Back-end Defect Research Articles

Local texture and back-end defect in hot extruded AZ91 magnesium alloy

Abstract Local texture in hot extruded MgAl9Zn1 alloy has been studied using individual grain orientation measurement (BKD, “Automated EBSD”) in the SEM. In the shaft of a partial extrudate through a circular die orifice, a <0001> ring fiber texture has formed which continuously changes into a spiral fiber texture in the bottom neck. The flow tube of the internal back-end extrusion defect has a distinctly different texture whereby the (0002) lattice planes are preferentially aligned parallel to the radial and the extrusion directions. The formation of this texture is explained by basal glide, the pile-up of the hard billet mantle, and the inhomo-geneous metal flow during the extrusion process.

Microstructures of longitudinal/transverse welds and back-end defects and their influences on the corrosion resistance and mechanical properties of aluminum alloy extrusion profiles

Aluminum alloy profiles manufactured by semi-continuous porthole die extrusion have three kinds of intrinsic features: longitudinal welds, transverse welds and back-end defects, which affect the corrosion resistance and mechanical properties of the profiles. In this work, the microstructure, corrosion resistance and mechanical properties of these intrinsic features were studied by microstructural characterization, chemical immersion and in-situ test, and the relationship between the material flow behavior and the evolution of the back-end defect and transverse weld was revealed through numerical simulation. It is found that, in terms of corrosion resistance and mechanical properties, both the transverse weld and back-end defect are poor, while the longitudinal weld is relatively good. Impurities and oxide particles are densely distributed in the aluminum alloy matrix in the back-end defect area. In the transverse weld area, the bonding interface has impurities and oxides, and grains are separated by the bonding interface; while in the longitudinal weld area, the grains across the bonding interfaces are formed. The bonding interface of the ring-shaped transverse weld which is far from the longitudinal weld has a low bonding degree. When the transverse weld becomes no longer ring-shaped and very close to the longitudinal weld, its bonding degree is greatly enhanced. The bonding degree of all the transverse welds is lower than that of all the longitudinal welds. During the tensile tests of the extruded profiles, strain concentration phenomena occur at the positions of the longitudinal welds, transverse welds and back-end defects, and there are two types of fracture modes: “uniform plastic deformation → strain concentration → fracture” and “uniform plastic deformation → strain concentration → debonding → fracture”.

Design of a novel dummy block by finite element simulation to eliminate back end defects in direct extrusion

Back-end defect can degrade the production yield and cause a significant quality problem in direct extrusion of the aluminum alloys. This study is concerned with two types of back-end defects, i.e., the funnel defect and the internal oxide ring, which are usually found in the extrudates of aluminum alloys. Hot top and beveled edge dummy blocks are designed to prevent these defects. Due to the high costs of the experimental studies, the effect of two designed dummy blocks on the flow pattern of a deformed billet of aluminum alloys is analyzed by finite element method (FEM) Deform 3D package software. Using simulation, a hot top dummy block with a concave-center is designed to eliminate the formation of the funnel defect. A hot top capacity C is defined at the fixed depth of the concave: C = 1 is a suitable criterion for the design of the hot top dummy block. A beveled edge dummy block is designed to affect and to impede the back flow of the surface layer of the billet into extrudates of aluminum alloys.

Evaluation of a pyramid die extrusion for a hollow aluminum profile using FE simulation

The pyramid die extrusion for a hollow aluminum profile was analyzed to investigate the potential of such innovative dies. For this purpose, the pyramid and conventional porthole dies were respectively designed for a given hollow aluminum profile. And the extrusion process was comprehensively studied by performing different types of finite element simulation, such as the analysis of steady state, transient state and billet skin tracking. The effects of pyramid angle on the evaluation parameters of extrusion, such as extrusion load, material flow, exit temperature, length of transverse weld, quality of longitudinal weld, back end defect and die stress were overall analyzed and compared with the conventional porthole die. Through this study, the advantages and shortcomings of pyramid die were well concluded, which should be important information for die designers and makers.

Effects of ram velocity on pyramid die extrusion of hollow aluminum profile

During the extrusion process of aluminum alloy, the ram velocity should be well controlled, since it is an important parameter affecting the profile quality and extrusion productivity. The effects of ram velocity on conventional porthole die extrusion have been investigated by some researchers, while its influence on pyramid die extrusion has not been fully clarified. Thus, in this study, the pyramid die extrusion process for producing a hollow rectangular aluminum tube was comprehensively investigated by performing analysis of steady state, transient state, and billet skin tracking. Importantly, the effects of ram velocity on some evaluation parameters of pyramid die extrusion, such as the material flow behavior, extrudate temperature, extrusion force, transverse weld, quality of longitudinal weld, and back end defect, were overall investigated. The results show that the flowing velocity, extrudate temperature, extrusion force, and welding pressure tend to increase as the increase of ram velocity, while the length of transverse weld is reduced at higher ram velocity. However, the effect of ram velocity on the back end defect is quite slight. Moreover, the advantages and shortcomings of pyramid die were also discussed by comparing with the conventional porthole die.

Finite element method simulation of internal defects in billet-to-billet extrusion

During billet-to-billet extrusion of aluminium alloys, internal defects occur at the location where the billets are welded together. These defects are unacceptable and must be cut out from the extruded section. Hence, the efficiency of this process significantly depends on the length of these internal defects. The defects that arise inside the extruded section are the so-called back-end defect, which occurs as a result of inflow of the lateral billet surface contaminated with oxide into the extrudate, and transverse welds, which occur as a result of inflow of the face surfaces of the billets pressed together. The length of these defects depends on different process parameters such as friction, billet temperature, geometry of the tools, geometry of the extrudate, etc. In this study, finite element method (FEM) simulation is done to quantify the main parameters influencing the back-end defect and transverse weld formation. The first part of this work deals with modelling techniques that have to be developed to obtain information about the length of internal defects in extruded products from FEM simulation. Comparison is done between the simulation results and experimental studies to verify the results obtained from finite element analysis. In the second part of this work, the model is used to determine the main parameters influencing the back-end defect and the length of transverse welds in a computational investigation.

A numerical sensitivity study to determine the main influence parameters on the back-end defect

Productivity of unlubricated direct extrusion of aluminium alloys depends significantly on the so-called “back-end” defect which always occurs in the center of an extruded section on the end of one extrusion cycle. In structural sections this defect leads to an unacceptable decrease in product quality and so the length of the extruded section which contains this defect must be scrapped. The length of the scrap due to the back-end defect depends on different process parameters like friction, initial temperature of the billet or tool geometry. The objective of this work is to determine the most important parameters influencing the length of the back-end defect in an extruded profile by a numerical sensitivity study with finite elements.

Local texture and back-end defect in hot extruded AZ91 magnesium alloy

Abstract Local texture in hot extruded MgAl9Zn1 alloy has been studied using individual grain orientation measurement BKD, “Automated EBSD”! in the SEM. In the shaft of a partaal extrudate through a circular die orifice, a ring fiber texture has formed which continuously changes into a spiral fiber texture in the bottom neck. The flow tube of the internal back-end extrusion defect has a distinctly different texture whereby the (0002) lattice planes are preferentially aligned parallel to the radial and the extrusion directions. The formation of this texture is explained by basal glide, the pile-up of the hard billet mantle, and the inhomogeneous metal flow during the extrusion process.