Microstructures and Mechanical
Properties of Al/Mg-Li/Al Multilayer byAccumulative Roll Bonding (ARB)
Asian Forum on Light Metals
ALMA Forum 2016 ,August 6, 2016, Kyoto, Japan
Jian-Yih Wang (王建義)
Department of Materials Science and Technology,National Dong Hwa University (NDHU;国立東華大学), Taiwan

Contents
 Background Research purpose Experimental
 Results and Discussion
 Summary

Background
Mg-9%Li-1%Zn(LZ91) alloy possesses ultra-low density and good
formability at ambient temperature. However, the low strength andpoor corrosion resistance are its disadvantages.
Pure Al shows good formability and excellent corrosion resistanceafter anodizing treatment.
Therefore, clad rolling producing LZ91 plate sandwiched by Al
sheets is one way to improve the poor corrosion resistance of LZ91
alloy plate.
Besides, by using the accumulate rolling bond(ARB) process, themechanical properties of multilayer of Al/LZ91/Al can beimproved.

Accumulative Roll Bonding (ARB)

First, two sheets are placed together. The two layers of material are joined by rolling,
as in a conventional roll-bonding process.
Then, the length of rolled material is sectioned into two halves.
The sectioned sheets are again stacked and roll-bonded.
The whole process is repeated again and again.
A high strength multilayered composite of Al/LZ91/Al will be obtained,
and the outer surface of Al will be as the corrosion protective layer
after a proper surface treatment.
Acta mater. Vol. 47, No. 2, pp. 579-583, 1999
Scripta Materialia 51 (2004) 1093-1097

Research purpose
The motivation of this research is to investigate ARB processed
Al/LZ91/Al multilayers, which is the combination of mechanicalproperties and formability as well as corrosion resistance improvement.
(LZ91 - low density and high deformation capacity at ambient
temperature; Al - good surface treatment ability and high strength).
The Al/LZ91/Al multilayers were fabricated by ARB process method.

The mechanical strengths of metallic Al/LZ91/Al multilayers will beincreased due to the grain refining and work hardening after ARBprocess.
Annealing of as-ARBed multilayers were performed in order toenhance the interface bonding between LZ91 and Al.

Experimental

④③
⑤

①

②First, the Al and LZ91 sheet surfaces were cleaned and stacked together.
The stacked sheets were kept in a furnace and then hot roll-bonded.
Then the roll-bonded sheet was cut into two parts with the same dimensions.
After the surface cleaning, the two parts were stacked together ready for another ARB.

Materials
The materials used in this study were commercial pure aluminum (99.31wt%) andLZ91 alloy.
LZ91 plates (80 mm × 20 mm × 4 mm) were cut from the as-extruded plate and thenrolled into sheets of thickness about 1mm at 473K.
The AA1050 was received in the form of a rolled sheet (80 mm × 20 mm × 5 mm)
and annealed at 673K for 2 h. Then it was cold-rolled into sheets of thickness about
0.4 mm.
The compositions of materials used in this study.
wt.%

SiFeCu

Mn

Mg

Zn

Ti

Al

AA1050

0.20

0.35

0.03

0.02

0.04

0.03

0.02

bal.

wt.%

LiZn

Mg

LZ91

8.85

0.75bal.

Primary sandwich of Al/LZ91/Al
Two Al (0.4 mmt) and one LZ91 (1 mmt) sheets were degreased and wire-brushed,
then stacked by the sequence of Al-Mg-Al.
The stack was held for 5min at 673K in a preheated furnace.
Then it was warm roll-bonded into the primary sandwich with about 50% reduction(thickness of 1.8mm rolled to 0.9mm).
Al0.4mm
LZ91 1.0mm
Al0.4mm(1.8mm)Primary sandwich preparation(0.9mm)Multilayered Al/LZ91/Al by ARB
The prepared Al/LZ91/Al primary sandwich was cut into two halves. After
degreasing and wire-brushing (on the outer Al layers), then fastened together.

This stack was held 5min at 673K and then roll-bonded directly with reduction of
50% in one pass followed by air-cooling.
This procedure was called one cycle ARB and repeated up to five cycles.
The total number of layers was calculated by 2n+1 + 1
where n is the number of ARB cycles.
All the experiments were conducted without lubricant on a roll-mill.
The roll diameter was 110mm with speed of 7m/min-1.
Multilayered composite produced by ARB process

Materials Science and Engineering A 527 (2010) 3073–3078

Results and discussion

βα

Optical microstructure of the starting LZ91(9wt.% Li-1wt.%Zn) sheet.
Dual phases of LZ91with -Mg and -Li. The grain sizes of -Mg and -Li were around10-20μm .

Results and discussion
abcdefOptical micrographs of longitudinal cross-section of Al/LZ91/Al multilayered composites,(a) after primary sandwich preparation, (b) after 1 cycle, (c) after 2 cycles, (d) after 3 cycles,(e) after 4 cycles and (f) after 5 cycles.

Results and discussion

SEM image of the LZ91/Al interface after primary sandwich preparation.

