Magnesium alloys laser (Nd:YAG) cladding and alloying with side injection of aluminium powder
Introduction
The continuous and increasing demand for lightweight materials in aerospace and automotive applications encourage the research laboratories to focus attention on the development of magnesium based materials.
The first main advantage of Mg comes from its low density of 1740 kg m−3, which is approximately 35% smaller compared to Al-based alloys and 65% smaller than that of Ti-based alloys. Other very good properties, like a high strength to weight ratio, good elastic modulus and conductivity as well as high damping capacity make magnesium one of the most studied materials. Magnesium also has high thermal conductivity, high dimensional stability, good electromagnetic shielding characteristics, high damping characteristics, good machinability and is easily recycled [1]. However, the ductility and toughness of Mg alloys are generally low, as a result of the few slip systems that are available for dislocation movement in its hexagonal close-packed structure [2].
On the other hand, magnesium has a number of undesirable properties including poor corrosion and wear resistance, poor creep resistance and high chemical reactivity. Enough reasons to limit its extensive use in many applications. The chemical reactivity of magnesium is also a very important handicap, knowing that as soon as it comes in contact with water or air, a hydroxide/oxide layer forms on the surface. Using magnesium or its alloys in outdoor applications is very delicate because they are extremely susceptible to atmospheric corrosion, which can cause severe pitting in the metal. As a result, its mechanical stability and appearance are damaged.
The solution most employed to prevent corrosion in general is to coat the base material. Coatings can protect a material by providing a barrier between the metal and its environment and/or through the presence of corrosion inhibiting chemicals in them.
Concerning magnesium and its alloys, a number of possible coating technologies available can be found, such as [1]:
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electrochemical plating (using zinc, chromium, copper, nickel, gold or silver),
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conversion coatings (such as chromate, phosphate-permanganate, fluorozirconate or stannate treatments),
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gas-phase deposition processes (such as thermal spray coatings, chemical vapour deposition, physical vapor deposition or diffusion coatings),
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laser surface alloying/cladding,
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organic/polymer coatings (such as painting, powder coating, cathode epoxy electro-coat, sol-gel process, polymer plating or plasma polymerisation).
In recent times, the search for more environmentally acceptable alternatives in surface modification processes, particularly in the case of aluminium alloys, has led to the identification of rare earth metal salts (REMS) and especially cerium species [3].
Section snippets
Al–Mg system
The addition of alloying elements is one of the solutions to improve mechanical properties of Mg alloys. Mg–Al alloys are most widely used as the addition of aluminium increases the strength of the Mg alloys by solid solution hardening, precipitation hardening and some grain refinement [2]. Aluminium is corrosion resistant in marine and industrial atmospheric conditions. It has good electrical and thermal conductivity. Relatively soft and ductile, it can be used to repair aluminium and
Equipment
The powder of Al arrives at the working zone being transported in an inert gas (argon) field (as shown in Fig. 2).
The laser beam provided by a Nd:YAG cw laser (with 3 kW maximum output power) is transmitted at the interaction zone by the mean of optical fiber with an inside diameter of 600 μm. The laser power ranges from 1500 to 3000 W and the working speed from 300 to 1300 mm/min.
Since nozzle diameter is selected at 4 mm, the laser spot must also be adapted to 4 mm, for a better coupling effect
General analysis
Before presenting the effective results concerning the obtained microstructures and the clad layer hardness, some general observations are needed to be underlined:
- (a)
A low level of laser power (1500 W, see even 2000 W) is not enough for laser beam—substrates coupling, in the case of WE43-B and ZE41-B. This is the reason for a greater number of experimentations in the case of the treated samples (T1 and T2) compared to the non-treated ones (B).
The delay and the difficulties in the laser
Conclusions
Single- and multiple-coat layers were successfully obtained by side injection laser cladding and alloying of aluminium powder on magnesium alloys substrates. The coatings present a very good general aspect, very regular and solid bond, few or none porosities and cracks and hardness average values around 200 HV0.05 in the first case or 120 HV0.05 in the second, due to the intermetallic phase formation, such as Al3Mg2 and Al12Mg17, as proved by X-ray diffraction and SEM–EDS analysis.
The results are
Acknowledgements
This work was performed in the frame of the PRESTIGE research project, financed by the French Research Ministry. The authors equally want to thank the industrial partners involved in this project (Honsel Fonderie Messier, PMA, CLAIRE) and the other partners (École Nationale Supérieure d’Arts et Métiers, École Centrale de Lyon and Université Aix-Marseille II) for their support and interest.
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