Date of Award

7-2008

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Mechanical Engineering

Advisor

Grujicic, Mica

Committee Member

Omar , Mohammed

Committee Member

Mears , Laine

Abstract

Traditionally, metals and plastics are fierce competitors in many automotive engineering applications. This paradigm is gradually being abolished as the polymer-metal-hybrid (PMH) technologies, developed over the last decade, are finding ways to take full advantage of the two classes of materials by combining them into a singular component/sub-assembly. By employing one of the several patented PMH technologies, automotive original equipment manufacturers (OEMs) have succeeded in engaging flexible assembly strategies, decreasing capital expenditures and reducing labor required for vehicle manufacture. The basic concept utilized in all PMH technologies is based on the fact that while an open-channel thin-wall sheet-metal component can readily buckle under compressive load, with very little lateral support, provided by a thin-wall rib-like injection-molded plastic subcomponent, the buckling resistance (and the stiffness) of the component can be greatly increased (while the accompanied weight increase is relatively small).
In the present work, the potential of direct-adhesion PMH technologies for use in load-bearing structural automotive components is explored computationally. Within the direct adhesion PMH technology, load transfer between stamped sheet-metal and injection-molded rib-like plastic subcomponent is accomplished through a variety of nanometer-to-micron scale chemical and mechanical phenomena which enable direct adhesion between the two materials. Multi-disciplinary computations are carried out ranging from: (a) computational investigation of the sheet-metal stamping process including determination of the residual stresses and the extent of stamped-component warping; (b) computational fluid mechanics of the filling, packing and cooling stages of the injection-molding process including determination of flow-induced fiber orientation in the molded plastic and the extent of residual stresses and warping in the injection-molded sub-component: and (c) structural-mechanics computational investigation of the effect of injection-molded component residual stresses and warping on their ability to withstand thermal loading encountered in the paint shop and mechanical in-service loading. The results obtained revealed that a minimal level of the polymer-to-metal adhesion strength (5-10MPa) must be attained in order for the direct-adhesion PMH technologies to be a viable alternative in the load-bearing body-in-white (BIW) components.
In the present work, also various PMH approaches used to promote direct (adhesive-free) adhesion between metal and injection-molded thermoplastics are reviewed and critiqued. The approaches are categorized as: (a) micro-scale polymer-to-metal mechanical interlocking; (b) in-coil or stamped-part pre-coating for enhanced adhesion; and (c) chemical modifications of the injection-molded thermoplastics for enhanced polymer-to-metal adhesion. For each of these approaches their suitability for use in load-bearing BIW components is discussed. In particular, the compatibility of these approaches with the BIW manufacturing process chain (i.e. (pre-coated) metal component stamping, BIW construction via different joining technologies, BIW pre-treated and painting operations) is presented. It has been found that while considerable amount of research has been done in the PMH direct-adhesion area, many aspects of these technologies which are critical from the standpoint of their use in the BIW structural applications have not been addressed (or addressed properly). Among the PMH technologies identified, the one based on micro-scale mechanical interlocking between the injection-molded thermoplastic polymer and stamped-metal structural component was found to be most promising.
Lastly, the suitability and the potential of various polymer-powder spraying technologies for coating metal stampings and, thus, for enhancing the polymer-to-metal adhesion strength in direct-adhesion PMH load-bearing automotive-component applications is considered. The suitability of the spraying technologies is assessed with respect to a need for metal-stamping surface preparation/treatment, their ability to deposit the polymeric material without significant material degradation, the ability to selectively overcoat the metal-stamping, the resulting magnitude of the polymer-to-metal adhesion strength, durability of the polymer/metal bond with respect to prolonged exposure to high-temperature/high-humidity and mechanical/thermal fatigue service conditions, and compatibility with the automotive BIW manufacturing process chain. The analysis revealed that while each of the spraying technologies has some limitations, the cold-gas dynamic-spray process appears to be the leading candidate technology for the indicated applications.

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