Investigation of Cracks on Internal Surfaces of Extruded Cold Worked Thick Walled Pipes of an Age Hardened Al-Alloy

Abstract

Thick seamless pipes of hardenable aluminum alloys demand close geometrical tolerances as well as high quality surface finish which are met by cold drawing after a series of different thermo-mechanical treatments. To meet the requirements of critical applications the final product undergoes stringent quality inspection procedures. State of the art quality assessment can detect even minor isolated defects. The production facilities develop their quality criteria suitable for specific applications. The present study investigated minute defects on the inside surface of thick seamless pipes, proposed mechanism of their formation and suggested the impact of defects on the end use. The root cause analysis was conducted, and measures were suggested to control the defects. Thick extruded seamless aluminum alloy pipes underwent a series of different thermo mechanical treatments; the final dimensions with required tolerances and the surface finish were achieved by adopting a 2-step cold drawing process. Cold drawing generated residual stresses which resulted in the formation of cracks in the material, preferentially at the defects generated during solidification and/or extrusion processes. The final product underwent stringent quality inspection, and the material was rejected if cracks of size 3 mm or larger were detected. The die scratches or notches generated on the inside surface of the pipes, during extrusion are assumed to grow if subjected to high stresses during subsequent processes, e.g. cold working. Observations at high magnification in SEM helped to determine the morphology of cracks. Radiographic testing did not detect any crack in the bulk material. Particles with faceted features indicated the presence of inclusion. Inclusions were detected in the form of strings along the direction of cold drawing. Energy dispersive spectrometry in SEM was used to determine the composition of inclusion detected in the vicinity of cracks. Almost all the inclusions were rich in silicon, iron, calcium along with carbon; it indicated that the inclusions were trapped particles of fluxes, slag, and brick powder. Particles rich in Ca, Na and/or Cl indicated entrapped flux, Fe and Si were mostly coming from aluminum scrap and refractory powder while presence of carbon indicated entrapped extrusion lubricant. Inclusions rich in a large variety of unwanted elements indicated presence of slag particles. Numerical analysis was conducted to develop a model in FEM in which scratches of different depths were introduced and autofrettage pressure was applied to determine the stresses generated according to the established Von Mises Model; the latter was used to establish the yield criteria. Finite Element Modelling concluded that when cold drawing pressure was applied on a pipe with a single notch of depth 0.3mm or three notches of depth 0.1 or greater at different locations the Von Mises stresses approached the yield strength of the pipe.