

Hot forging Temperatures

Product Quality Issues Surface cracks (forgeability limitation) Buckling Laps Internal cracks

Laps formed by buckling of the web during forging. Defects Laps formed by buckling of the web during forging. Internal defects produced in a forging because of an oversized billet. The die cavities are filled prematurely, and the material at the center of the part flows past the filled regions as deformation continues.

Defects Effect of fillet radius on defect formation in forging. Small fillets (right side of drawings) cause the defects.

Forging Machines Mechanical Presses – Hydraulic – Mechanical – Screw – Hammers – Gravity Drop – Power Drop – Counterblow – High Energy Rate

Hydraulic Presses Constant speed Load limited Compared to mechanical – Typically slower – Higher initial cost – Less maintenance Large amount of energy can be transmitted to the workpiece suited for extrusion-type forging Used for both open-die and closed-die forging The largest H-press in the world is 75000 tons, The largest of our country is 25000tons

Mechanical Presses Crank or eccentric types Stroke limited Energy dependent on that stored in flywheel Very large forces can be generated at bottom dead center Hence must be careful in die design and placement to avoid die fracture

Screw Presses Derive energy from flywheel like mechanical presses Flywheel drives a screw, not a ram Energy limited Process stops when flywheel energy exhausted Suitable for producing small quantities, for parts requiring precision (such as turbine blades), and for control of ram speed The largest screw press has a capacity of 16000 tons

Hammers Ram is raised by some mechanism and let fall onto workpiece Derives energy from potential energy of the hammer They are energy limited High speeds Minimal cooling Different types – Gravity drop – Power drop – Counterblow – High energy rate machines

Equipment Selection The selection of forging equipment depend on: The size and complexity of the forging The strength of the material and its sensitivity to strain rate The degree of deformation Guideline Presses are generally preferred for aluminium, magnesium, beryllium, bronze and brass Hammer are preferred for copper, steel, titanium and refractory alloys

Characteristics of Forging Processes

Forging Economics Setup and tooling costs are high initially Good for large production quantities Material costs as a fraction of total costs vary with material – High percentage for stainless steels (70-85%) – Low percentage for carbon steel (25-45%)