When Toyota Motor Corp. deputy chief engineer Takatomo Suzuki presented the all-new Vios’ improvements at the launching extravaganza last week, one of the first he mentioned was the increased number of spot welds. It underscored the major role that spot welds play in making a car safe. Spot welds not only make a car safe, they also help to reduce rattles and road noise.
Several criteria make a car safe, but the metal structure of a car is hugely important. Spot welds are the dominant joining method in the automotive assembly process. To create a spot weld, two or more metal sheets are pressed together by electrodes and an electric current is passed through. The resistance of the metal generates heat and the sheets are welded together by means of local metal fusion, creating a spot weld. No welding material is added in this process.
Since the 1950s, resistance spot welding has been the prominent assembly technique in the auto industry. Computer-programmed industrial robots are used to spot-weld metal parts together, making the joined parts stronger, safer and more uniform in appearance
INTEGRITY. There may be 3,000 to 5,000 spot welds in a typical car body. Assured reliability of the spot weld is important since the integrity of the spot welds is vital to the crash performance of a car and the safety of its occupants. Since the automated assembly process is not perfect, some spot welds may be absent when the vehicle rolls off the assembly line. Furthermore, spot welds are susceptible to fatigue so that a substantial number may fail during the vehicle’s lifetime.
In the manufacturing process, the amount of energy (heat) delivered to the spot is determined by the resistance between the electrodes and the magnitude and duration of the current. The amount of energy is chosen to match the sheet’s material properties, its thickness and type of electrodes. Applying too little energy will make a poor spot weld.
In developing countries where the cost of energy is high, one way to save on electricity and thus cut manufacturing costs is to either reduce the number of spot welds or use less energy for each spot weld made. But this affects structural performance in the event of a crash. Since the electricity used in building a car is about 20 percent of the cost of the structure, a manufacturer in a developing country can reap a 10-percent profit, compared with only 3 percent in North America and Europe, by applying less electricity to spot welds. A global manufacturer can easily cut production costs in emerging markets where consumers, knowing little about safety issues, are less demanding on safety standards.
BELIEVE. Since spot welds are done early in the car’s production, a good spot weld or a poor one can’t easily be seen in the finished vehicle. The consumer just has to believe that the manufacturer has made an adequate number of accurate spot welds to create a more stable platform for the car.
Aside from spot welds holding the vehicle together, other criteria that make a car safe are adequate body reinforcements, crumple zones, high-quality steel, an updated platform, three-point seatbelts, airbags and a strong steering column. In an online Associated Press report last May, an engineering professor at Brazil’s premier University of Sao Paolo said that in a car with a weak body structure and no airbags, a driver’s biggest danger is the steering wheel.
In frontal crashes, the wheel of a car with a fragile steering column can slam into the driver’s chest and abdomen, causing serious damage to vital organs. David Ward, director general of the London-based FIA Foundation for auto safety, said that steering wheels that break off and “float” during wrecks in poorly made cars move around the cabin in the driver’s area, so that even if an airbag is deployed, the steering wheel may go around or under it and directly hit the driver.
ABSORB. The omission of crumple zones, areas that absorb energy during a road crash, will endanger occupants’ lower limbs as foot wells rip off, and expose feet and legs to car parts slamming into them from the front. If a car’s body cannot absorb the energy of a crash, it will logically result in more damage, more injuries to passengers, a doctor in Brazil who specializes in traffic accident victims said. The secret of a car’s body being able to withstand a road crash is the spot welds.
It is a given that in some emerging markets in Asia, Africa and Latin America, consumer awareness of car safety is some 20 years behind the United States and Europe. Ward said he has watched the same battles play out over car safety—the only thing that changes is the location. In 1965, consumer activist Ralph Nader’s book, “Unsafe at Any Speed,” indicted the US auto industry and ignited a national discussion on auto safety that ultimately led to reforms in the industry’s standards. It helped lead to a 32-percent drop in road crash deaths by 2011.
The Associated Press quoted Ward as concluding, “The sad thing is, this has been the experience in the 1960s in the United States, in the 1990s in Europe and now in Latin America. The industry does the least it can get away with until they’re forced to do something different. It’s maddening.”