Lecture 17 Drawing practice and force Course Hour: 2 Basic requirements: Acquainting with different type of drawing processes, advantages and disadvantages; grasping the die pressure and drawing force Emphasis: drawing practice Difficulties: drawing force and die pressure

Drawing Process Like extrusion but material pulled out the orifice of the required shape instead of being pushed in. Resulting shape variety is more limited than extrusion – Rod (larger cross sections) – Wire (smaller cross sections)

Drawing Process Advantages – Very longer product than extrusion - wire, rod (total length can exceed miles) – Non circular cross sections can be produced Disadvantages – Higher forces required – If exceed tensile strength further deformation can be non-uniform – Limited size reduction per pass (Maximum 63%) – Need multiple dies

Drawing of round sections Drawing force – The drawing stress σd can be obtained by the same approach as that in extrusion, – For strain-hardening materials Y is replaced by an average flow stress. For a material that exhibits the true stress-true strain behavior of The average flow stress is given by the Drawing force is then F= Afln(A0/Af)

Die pressure The die pressure at any diameter along the contact The tension stress and die pressure distributions are shown below

Drawing at elevated temperature At elevated temperatures, the flow stress of metals is a function of the strain rate The average true strain rate in the deformation zone in drawing can be given by Optimum die angle There is an optimum die angle in drawing The optimum die angle for the minimum force increases with reduction Optimum angles in drawing are rather small

Maximum reduction per pass If the drawing stress reaches the yield stress of the material, the product will simply continue to yield further as it leaves the die Thus, the maximum possible drawing stress can only be equal to the yield stress of the exiting material. The maximum reduction per pass=(A0-Af)/A0=1-1/e=63% Effect friction and die angle on maximum possible reduction

Examples of Tube-Drawing Operations Examples of tube drawing operations, with and without an internal mandrel. Note that a variety of diameters and wall thicknesses can be produced from the same initial tube stock (which has been made by other processes)

Lecture 18 Drawing die and equipment Course Hour: 2 Basic requirements: Acquainting with drawing equipment, grasping the principles of die designs for drawing processes; acquainting with die wear and residual stress in drawing Emphasis: Die design principles Difficulties: Residual stress

Die for Round Drawing Die angles usually range from 6 to 15 degrees The purpose of the die land is to size and set the final diameter of the product When the die is reground, the die land maintains the exit dimension of the die opening A rod or wire is fed into the die by first pointing it by swaging

Die for Round Drawing Die material are usually tool steel, carbides, or diamond In addition to rigid die, a set of idling rolls are also used in drawing Tungsten- carbide die insert in a steel casing. Diamond dies, used in drawing thin wire, are encased in a similar manner

Die wear Die wear is the highest at entry The factors, which influence the wear are pressure, variation in diameter of the wire and vibration, and the presence of abrasive scale on the surface Roll Straightening

Cold Drawing Cold drawing of an extruded channel on a draw bench, to reduce its cross-section. Individual lengths of straight rod or of cross-sections are drawn by this method.

Multistage Wire-Drawing Two views of a multistage wire-drawing machine that is typically used in the making of copper wire for electrical wiring

Lubrication Dry drawing – The surface of the rod or wire is coated with various lubricant – The most common lubricant is soap – The rod is first treated by pickling – Soft metal may also coated high-strength material as lubricant – Conversion coating may consist of sulfate or oxalate coatings (soap, polymers) Wet drawing, Dies and the rod are completely immersed in a lubricant. Typical lubricants are oil and emulsions containing fatty or chlorinated additives, and various chemical compounds

Residual stress Cold drawn rod, wire, or tube usually contains residual stress due to inhomogeneous deformation For very light reduction, the surface residual stresses are compressive, which improves fatigue life of the components Residual stress can also be significant in stress-corrosion cracking over a period of time, and in warping of component when a layer is subsequently removed, such as by machining

Homework Why a cold-drawn rod, wire, or tube contains residual stress? In which situation the surface residual stresses are compressive? Which stress environment is desired in metal working operations? Why?