The study presents investigation of chemical composition, microhardness and electrical conductivity of Cu/Al laminated metal composite after heat treatment at temperatures higher than Cu–Al eutectic melting point. The Cu/Al bimetal was obtained via explosion welding. Chemical composition of the material after heat treatments was identified using EDS analysis. Eddy current testing was applied to investigate electrical conductivity of the composite’s components. Strain-hardened zones were identified in the explosion welded composite. The experimental value of electrical conductivity of explosion welded composite was in good coherence with calculated by additivity rule results. Heat treatments resulted in the formation of multiple interlayers which had high microhardness value and had intermetallics in composition. The electrical conductivity of the identified interlayers was significantly lower than of Cu and Al.
Clad metals are widely applied in a range of various industries. One of the main advantages of such materials is the ability to combine properties of their components. Often laminated metal composites have unique properties not represented in their components. Such features can usually be achieved by special design described for example by Gurevich
Al/Cu clad metals are usually used as conductors of electricity and heat (Veerkamp,
The contact melting significantly accelerates the interaction of components in the bond. The speed of contact melting is larger when one component is highly soluble in another. The relation of contact melting speed to temperature is given by:
where
The aim of this study is to investigate the properties of Al/Cu composites treated at temperature of contact melting.
Laminated metal Al/Cu composite was obtained via
Chemical composition of materials used in this study
Composition (wt.%) | ||||||||
---|---|---|---|---|---|---|---|---|
Al |
Fe |
Si |
Mn |
Ti |
Zn |
Cu |
Mg |
Al |
Cu |
Fe |
Ni |
Zn |
Sn |
S |
Pb |
O |
Cu |
The explosion welded specimens were subsequently heat treated at 570 °C for a range between 0.25 and 45 h. To avoid oxidation and outflow of molten metal, the specimen were coated with a mixture of sodium silicate (70%) and talc powder (30%).
The microstructure of the composite was studied using optical microscope Olympus BX-61 and scanning electron microscope Versa-3d. Hardness measurements were carried out using standard microhardness tester PMT-3 with the Vickers indenter test with loads of 10–50 g for 15 s.
EDS Analysis was used to determine chemical composition of the bond area after heat treatments.
Eddy-current testing was applied to investigate the electrical conductivity of the composite. Electrical conductivity measurement device Vihr-1M was used.
Microhardness distributions identified strain-hardened zones both in Al and copper layers of 50 and 30 µm thickness respectively. Microhardness value of Cu and Al outside of strain-hardened zones was 0.5 and 0.3 GPa respectively.
Heat treatment at 570 °C for 0.25 h contributed to the formation of two interlayers as shown in
Optical images of the bond area of explosion welded and heat treated at 570 °C for (a) 0.25 h, (b) 0.5 h and (c) 2 h Al/Cu composite.
Microhardness distributions of the bond area of explosion welded and heat treated at 570 °C for (a) 0.25 h, (b) 0.5 h and (c) 2 h Al/Cu composite.
The increase in heat treatment duration up to 0.5 h caused the growth of the thickness value of both interlayers up to 21 µm and 3675 µm for Cu and Al side interlayers respectively (
Heat treatment for 2 h terminated the contact melting and resulted in the conversion of the whole Al layer into crystallized melt. As shown in
The 30-35 µm thick layer adjacent to Cu with 7.8–8.0 GPa microhardness value (
Highly heterogeneous crystallized melt zone with arbitrary aligned dendrites. Microhardness value of the zone was measured to be 1.4–1.5 GPa (
Heat treatment for 45 h resulted in the conversion of most of the Cu layer volume into diffusion zone. As shown in
Optical image of explosion welded and heat treated at 570 °C for 45 h Al/Cu composite.
To determine the Al/Cu ratio in the obtained layers EDS chemical mapping analysis was applied across the line presented on
Explosion welded and heat treated at 570 °C for 45 h Al/Cu composite: (a) SEM image and (b) EDS map of Al/Cu ratio distribution across the white dotted line.
1st layer - solid solution of Al in Cu Cu(Al)
2nd layer - solid solution of variable composition with aluminide Cu9Al4 base (γ2-phase)
3rd layer - aluminide Cu1-xAlx (δ-phase)
4th layer - aluminide Cu11,5Al9 (ζ2-phase)
5th layer - aluminide CuAl (η2-phase)
The subsequent investigation of crystallized melt revealed that it had a mixture of CuAl2 intermetallic and Al-base solid solution in its composition (
Explosion welded and heat treated at 570 °C for 45 h Al/Cu composite: (a) and (b) SEM image with EDS map of Al/Cu ratio distribution in crystallized melt inclusions across the white dotted line.
