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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
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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 byappropriate 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
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 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
Impression and Closed DieForging • 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 workpieceat anyone time – Applicable to mostly cylindrical shapes • Incremental forging – Blank formed in several small steps likeorbital – 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 byreciprocating 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 toundergo deformation without cracking • Common test is the upset test – Upset cylindrical specimen to fixed, largedeformation – 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 extrusiondeformation capabilities
Product Quality Issues • Surface cracks (forgeability limitation) • Buckling • Laps • Internal cracks
Defects Lapsformed by buckling ofthe web during forging. Internal defects produced in a forging because of an oversizedbillet. The die cavities are filled prematurely, and the material at the center of thepart 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 bottomdeadcenter • Hence must be careful in die design andplacement to avoid die fracture
Screw Presses • Derive energy from flywheel like mechanicalpresses • Flywheel drives a screw, not a ram • Energy limited • Process stops when flywheel energyexhausted • 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 letfall onto workpiece • Derives energy from potential energy of thehammer • 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
Forging Economics • Setup and tooling costs are high initially • Good for large production quantities • Material costs as a fraction of total costs varywith material – High percentage for stainless steels (70-85%) – Low percentage for carbon steel (25-45%)
