FEM Numerical Calculations and Experimental Verification of Extrusion Welding Process of 7075 Aluminium Alloy Tubes

Abstract

Extrusion of AlZnMgCu alloys is associated with a very high plastic resistance of the materials at forming temperatures and significant friction resistance, particularly at the contact surface between the ingots and the container. In technological practice, this translates into high maximum extrusion forces, often close to the capacity of hydraulic presses, and the occurrence of surface cracking of extruded profiles, resulting in a reduction in metal exit speed (production process efficiency). The accuracy of mathematical material models describing changes in the plastic stress of a material as a function of deformation, depending on the forming temperature and deformation speed, plays a very important role in the numerical modelling of extrusion processes using the finite element method (FEM). Therefore, three mathematical material models of the tested aluminium alloy were analysed in this study. In order to use the results of plastometric tests determined on the Gleeble device, they were approximated with varying degrees of accuracy using the Hnsel-Spittel equation and then implemented into the material database of the QForm-Extrusion^® programme. A series of numerical FEM calculations were performed for the extrusion of Ø50 × 3 mm tubes made of 7075 aluminium alloy using chamber dies for two different billet heating temperatures, 480 °C and 510 °C, and for three different material models. The metal flow was analysed in terms of geometric stability and dimensional deviations in the wall thickness of the extruded tube and its surface quality, as well as the maximum force in the extrusion process. Experimental studies of the industrial extrusion process of the tubes, using a press with a maximum force of 28 MN and a container diameter of 7 inches, confirmed the significant impact of the accuracy of the material model used on the results of the FEM numerical calculations. It was found that the developed material model of aluminium alloy 7075 number 1 allows for the most accurate representation of the actual conditions of deformation and quality of extruded tubes. Moreover, the material data obtained on the Gleeble simulator made it possible to determine the limit temperature of the extruded alloy, above which the material loses its cohesion and cracks appear on the surface of the extruded profiles.

Keywords: FEM numerical modelling; aluminium alloys; extrusion; extrusion force; material models; product quality.

Figure 1

Test station for determining the rheological properties of aluminium alloys: (a) test chamber of the Hydrawedge module of the GLEEBLE 3800 simulator; (b) Gleeble 3800 physical simulator; (c) cylindrical aluminium samples with a diameter of 10 mm and a height of 12 mm.

Figure 2

The geometrical model of the extruded material with mesh elements: (a) side view showing the billet and the extruded material filling the die gaps; (b) cross-sectional view showing the extruded material in the die inlet channels.

Figure 3

Three-dimensional CAD model of the designed porthole die for extrusion of tubes of Ø50 × 3 mm from 7075 alloy in the cross-sectional view (a) and in the bottom view (b), in the cross-section (c) and in the cross-section with the bridge dimensions marked (height and thickness) (d).

Figure 4

Industrial trials of extrusion of Ø50 × 3 mm tubes made of 7075 aluminium alloy: (a) designed porthole die; (b) hydraulic press with a 7-inch container diameter and 28 MN pressure force.

Figure 5

Station for 3D optical scanning of extruded tubes using a GOM ATOS scanner—(a) samples of extruded tubes Ø50 × 3 mm made of 7075 aluminium alloy; (b) GOM ATOS scanner; (c) software for visualisation and processing of measurement data.

Figure 6

Actual and approximated plastic flow curves of 7075 aluminium using material model no. 1: (a) temperature 450 °C; (b) temperature 480 °C; (c) temperature 510 °C.

Figure 7

Actual and approximated plastic flow curves of 7075 aluminium using material model no. 2: (a) temperature 450 °C; (b) temperature 480 °C; (c) temperature 510 °C.

Figure 8

Actual and approximated plastic flow curves of 7075 aluminium using material model no. 3: (a) temperature 450 °C; (b) temperature 480 °C; (c) temperature 510 °C.

Figure 9

Distribution of velocity deviation while extruding tubes of dimensions of Ø50 × 3 mm made from 7075 aluminium alloy through porthole die for billet temperature of 480 °C and three different assumed material models—(a) material model no. 1, (b) material model no. 2, (c) material model no. 3.

Figure 10

Distribution of velocity deviation while extruding tubes of dimensions of Ø50 × 3 mm made from 7075 aluminium alloy through porthole die for billet temperature of 510 °C and three different assumed material models—(a) material model no. 1, (b) material model no. 2, (c) material model no. 3.

Figure 11

Distribution of material temperature while extruding tubes of dimensions of Ø50 × 3 mm made from 7075 aluminium alloy through porthole die for billet temperature of 480 °C and three different assumed material models—(a) material model no. 1, (b) material model no. 2, (c) material model no. 3.

Figure 12

Distribution of material temperature while extruding tubes of dimensions of Ø50 × 3 mm made from 7075 aluminium alloy through porthole die for billet temperature of 510 °C and three different assumed material models—(a) material model no. 1, (b) material model no. 2, (c) material model no. 3.

Figure 13

FEM calculated predicted product’s wall thicknesses while extruding tubes of dimensions of Ø50 × 3 mm made from 7075 aluminium alloy through a porthole die for a billet temperature of 480 °C and three different assumed material models—(a) material model no. 1, (b) material model no. 2, (c) material model no. 3.

