Investigation of different parameters on roll bonding quality of aluminium and steel sheets M. Buchner1 , B. Buchner1 , B. Buchmayr1 , H. Kilian2 , F. Riemelmoser2 1 Chair
of Metal Forming, University of Leoben, Franz-Josef-Strasse 18, A-8700 Leoben, Austria URL: www.metalforming.at e-mail:
[email protected] 2 ARC Leichtmetallkompetenzzentrum Ranshofen GmbH, Postfach 26, A-5282 Ranshofen, Austria ABSTRACT: One objective of the project ”Austrian Light Weight Structures (ALWS)” is a basic study of production, modelling and validation of roll bonded composites of AA6xxx-automotive alloy and IF-steel. In this context, the influence of surface preparation, pre-heating temperature of aluminium and steel sheet, and post-heat treatment on the bond strength of the composites is analysed by shear tests and reverse bend tests. Furthermore, the interface region is investigated by micro-hardness tests and metallographic observations. KEYWORDS: aluminium, steel, roll bonding, bond strength 1
INTRODUCTION
a 2.5 mm thick IF-steel, rolling experiments are performed. To analyse the influence of surface preparation, three different surface conditions are investigated:
An actual trend in the automotive industry is the production of components of aluminium and steel composites. Aluminium alloys are potential materials for light weight components due to their good mechanical properties, formability, corrosion resistance and low density. Furthermore, they have a good strength to density ratio. Steels, on the other hand, have an edge over other metals in the forming and welding characteristics [1]. As a consequence, aluminium cladded steel sheets play an important role as construction materials. A well known problem in the fusion welding of aluminium and steel are brittle intermetallic phases like Al3 Fe and Al5 Fe2 [2]. Due to this, attempts are made to join these metals by alternative methods as roll bonding, where the materials are joined by high pressure and surface expansion. The aim of the present work is to quantify the main parameters on roll bonding quality of an AA6xxx-automotive alloy and an IF-steel. Basic investigations of the effects of surface preparation as well as pre- and post-heat treatment on the bond strength are performed. 2 2.1
1. The sheets are only degreased (D). 2. The degreased aluminium sheets are ground perpendicular (P) and the degreased steel sheets are ground longitudinal (L) to the rolling direction. 3. Both degreased sheets are ground longitudinal (L) to the rolling direction (standard condition). After the surface preparation, the aluminium sheets are pre-heated at 400 °C for 20 min and/or the steel sheets are pre-heated at 400 °C for 30 min in air atmosphere without prevention of oxidation. Then the composites are rolled immediately to a thickness reduction of 45 % in a 250 mm diameter two-high mill with a roll temperature of about 130 °C at 3.9 m/min (cp. [3]). After the rolling process, pieces of the composites are heat treated. The literature [2, 4, 5, 6] describes postheat treatments at a constant temperature for various times in order to investigate the growth of the intermetallic phases. However, in the present analyses, the specimens are solution heat treated at 540 °C for 2 min and aged at 170 °C for 16 h (T6) to optimise the material properties of the aluminium alloy.
EXPERIMENTAL Manufacturing of the composites
To quantify the main parameters on bonding quality of a 2 mm thick AA6xxx-automotive alloy and 1
140
Experimental details bond shear strength, N/mm²
2.2
The bond strength is measured using shear tests (the corresponding specimen geometry is presented in Figure 1) and reverse bend tests (see Figure 2, [7]). In the latter, a specimen (20 mm × 80 mm) is bent alternately to ± 90 ° until delamination occurs in the interface or fracture occurs in the aluminium sheet. The number of bendings before failure is a measure for the bond strength of the specimen. 3
3
120 100 80 60 40 20
no post-heat treatment post heat-treatment
0
/ / / / / °C °C °C °C RT 00 0 °C l (L) 0 °C 00 ) RT 00 0 °C 00 0 °C 4 4 4 4 ) ) ) A ) 40 l (L) e (L (D 40 l (P L) 40 l (L L) 40 F A e( A e( A Al e (D) (L F Fe F F
10
1.5 100
Figure 3: Bond shear strength as a function of surface preparation, pre- and post-heat treatment.
Figure 1: Specimen for testing of the bond shear strength.
The reverse bend tests show similar results; grinding leads to a doubling of the number of endured bendings. The degreased specimens fail due to delamination in the interface whereas the ground specimens fail due to fracture in the aluminium sheet. From these results, it can be concluded that the ground specimens are bonded very well (see Figure 4).
bending lever
axis of rotation bending cylinder
18
3
10
1
max. 0.1
number of bendings before failure
37.5
actuator
clamping jaw
specimen Figure 2: Set-up for testing of the number of bendings before failure.
Micro-hardness was measured using a Vickers pyramid indenter under 150 g load for 10 s in a distance of 0.5 mm from the interface. Furthermore, the average distance between the sheets and the existence of oxides, respectively, are observed by SEM.
16 14 12 10 8
no post-heat treatment post heat-treatment
6 4 2 0
/ / / / °C °C °C °C 00 0 °C 00 ) RT 00 0 °C 00 0 °C 4 4 4 4 ) (P) 40 (L) 40 (L) (L (D 40 Al e (L) Al e (L) Al Fe Al e (D) F F F
Figure 4: Number of bendings before failure as a function of surface preparation, pre- and post-heat treatment.
