Taisei Plas Co., Ltd (Tokyo, JP) inventors Masanori Naritomi and Naoki Andoh  developed a patent pending method to manufacture lightweight and strong composites of a magnesium alloy and carbon fiber reinforced plastics (CFRP), by strongly bonding the magnesium alloy and the CFRP using an epoxy adhesive. The magnesium alloy has specific ultra-fine irregularities is compatible with an epoxy resin adhesive and exhibits thus strong adhesion.  The manufacturing processes are detailed in U.S. Patent Application 20100112287.

A magnesium alloy composite plate material, in which magnesium alloy plates and a CFRP are integrated by exploiting bonding techniques, can be used in ordinary assembly structures with other metal members and bolts. The magnesium alloy plates can withstand strong local forces, and hence the CFRP is not damaged. As a result, the composite is effective for applications in casings, bodies and parts in mobile equipment such as automobiles or in mobile devices, where lightweightness, toughness and ease of assembly are required.The materials may be used in transport equipment where lightweight is required, such as automotive components, aircraft components, and bicycle components. 

Carbon fiber reinforced plastics have the highest strength among structural materials, including metals, and are lightweight, having a specific weight of 1.6 to 1.7, i.e. a specific weight comparable to that of magnesium alloys. Ultra-lightweight and high-strength structural members could be manufactured if both CFRP and magnesium alloys could be strongly bonded to each other. Fortunately, CFRP prepregs, which are the precursors of CFRPs, are fabrics or aggregates of carbon fibers impregnated with an uncured epoxy resin, and thus integration simultaneous with curing can be made simple by tweaking the affinity of CFRP prepregs and an epoxy adhesive coated on the metal.

In order to make the magnesium and CFRP composites,  the inventors felt that first of all it was necessary to conduct diligent research and development on how to improve and stabilize bonding forces (bonding strength) between magnesium alloys and epoxy adhesives. Thus, they endeavored to develop a method that affords strong bonding with fiber-reinforced plastics, in particular CFRPs, by focusing on the development of surface treatment techniques for magnesium alloys. 

A specific preferred treatment method involves immersing the already-pretreated magnesium alloy part in a very dilute acidic aqueous solution for a short time, followed by water rinsing. Residual sodium ions that were not washed away during the pre-treatment are neutralized and removed thereby. The magnesium alloy part is then immersed in the aqueous solution of the conversion treatment, followed by water rinsing. The dilute acidic aqueous solution used is preferably a 0.1 to 0.3% aqueous solution of citric acid or malonic acid. Immersion takes place preferably around normal temperature for about 1 minute. 

The aqueous solution used for the conversion treatment is preferably an aqueous solution containing 1.5 to 3% of potassium permanganate, about 1% of acetic acid and about 0.5% of sodium acetate, at a temperature of 40 to 50.degree. C. Immersion in this aqueous solution lasts preferably about 1 minute. As a result of the above operation, the magnesium alloy becomes covered with a conversion coat of manganese dioxide. The surface morphology of the skin exhibits micron-scale roughness (surface roughness), and also nano-scale ultrafine irregularities when observed by electron microscopy.

U.S. Patent Application 20100112287 FIGS. 6 and 7 are 100,000-magnification electron micrographs of such nano-scale ultra-fine irregularities. The surface morphology of the ultra-fine irregularities is difficult to describe in a straightforward manner. 

The surface shown in the electron micrograph of FIG. 7 can be approximately described as being covered with innumerable tangled rod-shaped or spherical ultra-fine irregularities having a diameter of 5 to 20 nm and a length of 20 to 200 nm. Meanwhile, the ultra-fine irregularities on the surface in the electron micrograph of FIG. 6 look like irregular stacks of spherical bodies which have a diameter of 80 to 120 nm, and from which there grow innumerable rod-shaped or spherical protrusions having a diameter of 5 to 20 nm and a length of 10 to 30 nm. As far as can be observed by electron microscopy, all the rod-like (needle) shapes having a diameter of about 10 nm appear to be crystals, although no diffraction lines for manganese oxide were observed using an X-diffractometer (XRD). 

FIG. 6 is an electron micrograph at 100,000 magnifications of a test piece of an AZ31B magnesium alloy having had a surface treatment developed by Taisei Plas.