Course:MTRL466-adhesivebonding/Group1/Adhesive Bonding Review

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Review of joining of aluminum and applications of adhesives in the automotive industry

Welding Aluminum

Currently used in frame manufacturing is spot welding. Spot welding requires access to both sides of the joint and large specialized apparatus, but has the advantage of being well established in the industry with capital and expertise already invested. Aluminum, however, is not as favourable a candidate for spot welding due to the high melting point of the oxide film. The welding parameters of Aluminum are different because considerable energy is required to breakdown the oxide and allow the weld to take place.(Barnes)

Parameters Bare Al Bare steel Zn-coated (hot-dipped)
Weld time (50 Hz cycles) 3 7–10 9–12
Current range (kA) 18.0–23.0 7.0–10.0 9.0–11.0
Force (kN) 4.1–5.0 1.9–2.6 2.2–2.9

Spot welding of aluminum requires heavier equipment because "The current and force requirements remain significantly higher than for steel even after surface pre-treatment". Alternative welding methods are considered for aluminum frame production. Arc welding, in the case of MIG (Metal Inert Gas) welding, can use a flux which carries the oxide layer away from the weld. Unfortunately, it can also result in weld cracking: cracks caused by thermal stresses during cooling of the weld pool after impurities and alloying elements segregate at grain boundaries. The 6000 series alloys are particularly susceptible. The intense heat of the arc weld also affects the surrounding area, called the Heat Affected Zone (HAZ), which can be weakened by changes in the microstructure. Friction stir welding avoids high heat by using pressure and friction to "plasticize" the abutting edges and join them together, but this is a rather slow process that only works for simple joint configurations.

Development of Adhesives

Development in the automotive industry is driven by competition and regulation, which leads to a focus on reducing cost and weight without compromising safety. Aluminum is being developed to replace steel in various parts of automobiles, but the difficulties in welding have increased interest in assembly using adhesives, which already presents several advantages over welding. "Adhesive bonding" has technically been in use in the automotive industry since the 60's and the primary novelty of the current technology is its structural applications in the Body In White stage. The BIW is the car body when the frame has been fully welded/bonded together, before any moving parts or "trim" have been added. The desired structural properties of the bonds in the BIW are stiffness and strength.

Continuous Weld Vs Spot Weld

As previously discussed, current technology used in the BIW is Spot Welding, which joins components at several discrete sites. A continuous bond (which can be achieved through laser welding or brazing as well as adhesives) represents a significant (+20%) increase in stiffness, especially in torsion. An analysis of a spot welded joint shows that the "geometry" of the smaller joined area leads to higher stress. A low modulus adhesive would have the same effect as the high strength "structural" adhesives used, but Fays (2003) notes that the thickness of the low modulus adhesive is too variable to be used in the precision engineered automobile frame.

Failure (Fatigue, Vibration)

The continuous joint allows a significant improvement in fatigue resistance due to the "work" being distributed evenly along the joint and not concentrated by the fasteners. Though continuous joints are preferable, a combination of both adhesive and welding increases stiffness, as failure of the adhesive (through delamination) was reduced at the site of spot welds which stopped delamination propagation.

Engineers worry about the reliability of the adhesive (especially its behaviour during and after "ageing"). Despite sophisticated modeling of the system, the auto industry needs assurance that it is consistent on the "industrial model" (i.e. easily reproduced with acceptable tolerance for manufacturing differences). For this purpose, a crash test with an "aged" adhesive bond was performed, and the analysis shows that the adhesive failed only after the entire structure had impacted on it. (Fays 2003) Like all adhesive applications, automotive adhesives are best suited for sites that normally experience compression or shear stresses. In tension the joint is vulnerable to “peel”. Currently adhesives are used in closure panels (?). Besides the lifetime of the component, designers must always keep in mind the final commercial appeal of car. An important commercial consideration relevant to adhesive use is vibration; customers will not buy a car that vibrates excessively. The adhesive also improves vibration resistance, not through "dampening" as may be assumed, but rather because the increased stiffness leads to a "mode shift" (?) towards "higher frequencies". Lower natural resonance frequencies are eliminated. A further analysis on the advantages of adhesive bonding in point form:

  • Continuous bonding results in high stiffness in torsion
  • No deformation of substrate (unlike arc welding)
  • "... good noise and vibration damping properties" (Barnes) (Conflicts with Fays)
  • Also acts as a sealant.
  • Smooth joint without abrupt edges reduces stress concentrations in general
  • Inherently strong strength in shear (Grant et al suggest that this is inversely related to thickness of bond)
  • Permits joining of metals that would otherwise result in "galvanic corrosion"

Disadvantages:

  • Environmental costs of epoxies and solvent based adhesives
  • Difficult to coordinate for high volume production since adhesives often have very limited shelf lives
  • Delamination/"peel" failure is not accounted for in current automobile designs. Structural use of adhesives requires re-evaluation of crash safety.
  • Require fixtures for curing, and/or heat curing to be used in a timely fashion.

