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Joining-Aluminum-MMC:Is Welding Suitable?SOLUTIONS with Effective, Practical Advice
Joining-Aluminum-MMC is not straightforward. It must be devised and implemented for discontinuously reinforced aluminum (DRA) metal matrix composites (MMC) that have to be connected. To realize functional structures they may have to be joined to similar or dissimilar materials. Various studies were dedicated to research feasibility and properties of such weldments. It is affirmed though, that the welding technology is not yet mature and freely applicable. The relevant information is restricted because of proprietary rights or for security reasons if military applications are involved. Aluminum matrix reinforcements are generally particles of aluminum oxide, of silicon carbide or carbon whiskers. The particulate structure is preferred to fiber reinforcements because of their improved ease of manufacturing, lower cost and almost isotropic properties. Aluminum MMC have increased stiffness, strength and wear resistance, when compared to their respective unreinforced monolithic aluminum alloys. They also have with higher thermal conductivity and lower coefficient of thermal expansion.
Joining-Aluminum-MMC is quite unlike from welding monolithic metal, because of the presence of the microstructurally different inclusions. However the usual cleaning procedures and inert gas shielding applied for un-reinforced metal must be implemented. The reinforcement additions to the aluminum matrix modify physical and chemical properties that influence weldability and require understanding. When arc welding by classic means, viscosity of the molten puddle is increased by the solid un-fused particles present. Flow and wetting are found to be lower than those of un-reinforced aluminum, as heat flow is less efficient.
These characteristics make welding practice more difficult to unaccustomed welders, besides affecting final microstructures and residual stress distribution. Joining-Aluminum-MMC is influenced also by chemical reactions between matrix and reinforcements. Molten aluminum contacting solid particles can bring about the formation of new and unwanted phases that reduce mechanical properties. If fusion welding is applied, it is preferred in general to use rapid thermal cycles with limited heat input. Otherwise solid state welding should be selected. Depending on the specific dispersed phase, aluminum carbide (Al4C3) could form. The amount produced of this substance should be kept low by limiting the time at matrix melting temperature. That is because its shape in clustered or coarse form is detrimental to mechanical properties. Furthermore because, if exposed to water it will dissolve, leaving porosity and loss of integrity in the weld. Alternatively increased presence of silicon would limit the formation of aluminum carbide. In aluminum-magnesium or aluminum-copper reinforced matrix materials, the formation of magnesium or copper spinel at the interface may occur between metal and reinforcing particles. That is likely to appear, but those phases are not considered to affect weld mechanical properties. The solidification process in Joining-Aluminum-MMC by welding is affected by the presence of reinforcing particles in the weld pool that are pushed ahead of the solidification front. This fact locally disturbs heat flow and mass transport and seems to improve resistance to solidification cracking. A few modifications may be needed to adapt geometry and dimensions of joints to the reduced fluidity of the molten matrix material. Several welding processes have been used to perform Joining-Aluminum-MMC, including GTAW, GMAW, EBW (Electron Beam), LBW (Laser Beam) and RW (Resistance Welding). Also friction welding (FRW), friction stir welding (FSW), transient liquid phase (TLP) bonding, and capacitor discharge welding (CDW) have been applied. GTAW is applied with the same equipment and consumables used for monolithic aluminum alloys with low and controlled heat input. However the formation of secondary phases during welding should be monitored by metallographic examinations and by mechanical testing. Welders may find it difficult, at least initially, to form properly the weld pool and to advance it correctly. GMAW too can be used, with special attention in preparation and cleaning of the surfaces. The application of a grooved backing bar, of back gouging and of vigorously scrubbing with a clean stainless steel wire brush between passes is recommended. Laser beam and electron beam welding were successfully employed for Joining-Aluminum-MMC. Sometimes shims of filler metal were preplaced in the joints. Any new application would need a thorough development program with complete review of the results. An Article on Dispersion Strengthened Aluminum Alloys was published (7) in Issue 89 of Practical Welding Letter for January 2011.
The Aluminum Metal Matrix Consortium Read more on Joining-Aluminum-MMC in the following online publication:
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