Damage analysis of aluminium--steel bimetals.

Maronek, Milan ; Caplovic, Lubomir

1. INTRODUCTION

Aluminium steel bimetals are often used in metal production industry due to their good mechanical properties and very low transient resistance between aluminium and steel. They can be made by several technologies among which belongs also an explosion welding. However, the combination of aluminium and steel is difficult to weld due to possible formation of intermetallic compounds between aluminium and iron which are hard and brittle. There is why the some bimetal manufactures uses titanium as an interlayer between aluminium and steel.

An explosion welding process uses high speed impact and despite of extremely short welding time (typically milliseconds) there is generated enough heat in the joint interface to produce melted areas and to form intermetallic compounds if the welded metals are prone to their creation. That is why there is necessary to control the heat input by explosion welding parameters, e. g. detonation velocity, dynamic collision angle, set-up distance etc. (Hudak, 1995), (Kwang-Jin, 2007).

The bimetals subjected to the analysis in this paper were damaged after a very short time of their work. Having known some of the process variables, there could have been calculated other variables allowing to estimate whether welding parameters had not been set too high with a risk of intermetallic coumpound formation (Maronek. 1995). The proof of their presence in the weld joint interface was carried out by examination methods described below.

2. INVESTIGATION PROCEDURE

The first part of analysis was to determine the detonation velocity of an explosive charge. In order to get the straight propagation of detonation wave, the detonator initiates the primary charge with high detonation velocity at first and this charge consequently initiates the main charge having detonation velocity significantly lower. The front of detonation wave forms an angle y with an edge of primary charge (Fig. 1). Detonation velocity of the main charge is described by formula:

[v.sub.d2] = [v.sub.d1] x sin [gamma] (1)

where [v.sub.d2] is the detonation velocity of main charge and [v.sub.d1] is the detonation velocity of primary charge. Having known the angle [gamma] and detonation velocity of primary charge [v.sub.d1], there is possible to estimate the main charge detonation velocity and to compare it with the recommended detonation velocity.

[FIGURE 1 OMITTED]

The next step used light microscopy, microhardness measurements in the joint interface and electron probe microanalysis (EPMA).

3. RESULTS

A surface of damaged bimetals had wavy pattern typical for explosion welding joints (Fig. 2).

[FIGURE 2 OMITTED]

The value of [gamma] angle measured on damaged bimetal (Fig. 3) was 24[degrees].

Detonation velocity of plastic explosive used as the primary charge (Tvarex 4A) was 7300 [m.s.sup.-1]. Hence the detonation velocity of the main charge (Sypex 14A, loose consistency) was determined according to formula (1) to 2969 [m.s.sup.-1].

[FIGURE 3 OMITTED]

The recommended detonation welocity according to welding procedure specification was 2200 to 2300 [m.s.sup.-1].

It was obvious the detonation velocity used was 29 to 35 % higher than recommended and the occurrence of intermetallic compounds could have been predicted.

Predicted presence of intermetallic compound was proved by light and electron microscopy analysis. There was observed continual layer of approximately 50 [micro]m thick brittle phase (Fig. 4) with. This layer included cracks and there had been observed significantly plastically deformed islands of metal matter.

[FIGURE 4 OMITTED]

The microhardness measurements showed, that the hardness of brittle phase was 400 to 700 HV 0,1. This microhardness values significantly exceeded the hardness of both basic materials (Fig. 5).

[FIGURE 5 OMITTED]

A test sample for the impact test had been prepared from the weld joint. Having broken the test sample, the fractographic analysis of revealed fracture surface as well as the the chemical composition of the fracture surface by EDS analysis had been carried out. The morphology of fracture surface (Fig. 6) was transcrystallic with brittle splitting. The chemical analysis detected that the element composition corresponded to intermetallic phase FeAl (Brundle, 1992), (Westbrook, 2000).

[FIGURE 6 OMITTED]

4. DICSUSSION

Detailed analysis of thin intermetallic phase showed, that a fine microcrystalline metal compound was squashed into the crack area. That means the created layer cracked immediately after its formation due to intensive deformation. Consecutively small volumes of welded materials were pressed into the cracks and aterwards in the process of impact load they represented only a small barrier during the crack propagation. That is why the fracture surface had such a heterogenous character. From the chemical composition point of view the layer was unambiguously identified as the intermetallic phase FeAl, which has low mechanical properties.

5. CONCLUSION

The obtained results prooved, that the main cause of continuous highly brittle phase FeAl formation having significant influence on mechanical properties degradation of weld joint fabricated by explosion welding with parameters mentioned and revealed above was the high kinetic energy of used explosive charge. This energy caused the local overheat of weld interface with temerature exceeding the aluminium melting point. Due to this fact there came up appropriate cicrcumstances for brittle phase formation.

The reason of high energy input during explosion welding could have been in using explosive charge with higher detonation velocity, thicker explosive charge or higher charge density. This finally led to higher flyer plate impact velocity and thus higher kinetic energy transforming after collision with parent plate to heat.

This paper was realised with the support of KEGA 3/4157/06 and APVV 0057-07 grants.

6. REFERENCES

Brundle, C.Richard; Evans, Charles A. Jr. (1992). Encyclopedia of Materials Characterization--Surfaces, Interfaces, Thin Films, Butterworth, ISBN 0-75069-168-9, Boston

Hudak, J., Maronek, M., Turna, M. (1995). Quality evaluation of Al- steel weld joitnt fabricated by explosion welding. Zvaranie, Vol. 44, No. 6, 131-133. ISSN 0044-5525

Kwang-Jin L, Shinji K, Takashi A, Tomokatsu A. (2007) Interfacial microstructure and strength of steel/aluminum alloy lap joint fabricated by magnetic pressure seam welding, Materials Science and Engineering A 471, 95-101

Maronek, M.: Presence of melted areas in the interface of weld joint fabricated by explosion welding.(1995). In.: Zbornik vedeckych prac MtF STU v Bratislave. Vol 3., 45-50. ISBN 80-227-0810-0

Westbrook, J.H., Fleischer, R.L.. (2000). Intermetallic Compounds, Volume 2--Basic Mechanical Properties and Lattice Defects of Intermetallic Compounds, John Wiley & Sons, ISBN 978-0-471-61175-2