3 3.1
RESULTS In the evaluation of the micro-hardness tests, no significant difference between the surface preparations can be found (see Figure 5), because all composites have approximately the same material properties due to the same forming conditions like temperature and thickness reduction.
Influence of the surface preparation
In the shear tests, the ground specimens show a significantly increased bond strength compared to the only degreased interfaces. Thereby, the grinding direction has no influence (see Figure 3). 2
From these results, it can be concluded that the surface condition in the interface has a significant influence on bonding: Even after the rolling process, the ground surfaces are rougher (also in a microstructural sense) than the degreased surfaces. This leads to a mechanical interlocking of the materials and thus to an increased bond strength (cp. [8]).
verse bend tests. While the decrease of bond shear strength is marginal due to the effect of interlocking asperities, the number of bendings before failure decreases significantly (see Figure 3 and 4). Al
200
micro-hardness HV0.15
30 µm
Fe
180
(a) steel pre-heated
160
Al
140 120 100
(b) steel not pre-heated
60 40 20
no post-heat treatment aluminium steel
Al
post-heat treatment aluminium steel
0
Fe
/ / / / / °C °C °C RT °C 00 0 °C l (L) 0 °C 00 0 °C 00 0 °C 4 4 4 400 ) RT ) ) ) ) 0 0 0 0 P A L 4 4 4 ( ( (L (D 4 (L Al e (L) Al e (L) Al e (D) Al Fe (L) F Fe F F
30 µm
(c) steel not pre-heated – T6 Figure 6: SEM-investigations of the weld interface as a function of pre- and post-heat treatment.
Figure 5: Micro-hardness as a function of surface preparation, pre- and post-heat treatment.
3.2
30 µm
Fe
80
The micro-hardness tests show, that according to expectation, the difference of the hardnesses is least between cold aluminium and pre-heated steel (see Figure 5).
Influence of the pre-heat treatment
As proposed in the literature, both metal sheets are pre-heated in the standard test condition (see e.g. [4]). However, if only the steel sheet is preheated, the bond strength increases significantly due to the more similar flow characteristics of the metals (see Figure 3). As a consequence, the steel is deformed earlier in the roll gap and a greater surface expansion of the steel is reached. Furthermore, due to local heating of the aluminium in the interface, the surfaces align completely and so there is a metallic bond between the sheets (see Figure 6 (a)). On the contrary, if only the aluminium sheet is preheated, the bond strength is reduced because of the following reasons: On the one hand, the additional surface expansion of warm aluminium does not contribute to a higher bond strength. On the other hand, the aluminium cools rapidly in the interface due to the high heat conductivity and a complete alignment of the surfaces is not possible (the average distance between the sheets is approximately 1 µm, see Figure 6 (b)). As a result, there is only mechanical interlocking between the surface asperities [8]. This effect is obvious when comparing shear tests and re-
3.3
Influence of the post-heat treatment
Due to the hardening process (T6) described in Section 2.1, the highest bond shear strength is reached – independent of the initial bond properties – after the post-heat treatment (see Figure 3). The very low value of the only degreased specimen is assumed to be caused by wrong specimen preparation and should not indicate another behaviour. In contrast, the number of bendings before failure decreases significantly in the reverse bend tests after post-heat treatment (see Figure 4). Thereby, all specimens fail due to fracture in the aluminium sheet what is caused by the reduced ultimate strain of the hardened aluminium sheet. The micro-hardness tests show the expected increase of the hardness of the aluminium sheet due to the hardening process, whereas the steel sheet is weakened marginally (see Figure 5). Futhermore, it is found that the porosities disappear between the sheets after the post-heat treatment (see 3
Figure 6 (c)) to reduce the surface energy of the sheets. This phenomenon is described in detail in the sintering technique (see e.g. [9]). 4
ACKNOWLEDGEMENT The authors would like to thank the Austrian National Foundation for funding this research work in the frame of the project Austrian Light Weight Structures.
CONCLUSIONS
In this investigation, the significant influence of the surface preparation, pre- and post-heat treatment on the bond strength is verified according to expectations. However, partially, dissimilar results are measured by the shear tests and reverse bend tests because of the different load characteristics. Due to this, the test method should be adjusted to the corresponding forming technique and to the operational conditions. Generally, it can be stated that the shear test characterises the loads at deep drawing and stretch forming conditions very well, whereas the reverse bend test identifies the limit of formability and the durability in cyclic loading. Due to the low bond shear strength, the low number of bendings before failure and the scatter of the results, degreasing without further grinding is not recommended as surface preparation. In contrast, good results are obtained when the surfaces are modified (and hardened) by grinding whereby the grinding direction has no influence on the result. Basically, it can be said that, the rougher the surface, the stronger the interlocking effect between the surfaces. Additionally to grinding, a pre-heating of the steel sheet is advantageous to prevent porosities at the interface without the need of post-heat treatment. However, if a post-heat treatment as described in Section 2.1 is performed, porosities are dissolved by sintering processes and the bond strength is enhanced. The thin intermetallic layers that are observed in the aging of specimen that were bonded with pre-heated steel have no effect on the bond strength. However, because of the high strength caused by the T6 condition, the formability of the post-heat treated specimen was significantly reduced. Due to this, in the production process, components should be first annealed, then formed and finally aged to T6.
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