Defects in ExtrusionExtruded Products 1 Centerburst 2 Piping

Defects in Extrusion/Extruded Products 1) Centerburst 2) Piping 3) Surface Cracking

Contd… a) Centerburst b) Piping c) Cracking

Centerburst • This defect is an internal crack that develops as a result of tensile stresses along the centerline of the workpart during extrusion. • Tensile stresses may seem unlikely in this compression process but they tend to occur under conditions that cause large deformation in the regions of the work away from the center of the workpart.

Contd… • Conditions causes centerburst : a) High die angles. b) Low extrusion ratio. c) Impurities in the work metal that serves as a starting points for crack defects. (Other names for this defect : arrowhead fracture, center cracking, chevron cracking) Note : It is an internal defect that is usually noticeable by visual observation.

Piping • It is a formation of sink hole at the end of the billet. • It is associated with direct extrusion. • The use of dummy block whose diameter is slightly less than that of the billet helps to avoid piping. • Other names given to this defect are tailpipe and fishtailing.

Surface Cracking • This defect results from high workpart temperatures that cause cracks to develop at the surface. • They often occur when extrusion speed is too high, leading to high strain rates and associated heat generation. • Other factors contributing to surface cracking are high friction and surface chilling of high temperature billets in hot extrusion.

Assignment • • • Swaging and Radial Forging Roll Forging Upsetting and Heading Orbital forging and Hubbing Isothermal hot die forging and Trimming

Drawing

Contd… • It is an operation in which the cross-sectional area of a bar, rod or wire is reduced by pulling it through a die opening. • The general features of this process are similar to those of extrusion. • Presence of tensile force causes deformation. Note : Compressive force plays a significant role as the metal squeezes through the die in extrusion.

Contd… • Drawing is a term used in sheet metal working. • The term wire and bar drawing is used to distinguish the drawing process from the sheet metal process of the same name. • Bar drawing is the term used for large diameter bar and stock, while wire drawing applies to small diameter.