February_AMP

Extrusion Welding

in a Magnesium Alloy

(AM30) Hollow Section Extrudate

he number of magnesium extruded

parts used in the automotive industry is

increasing due to their lightweight and

high specific mechanical properties that facili-

tate reduced fuel consumption. However, tech-

nical challenges in the deformation processing

of magnesium alloys exist and include low

formability, which is attributed to their hexag-

onal close packed (hcp) crystal structure. The

extrusion process is a promising metal forming

process because it applies the hydrostatic com-

pression state of stress in the deformation zone,

enabling improved workability.

Producing hollow section profiles, such as

the double hat extrudate shown in Fig. 1, uses a

porthole die as shown in Fig. 2. When a magne-

sium alloy billet is extruded through the port-

hole die, material is separated by eight die arms

into strands, which are then joined together

under high temperature and high pressure con-

ditions in the die’s welding chamber. This join-

ing process occurs as solid-state bonding and is

known as extrusion welding. The main objec-

tive of this work is to better understand the in-

tegrity and microstructure of extrusion welding

in a magnesium alloy (AM30) hollow section

extrudate (See Table 1).

Metallographic sample preparation

Preparing metallographic samples of mag-

nesium alloys is challenging because the alloys

are relatively soft and can be easily scratched

during preparation. In addition, twin deforma-

tion can occur during polishing and will be vis-

ible after etching. Further, some intermetallic

particles always contaminate the polishing

cloths and can cause additional scratches.

Water should be avoided during the final pol-

ishing process because magnesium alloys have

a tendency to stain when exposed to water-

based liquids.

Due to the extrusion profile symmetry, only

three locations (A, B, and C) are shown in Fig.

1, which are equivalent to all eight extrusion

welds that were metallographically investi-

gated. Specimens were sectioned transversely

to the extrusion direction, mounted, ground,

and polished using standard metallographic

specimen preparation techniques. Specimens

were then chemically polished using 100 mL

methanol, 12 mL hydrochloric acid, and 8 mL

nitric acid for 10 seconds. Finally, specimens

were etched using a solution of acetic picral (4

g picric acid, 5 mL acetic acid, 10 mL H

O, 100

mL ethanol) for 5 seconds

[1]

Results

Figures 3, 5, and 6 are each a collage of po-

larized light optical microscopy (LOM) images

in locations A, B, and C, respectively, and show

the different types of extru-

sion welding.

Figure 3 shows the differ-

ence in grain sizes between

the middle of the image

where the extrusion welding

occurred and the rest of the

extrudate near the edge of

the image represents a less-

modified alloy microstruc-

ture in the welding chamber.

Figure 4 is a magnified

image captured in the mid-

dle of the specimen in loca-

tion A (see Fig. 3), which

shows good extrusion weld integrity with no vis-

ible weld line. On the other hand, Figs. 5 and 6

show different types of extrusion welding with a

curved weld line. The presence of the line is

most likely due to oxidization of the strands’ sur-

faces or insufficient pressure and/or tempera-

ture. These welding lines could be straight or

tortuous, depending on the velocities of the two

Nabeel H. Alharthi*

Lehigh University

Bethlehem, Pa.

King Saud University

Riyadh, Saudi Arabia

Wojciech Z.

Misiolek, FASM*

Lehigh University

Bethlehem, Pa.

ADVANCED MATERIALS & PROCESSES •

FEBRUARY 2014

*Member of

ASM International

JACQUET-LUCAS AWARD

This entry won

the prestigious

Jacquet-Lucas

Award for

Best in Show

at the 2013

International

Metallographic

Contest held in

conjunction

with the

M&M 2013

conference in

Indianapolis.

TABLE 1 — NOMINAL CHEMICAL COMPOSITION

OF ALLOY AM30 (wt%)

Balance 3.4

0.33

0.16 0.0026 0.0006 0.0008

Fig. 1 —

Investigated extrusion welding locations

of the AM30 profile.

Fig. 2 —

Porthole die

used for the double

hat extrusion.

102 mm