Story and Photos provided by I-CAR
Like it or not, aluminum is here. Aluminum use in vehicle production has steadily increased during the last several years. Aluminum is quite frequently being used for exterior body panels and is making its way into the structural role of vehicles. Are your technicians ready to repair aluminum when it shows up in your facility? With the proper training, aluminum repairs can be less intimidating than you may think.
Differences from steel
A collision repair technician must first understand that steel and aluminum have different characteristics. Aluminum, in the pure state, is a much softer metal than steel. Therefore, the aluminum used in the vehicle manufacturing process is generally alloyed with other elements to create a metal with the physical properties that are needed.
Aluminum alloys commonly used for vehicle production can be broken down into two groups, heat-treatable alloys and non-heat-treatable alloys. The heat-treatable alloys gain mechanical properties when thermally treated during the manufacturing process. In some cases, additional heat-treatment may occur during the bake cycle for the finish at the assembly plant. Other applications are heated following the forming process of aluminum. Non-heat-treatable alloys gain mechanical properties by the cold-forming process. Forming the aluminum into a shape strengthens it by compressing and stretching the molecules. This is known as work hardening. Steel work hardens as well but not to the point that aluminum does.
Aluminum has less memory than steel, meaning it will tend to stay in its existing shape. The aluminum molecules lock into place and are very difficult to unlock. Additional force, as well as the application of heat, may be required to straighten the aluminum structure compared to a steel structure.
Aluminum structural use
Structural aluminum parts generally come in the following categories: stampings, extrusions and castings. These parts are attached using a variety of methods at the assembly plant to create the vehicle structure. Depending on the design, these methods can be rivet-bonding (using rivets in conjunction with adhesives), riveting, gas metal arc welding (GMAW) or metal inert gas (MIG) welding, or laser welding.
Repair versus replace
Now that we understand some of the principles of an aluminum vehicle, let’s see what can be straightened. There is no general rule for repair versus replace on aluminum vehicles like the kink versus bend rule for steel structural repairs. In most cases, damaged structural aluminum will be straightened to repair indirect damage and the parts with visual deformation will be replaced.
Stampings and extrusions offer the most repair options. Castings will generally crack when bent and usually require replacement when damaged. As a general rule, if the part is cracked, either as a result of the collision or during the straightening process, it must be replaced. In most cases, the straightening of aluminum structural parts will be to remove the indirect damage and to restore the dimensions within tolerance. If visual deformation cannot be removed, the part will generally require either partial or complete replacement.
Another item we need to look at is what the vehicle maker says about structural straightening. Some vehicle makers are very strong on their stand of not allowing straightening to an aluminum structure. So strong, in fact, that they have published information warning against it. Regardless, the vehicle will usually require a trip to the frame rack for replacement of damaged parts. Other vehicle makers recommend only minor straightening, while others say straightening is acceptable as long as any areas of visible deformation are repaired. Those vehicle makers that say straightening may be attempted generally may have recommendations for anchoring and heating as well. There are also some vehicle makers who have restrictions on part availability and equipment that is required to repair the vehicle. The concern is the repair technicians need to be trained in the proper repair techniques required to restore the vehicle to its pre-damaged condition. Before attempting structural repairs to an aluminum-intensive vehicle, be sure to locate the collision repair information provided by the vehicle maker. Attempting to repair an aluminum vehicle without it could lead to some serious problems.
Anchoring
The first step when structurally straightening an aluminum-intensive vehicle is anchoring the vehicle to the frame rack. Some vehicle makers have specialty fixtures that will be required to secure the vehicle to the rack to prevent additional damage during the pulling process. Frame repair equipment providers may have clamps available for aluminum vehicles.
Due to the fact that aluminum will tend to stay in its existing shape, a greater amount of force may be required to straighten the vehicle structure when compared with a steel structure. The added amount of force makes anchoring that much more important. Improperly anchoring the vehicle can cause extensive damage to the vehicle structure due to the anchors tearing out. Inspection of the anchoring points during the straightening process is a requirement whenever pulling is being performed. This is especially important on an aluminum-intensive vehicle.
Heating during straightening
Now that the vehicle is on the frame rack, the vehicle can be measured and a repair plan can be developed. Here is where the heating comes into play. When pressure is applied, heat will help the aluminum become temporarily more elastic, allowing it to more easily return to the original shape without cracking or tearing. Before heat is applied, there are a few characteristics of aluminum that must be pointed out. Aluminum will not change color when it reaches the melting point, as steel does. Also, heat travels much quicker on aluminum than it does with steel.
It is crucial that the temperature be monitored when heat is applied to aluminum. There are several ways to do this, but the most likely choices are thermal indicating paints or crayons or using a noncontact thermometer. Noncontact thermometers read the temperature that the part is emitting, known as emissivity, not the surface temperature. If the part is shiny, reflections can make the reading inaccurate. To effectively measure temperature and to receive accurate readings, the coating of the aluminum must be intact. Some noncontact thermometers have an emissivity adjustment that can help the readings become more accurate, but the best method to get an accurate reading is to keep the coating intact or apply a coating to it. An aerosol primer is an effective coating that can be applied to any bare or shiny aluminum to make the noncontact thermometer read accurately.
When straightening aluminum, there are some temperature ranges that can be very effective. The general repair temperature guidelines are to heat the aluminum to a temperature range of 200-300°C or 400-570°F. Exceeding this range can approach the annealing temperature, which could compromise the strength of the metal. Also consider the effect of heat being applied and how it will travel into surrounding areas of the vehicle. This is of significant concern when dealing with rivet-bonded parts. Heat can travel to the adhesive and compromise its strength.
Following the repair plan
After developing the repair plan, straightening can be approached following the same techniques used to repair a steel structured vehicle except for one key factor. The areas that are dimensionally correct must be held in the position to prevent pulling the vehicle in the wrong location. Additional anchoring or blocking may be required to do this. Otherwise, the repair techniques are the same. After straightening, a dye penetrant test should be used to inspect the repaired areas for any cracking of the parts or the welds that may have occurred.
Planning and preparation
The fact is, aluminum has arrived in the vehicle manufacturing process. If you have not yet had the opportunity to repair an aluminum-intensive vehicle, chances are you will in the future. Having the knowledge and skills required for repairing aluminum before it shows up in your facility can help you be prepared when it does.
There are several I-CAR Enhanced Delivery Programs dedicated to informing all segments of the collision industry of the repairs to aluminum. Visit www.i-car.com and conduct a class schedule search to see what is available in your location. Also, be sure to locate the collision repair procedures from the vehicle maker before attempting structural repairs.