A top mark in welding

A top mark in welding
Marginal Column

Photo: Qoros Automotive Co. Ltd.

Content

November 2014

 

A rugged passenger safety cell with perfect spot welds makes the Qoros 3 a crash test winner – due in part to the use of Rexroth controls.

 

A new carmaker from China, Qoros is attracting global attention. All the more so after its compact Qoros 3 Sedan achieved the maximum of five stars in the Euro NCAP crash tests in September 2013. This was the first Chinese model to do so, racking up the highest point value for all the vehicles tested in 2013. The basis for these superior crash test scores are ingenious engineering and perfect fabrication of the passenger safety cell. In designing its production, Qoros counts upon individuals with many years of experience. Thus Dr. Martin Messler, formerly of Ford Motor Company, is the director of manufacturing engineering and thus responsible for the design of the welding and assembly line. An expert in vehicular welding techniques, Bernd Riemann, was employed by renowned automotive industry firms in Germany for more than 40 years. He provides consulting support to the seven-member team of welding experts at Qoros. When selecting subcontractors, the new Chinese car builder also puts its faith in familiar faces. More than 50 medium-frequency welding controllers from Rexroth guarantee the quality of the spot welds.

Correct joining technology

The stringent demands of the Euro NCAP crash test are determinant for the design and quality standards to be met by the Qoros 3 bodywork. More than 70 differing sheet metals and steel alloys are used to achieve the required performance. The engineers have defined more than two hundred points that impact safety. Resistance spot welding at medium frequencies satisfies the exacting demands for the welding process. This joining technology is widely used in the automotive industry. Not only does it make for the best welding of heterogeneous materials but, thanks to dynamic power application, it offers higher welding quality than AC technology.

Many factors and influences are decisive for the quality of the spot weld. The quality of the materials, the nature and thickness of the parts being welded, their fit, surface properties and strength play a role, as do the degradation of the welding electrodes, their cooling and the coolant temperature and flow rate. The welding controls by Rexroth consider these parameters in their algorithm. This is augmented by many years of experience in widely diverse applications, accumulated by the engineers at Bosch Rexroth.

Perfect welding quality

The adaptive process control system incorporates quality monitoring matched to the needs of mass production. With intelligent U/I control for maximum process reliability, it makes possible online monitoring of the spot welds. The adaptive control logarithm thus ensures uniform welding quality and reduces spattering, even during demanding welding assignments. This significantly lowers the need for costly reworking. Tolerance limits can be set and monitored for a number of basic process parameters: current, phase angle, electrode voltage, resistance, current time cycle, power, and energy are among them. Rexroth provides two consolidated indicators to give the operator a quick survey of the operating status. One is the process stability factor, PSF, which indicates the stability of the welding process, while the UIP quality factor provides an indication of actual product quality. This is summarized with a process control display showing a bar graph that reflects the status of all the welding controls in the system.

Transferring know-how during the project

The welding technology team at Qoros had set itself an ambitious goal. The specifications provide for less than five percent spattering loss. This is a real challenge for optimizing the welding process. Xuejun Li, senior engineer for welding and assembly technology at Qoros, is very satisfied with the functions of the Rexroth products. “The medium-frequency welding control by Rexroth records information on all the spot welds. We use the logged data to analyze weld quality and to identify deviations. We gain experience in test series and in this way find the best parameters for each and every welding point.” In addition to the products themselves, Xuejun Li is enthusiastic about the expertise possessed by the engineers at Bosch Rexroth. The technical team there shared with Qoros valuable practical experience in welding technologies during training sessions and project services. Xuejun Li reviews: “Thanks to our excellent cooperation, the medium-frequency welding controls by Rexroth will continue to be our preferred choice in the future, too.”