Thickness deformation ratio(%)

Results and discussion

The thickness reduction ratio of the Al and LZ91 layers during each rolling process.
Thickness deformation ratio :(original (previous) thickness - rolled thickness) / original (previous ) thickness x 100%

Results and discussion
aLZ91

bedLZ91

cAlf

LZ91

AlLZ91

LZ91

LZ91

SEM microstructures of the LZ91 side in Al/LZ91/Al composite: (a) after primary sandwich
preparation, (b) after 1 cycles, (c) after 2 cycles, (d) after 3 cycles, (e) after 4 cycles and (f)after 5 cycles.

Results and discussion

The grain size of  phase in LZ91 side of composite after different number of ARB cycles

Results and discussion

Variation of microhardness of Al layer, interface and LZ91 layer with increasing
numbers of the ARB cycle.

Results and discussion

The YS of 3 cycle ARBed was230MPa which was 2 times of LZ91.

True stress-strain curves of the starting materials and the Al/LZ91/Al multilayred composite
after different number of ARB cycles

Results and discussion

Specific yield strengths of the starting materials and the Al/LZ91/Al composite after
different number of ARB cycles

Results and discussion

XRD patterns of LZ91/Al primary sandwich and after different number of ARB cyclesSummary for as-ARBed Al/LZ91/Al
AlLZ91

AlLZ91 Al

LZ91 AlSandwich
preparation

ARBprocess

The size of α phase in LZ91 was thinned after primary sandwich and broken down to small
particles after ARB process. Therefore, the grain refining and strain hardening of bothmaterials increased the strengths of multilayered Al/LZ91/Al.
The yield stress after 3 cycles ARB process is about 2 times higher than that of LZ91.
Besides, the elongation can be maintained around 20%.

Annealing of ARB processed LZ91/Al
In order to improve the bonding strength between LZ91 and Al, annealing of as-ARBed
Al/LZ91/Al was performed.
LZ91 + AA1050

Primary sandwichannealing

Mechanical Properties

1 cycle ARB(473K, 523K, 573K for 1h)

Microstructures

Annealing of multilayered LZ91/Alabc

LZ91

LZ91

AlAl

LZ91

Al

Cross-section optical micrographs of Al/LZ91/Al primary sandwich after annealingat (a) 473K, (b) 523K and (c) 573K.

bac

LZ91

LZ91

LZ91
AlAlAlCross-section optical micrographs of 1 cycle ARBed Al/LZ91/Al after annealing at (a)473K, (b) 523K and (c) 573K.

Annealing of as-ARBed LZ91/Ala180

primary sandwich
annealed at 473K
200
annealed at 523K
annealed at 573K 180

160160140140

True stress (MPa)True stress (MPa)

200

120
100
80
60

120
10080
60

40

4020200

1st cycle
annealed at 473K
annealed at 523K
annealed at 573K

b002468

101214

True strain (%)

161820222402468

101214161820

2224

True strain (%)

True stress-strain curves of (a) primary sandwich and (b) 1 cycle ARBed Al/LZ91/Almultilayers after annealing at different temperature.

Annealing of ARBed LZ91/Al(a)SEM cross-section images of 1 cycle ARBed Al/LZ91/Al after annealing at(a) 473K, (b) 573K.

(b)

Annealing of ARBed LZ91/Al

Micro-Vickers hardness distribution of 1 cycle ARBed Al/LZ91/Al after annealing at 573K.

Annealing of ARBed LZ91/Al

XRD spectrums of 1 cycle ARBed Al/LZ91/Al after annealing at (a) 473K, (b) 523K and(c) 573K.

Annealing of ARBed LZ91/AlabMg

cMg
AlMg

AlAlBSE images and EDS line scan of 1 cycle ARBed Al/LZ91/Al after annealing at (a) 473K,(b) 523K and (c) 573K.

Annealing of ARBed LZ91/Al
Alinterface
Al12Mg17
LZ91

The TEM image of interfaceof 1 cycle ARBed
Al/LZ91/Al after annealing
at 573K.

Summary for annealing of LZ91/Al

annealing

Intermetalliccompounds

Improving the bond interface by annealing:
1. Annealing at higher temperature will promote occurrence of intermetalliccompounds.
2. At lower temperature, diffusion layer only might improve the bond condition.

Summary1.

The Al/LZ91/Al composites were successfully processed by ARB up to fivecycles. Necking or rupture of both LZ91 and Al layers did not take placebecause the similar flow properties between them. The LZ91 and Al layers
underwent different applied strain during each rolling process.
2.

 phase of LZ91 was refined after primary sandwich preparation. During thefollowing ARB process, the grain size of  phase further decreased withincreasing of the accumulated strains.

3.

Although the ARB process was conducted at warm temperature, nointermetallic compound can be found in the interface.
4.

Both the yield strength and ultimate tensile strength of the Al/LZ91/Al
multilayered composite increased gradually while the elongation still
maintained around 20% with increasing of the ARB cycles. The yield strengthof primary sandwich was 168MPa. However, the yield strength increased up
to 230MPa after 3 cycles ARB process.

Summary5.

It is inferred that the grain refining of α phase and work hardening of LZ91

and Al are effective strengthening mechanism in these alloys.
6.

After annealing treatment, the compounds hard to be found at the interfaceof LZ91/Al if the annealing temperature is below 523K. However, thecompounds of Al3Mg2 and Al12Mg17 appeared at the interface after higher
annealing temperature.

7.

Because the compounds at the LZ91/Al interface are brittle, theyconsequently could not increase the strengths. However, the annealingtemperatures below 523K might improve the bonding characteristic.

National Dong Hwa University