Working with Aluminum
Aluminum is the most abundant metal and third most abundant element in the earth’s crust. Vehicle manufacturers are producing more models with this beneficial metal everyday. Demand for aluminum repair has increased rapidly in recent years and will continue to do so. Already 30% of all the hoods and over 20% of the bumper beams are aluminum. Body, bumper & closure parts account for 58% of the aluminum content growth from 2009 to 2012 (»). Shops eager to capitalize on this market need to do two things: learn how to work with aluminum and re-tool with tools specific to this type of repair.
Reasons for Usage
More than ever automotive manufactures are using lightweight materials to improve fuel economy and turning to aluminum to create these lighter components. Lighter can result in better mileage, smaller engines moving less mass and greater acceleration. US Governmental initiatives has accelerated the weight loss of production vehicles by mandating the Corporate Average Fuel Economy (CAFE) standards of 54.5 mpg by 2025. Companies realize it makes economic sense to utilize this abundant, recyclable resource that makes up about 8% of the earth’s crust. The top selling pickup in history, the Ford F-150, as well as the F-250 has been redesign and features aluminum in its build materials. Other notable shifts to aluminum include GM with their Cadillac CT6.
Things to Know
Aluminum behaves differently than steel. Change the shape of a steel part; it remembers the form it used to be in. The opposite is true when working with aluminum. Once it is reshaped it forgets its original form and will take on a new shape.
It is a work hardened material. When aluminum is stamped into shape it becomes stronger. After the panel is bent in a collision it becomes stronger still. Flex it too much and it breaks or cracks like a bent spoon. In order to repair and pull the damaged area, the panel must be heated to 400°F to allow the metal to soften. Aluminum dissipates heat very quickly but can become permanently changed if heated past a certain point, approximately 750°F. The heated panel can now be pulled and while pulling the material it is being work hardened to set the new memory.
A thin coating of aluminum oxide forms after being exposed to the air in as little as 15 minutes. This oxidation has a much higher melting temperature than the original aluminum material and a stud will not fuse correctly to the panel. Aluminum’s melting point is 1,200°F and aluminum oxide is 3,700°F. The oxide can be removed using a stainless steel brush.
Galvanic corrosion (left) will occur when a less “noble” metal like aluminum, comes in contact with a more “noble” metal like steel, in the presence of an electrolyte suck as water. The aluminum will corrode around the steel contamination, ruining the finish paint. In order to avoid contamination a separate set of working tools and separated work area must be maintained. Steel bolts, screws or rivets should not be used when in direct contact with aluminum unless properly coated.
A quick method is to use a magnet, it will not be attracted to an aluminum panel. There are alloys and different manufacturing methods that determine the series of aluminum. It is a good idea to refer to the manufacturer literature for series identification of cast and panel pieces. In all cases manufacturer methods and guidelines on whether to repair or replace should be always followed. However, not all companies make this information available. Another great resource to utilize is the internet. A basic search for high aluminum content brought me to the free information provided by the Aluminum Association (www.autoaluminum.org). The site contains loads of information about the aluminum industry and a database of vehicles with high aluminum components. Once it is determined that the damaged part is made of aluminum, the next step is to understand what the 4 digit series means.
Aluminum alloys are designated into 4 digit series based upon the main element used in combination of the aluminum. The most common use for automotive applications are 5000 and 6000 series which mostly contain Magnesium and Silicon. Any number that follows the first digit identifies the percentage of alloying agent used. These alloys are further classified as Heat-Treated and Non-Heat-Treated. This means that during the manufacturing process they have the ability to gain strength from being exposed to elevated temperatures or from being stamped or bent. Click the chart below to enlarge.
Selecting the Appropriate Tools
First thing you will need is an abrasive grinding disc, inline removal tool and stainless steel brushes to remove the paint and oxide coating. To shape the metal it must be softened. Heat to the to the appropriate temperature with a digital temperature set heat gun for heat control. Aluminum has a high thermal conductivity and heat travels throughout the panel rather than in a concentrated spot. It is critical to maintain a temperature range of 400-500°F. Some technicians use the aid of coworker to heat the area while the other uses the dent pulling tool. If you go above 750°F the structural rigidity of the part could be permanently annealed. A good tool to monitor temperature is a infrared thermometer that determines surface temperature without having to touch anything to the aluminum. Infrared has difficulty with reflective surfaces so measurement must be taken off nearby painted surface or use a permanent marker. Heat indicating crayons can also be used if an accurate reading is not being obtained. Visual cues like color change in the metal are non existent when aluminum is heated.
Once you have prepped the damaged area by heating to the appropriate temperature, repair techniques are similar but differ subtly from steel. Weld on dent pulling can still be performed as long as you have the appropriate tools. A Capacitor Discharge Welder (CDW) is needed to overcome the high thermal conductivity property of the aluminum and fuse the stud to the surface. It must produce a higher level of power and deliver it more rapidly that a traditional stud welder. An important factor to consider when selecting a CDW is to look at how the unit is grounded. When working on a dent at the center of an aluminum hood, less desirable units with wire grounds require two exposed metal areas near the edge as ground points. More work is now needed to prepare and later properly blend the repair in three separate locations with the rest of the hood and fenders. Look for the grounds to be on the front of the gun on both sides of the electrode (left). This eliminates extra areas to blend and less work in general; a benefit everyone can appreciate.
Once the studs are attached a T-Puller, Bridge Puller and Squeeze Puller offer a wide range of pulling options. The puller used is determined by the location and depth of the repair area, but in general the squeeze puller is the most commonly used. Squeeze pullers allow for a massaging motion to work the damage out and this puts work hardening back into the panel. If necessary the puller can be locked in position and the panel reheated then massaged back to hardness. The repair can be further smoothed by using a hammer and off dolly technique. Dollies best suited for this include rubber, highly polished steal and metal bearing filled “beanbags”. Body files can be used to remove any residual stud stumps.
It is critical to prevent galvanic corrosion when working with the unique metal. The best method of prevention against contamination is to keep tools that are used for aluminum work, separate from tools used on steel. A separate enclosed storage cart should be used. Besides contaminated tools another factor to consider is accidental airborne contamination. Metal is often kicked into the air by sanders, grinders and cutters from other locations in the repair facility. Curtain walls, at a minimum, are needed to separate the work area. The enclosed tool kits keep the particles and debris out of the station and away from the tools. A mobile cart allows every tool needed for the job to be transported to the desired location in one trip.
A well thought out system organizes all the tools needed. Foam drawer cutouts eliminate misplaced tools. Each tool has its own place and can be easily identified when it is missing. This system locks away to maintain the integrity of the repair process.
Aluminum repair is not new but is becoming much more prevalent. It used to be higher end manufacturers who used aluminum; not true today. The thing to remember is that lightweight is the name of the game and more aluminum panels and components are showing up on an increasing number of mass produced models from all over the world. Shops can either ignore the increase demand, at there own peril, or capitalize on the repairs most other shops cannot do. Where do you stand?