The formation of the identified layers can be explained by following mechanisms:
At the beginning of the contact melting process solid state diffusion leads to the formation of Al(Cu) and CuAl2 phases in adjacent to Al/Cu interface areas of aluminum layer and copper layer respectively. The formation of CuAl2 is apparently accompanied by the formation of aluminides which exist at 570 °C (η1-phase, ζ2-phase, δ-phase, γ2-phase and possibly β-phase). The thickness of the identified layers depends on the phase enthalpy of formation and solubility range. When the contact between Al(Cu) phase and CuAl2 phase is reached, free energy in the contact zone is higher than the eutectic melt energy. Thus liquid interlayer starts to form. The adjacent to the liquid interlayer phases dissolve in the interlayer while maintaining its eutectic composition. The solid state diffusion of Al and Cu atoms in copper and aluminum layer respectively contributes to the formation of various intermetallic layers as well as solid solution Al(Cu) layer. During the cooling process liquid solidifies into eutectic with CuAl2 crystals (
Three processes should occur in order for various intermetallic compounds to form: the penetration of Al atoms in existing intermetallic layer, transition of Al atoms across the layer, intermetallic formation reaction. During the contact melting process the thickness values of existing intermetallic layers are lower than those formed during solid state diffusion. Thus diffusion processes are accelerated compared to solid state diffusion.
Due to the possible applications of obtained materials in electrotechnical devices, which require electrical conductivity of the composite to have appropriate values, the electrical conductivity measurement techniques were developed for the investigated materials.
Eddy current method was applied to determine the electrical conductivity of the composite. The probe which contained two solenoids was placed on Al side of the composite. Then AC flowing through one of the solenoids induced magnetic field in the thin upper adjacent to the probe layer of the specimen, which contributed to eddy currents flow in the layer. The eddy currents induced magnetic field which affected the field induced by the solenoid. The change in the magnetic field can be registered by another solenoid. The eddy currents value depends on the conductivity of the investigated material and thus assessment of the conductivity was carried out. During the investigation thin layers of the material on Al side were successively removed as the measurements were repeated. The approach allowed creating distribution of the electrical conductivity value across the composite.
The depth of penetration is given by:
where σ – denotes electrical conductivity (Sm.m−1),
The value of f for Vihr-1M device is 109–110 kHz, which corresponds to the depth of penetration of 190–210 µm and 240–260 µm for Cu and Al alloys used in this study respectively. The values of the depth of penetration correspond with the experimentally obtained results of the electrical conductivity distribution across the composite before heat treatments and are presented on
Electrical conductivity distribution across the bond area of explosion welded Al/Cu composite: 1 – experimental data, 2 – calculated via (3) data.
where σ
Pronichev
Electrical conductivity distribution across the bond area of explosion welded and heat treat: (a) at 530 °C for 30 h and (b) at 570 °C for 45 h Al/Cu composite; 1 – experimental data, 2 – calculated via (3) data.
Heat treatment at 570 °C and subsequent cooling to 20 °C resulted in the crystallized melt and diffusion layer emergence. The crystallized melt had a mix of Al(Cu) and Θ-phase in its composition, while diffusion layer consisted of γ2, δ, ζ2 and η2 phases. As reported by Braunovic and Alexandrov (
The regions of electrical conductivity distribution related to the emergence of crystallized melt and diffusion layers are linear and are in coherence with the additivity rule.
Heat treatment at 570 °C of explosion welded Al/Cu bimetal leads to contact melting in the bond area of the composite. The diffusion processes are significantly accelerated compared to solid state diffusion.
Crystallized melt formed during contact melting is a highly heterogeneous mix of CuAl2 intermetallic (85%) and Al(Cu) (15%) grains. The reactive diffusion processes in the bond between Cu and crystallized melt result in the formation of Cu9Al4, Cu1-xAlx, Cu11,5Al9 and CuAl intermetallics which were identified during EDS analysis.
The method of the electrical conductivity of the composite’s components measurement was proposed. The electrical conductivity of intermetallics and crystallized melt was measured to be 14–16 MS.m−1 and 25–27 MS.m−1 respectively.
The present study was carried out according to the assignment of Russian Ministry of Education No 11.1865.2014/K.