The combination of fasteners and adhesives is again suggested as a solution to the combined problems of a need for fixtures during curing and weakness in delamination. Aluminum requires chemical preparation before bonding (with the adhesives discussed here) to remove the passive oxide layer. The quality of this preparation and of the bond is key to determining the strength of the adhesive. NDT can be used to detect delamination faults and other voids, but cannot be used to determine the strength of the adhesive/bond/preparation, which makes it less attractive and reliable.

Cure Process

Epoxies are the primary adhesive/polymer used in structural applications. Epoxies are a thermoset polymer produced by mixing a resin with a (usually) polyamine monomer. The polyamine reacts with the resin resulting in a highly crosslinked and (usually) rigid structure. The rate of reaction between the components, known as curing, depends on the type of epoxy but can take several hours to days at room temperature. The addition of heat will speed the process up considerably. In the automotive assembly line, cure conditions will be sped up as much as possible. Heating rates of 6000C/min to reach the cure temperature, with hold times of 15 minutes. In lab tests with commercially used adhesives, hold times of 140h at room temperature still did not result in the maximum cure/strength from accelerate heating and cured specimens (11 MPa vs 18 MPa in shear tests, (Wu, 2005))

When working to design the optimal cure cycle for an epoxy, the decomposition temperature of the resin supplies the upper temperature limit. With imprecise heating methods, there is a greater possibility of uneven temperature concentrations (especially, one would imagine, in an irregular car frame) and gas evolution where the resin decomposes. Hao and Wu (2005) examine a variety of temperature profiles and for the epoxy-based adhesive supplied by the automaker. The results show that in all conditions, a rapid heating rate and a moderate hold temperature (80C, almost half of that specified by the automaker) result in the highest strength.

The heating rate relationship is complex. The SEM analysis suggests that void formation caused by decomposition of the resin while the epoxy is still at relatively low viscosity. The rapid heating causes low viscosity quickly, minimizing the deleterious effects of voids in the rapid high temperature cure.

Cure kinetics
(image from Wu, 2005)

The equation shown summarizes what could be described as the phenomenological model of curing. Rate of cure (dα/dt) is a function of temperature and degree of cure in the adhesive. The effect of temperature is included in the form of an Arrhenius rate equation for the cross linking reaction.

αm(1–α)n

The (1- α) term models the effect of movement limitation in the cross linked network on the curing rate. The m and n terms combine to form a bell shape, with the fastest curing rate occurring in the middle of the cure. These constants have basis in actual phenomena, but in practice they are often experimentally determined. In the case of an unknown, or proprietary, epoxy, Differential scanning calorimetry (DSC) could be used to detect curing rates and generate data to which one could fit a model.

Sources

Joining techniques for aluminium spaceframes used in automobiles: Part I — solid and liquid phase welding T.A Barnes, I.R Pashby Journal of Materials Processing Technology Volume 99, Issues 1-3, 1 March 2000, Pages 62-71

Adhesive Bonding Technology in the Automotive Industry Fays, Samuel. Adhesion and Interface Vol 4-2, 2003

Joining techniques for aluminium spaceframes used in automobiles: Part II — adhesive bonding and mechanical fasteners T.A Barnes, I.R Pashby Journal of Materials Processing Technology Volume 99, Issues 1-3, 1 March 2000, Pages 72-7

The Effect of Adhesive Curing Condition on Bonding Strength in Auto Body Assembly J. Manuf. Sci. Eng. -- May 2005 -- Volume 127, Issue 2, 411 (9 pages)

Comments from Chad

Overall nice job, it would be nice if you could try now to summarize this -- rather than simply review each paper in turn, can you organize based on main ideas then use the papers to support the ideas. It would be also good if you could give some more detail on the adhesive bonding itself -- for what parts is adhesive bonding used? What kind of adhesives are used? What is the typical process cycle involving bonding? This is the sort of information that will be useful for you in developing your methodology here.