Figure 14

FEM calculated plastic strain while extruding tubes of dimensions of Ø50 × 3 mm made from 7075 aluminium alloy through a porthole die for a billet temperature of 480 °C and three different assumed material models—(a) material model no. 1, (b) material model no. 2, (c) material model no. 3.

Figure 15

FEM calculated mean stress while extruding tubes of dimensions of Ø50 × 3 mm made from 7075 aluminium alloy through porthole die for billet temperature of 510 °C and three different assumed material models—(a) material model no. 1, (b) material model no. 2, (c) material model no. 3.

Figure 16

FEM calculated extrusion force vs. ram displacement while extruding tubes of dimensions of Ø50 × 3 mm made from 7075 aluminium alloy through porthole die for billet temperature of 480 °C and three different assumed material models—(a) material model no. 1, (b) material model no. 2, (c) material model no. 3.

Figure 17

FEM calculated extrusion force vs. ram displacement while extruding tubes of dimensions of Ø50 × 3 mm made from 7075 aluminium alloy through porthole die for billet temperature of 510 °C and three different assumed material models—(a) material model no. 1, (b) material model no. 2, (c) material model no. 3.

Figure 18

Photographic documentation of extrusion tests of Ø50 × 3 mm tubes made of 7075 aluminium alloy—(a) tubes after extrusion at a billet temperature of 480 °C, (b) tubes after extrusion at a billet temperature of 510 °C.

Figure 19

The real wall thickness deviations for tubes of dimensions of Ø50 × 3 mm made from 7075 aluminium alloy extruded through porthole die (for billet temperature of 480 °C) (Red indicates dimensional deviations that are unacceptable according to the standard).

Figure 20

The registered extrusion technological parameters while extruding tubes of dimensions of Ø50 × 3 mm made from 7075 aluminium alloy through porthole die for different billet temperatures—(a) extrusion force for billet temperature of 480 °C, (b) extrusion force for billet temperature of 510 °C, (c) extruded material temperature for billet temperature of 480 °C, (d) extruded material temperature for billet temperature of 510 °C.

Figure 21

A comparison of the results of the plastometric tests on the 7075 alloy in the Gleeble simulator and the results of the industrial process of the extrusion of tubes of dimensions Ø50 × 3 mm made from the 7075 alloy by using porthole dies: (a) stress–strain curves determined on the Gleeble device at 480 °C with different strain rates, and the sample after compression at 480 °C with a strain rate of 5 s⁻¹; (b) image of extruded tubes of dimensions of 50 × 3 mm made from alloy 7075 for a billet heating temperature of 480 °C; (c) stress–strain curves determined on the Gleeble device at 510 °C with different strain rates, and the sample after compression at 510 °C with a strain rate of 5 s⁻¹ (d) image of extruded tubes of dimensions of 50 × 3 mm made from alloy 7075 for a billet heating temperature of 510 °C.

Figure 22

Comparison of metal flow in the industrial extrusion process of Ø50 × 3 mm tubes of 7075 aluminium alloy using porthole dies for a billet heating temperature of 480 °C: (a) actual bending of the ends of extruded tubes and (b–d) numerically calculated FEM bending of the ends of extruded tubes for 3 different mathematical material models (Red indicates faster metal flow from the die hole, while blue indicates slower flow).

Figure 23

Comparison of metal flow in the industrial extrusion process of Ø50 × 3 mm tubes of 7075 aluminium alloy using porthole dies for a billet heating temperature of 510 °C: (a) actual bending of the ends of extruded tubes and (b–d) numerically calculated FEM bending of the ends of extruded tubes for 3 different mathematical material models (Red indicates faster metal flow from the die hole, while blue indicates slower flow).

Figure 24

FEM numerically calculated dimensional deviations in the wall thickness of Ø50 × 3 mm tubes extruded from 7075 aluminium alloy using porthole dies for a billet heating temperature of 480 °C and 3 different mathematical material models: (a) material model no. 1; (b) material model no. 2; and (c) material model no. 3.

Figure 25

Comparison of wall thickness deviations for tubes Ø50 × 3 mm extruded from 7075 aluminium alloy using porthole dies for a billet heating temperature of 480 °C: industrial extrusion process and FEM numerical calculations (3 different mathematical material models).

Figure 26

A summary of numerically calculated FEM stress conditions in the die clearance, three different tube segments during the extrusion of 50 × 3 mm tubes made of 7075 aluminium alloy for three different material models (a) with images of the surfaces of the tubes extruded under industrial conditions (b).

Figure 27

Comparison of the extrusion force as a function of the punch travel—numerically calculated FEM and recorded in industrial tests for extrusion welding of tubes Ø50 × 3 mm made of 7075 aluminium alloy for ingot heating temperatures of 480 °C (upper figure) and 510 °C (lower figure).

Figure 28

The mind map showing the accuracy of reflecting the actual state of process and product parameters using FEM numerical calculations: (a) material model no. 1, (b–d) material model no. 2.