Korrosionsuntersuchungen von verschraubten Mg-Komponenten

Translated title of the contribution: Corrosion tests of bolted magnesium components

Thomas Jakob

Research output: ThesisDiploma Thesis

1734 Downloads (Pure)

Abstract

Aluminium threadforming screws for fastening magnesium components offer a high potential regarding cost- and weightreduction. For a safe threadforming process in magnesium the bolt material EN AW 7xxx is necessary. EN AW 7075 alloys are often referred as not applicable for high load applications in the literature due to negative corrosion performance. The own of this diploma thesis is the creation of a guideline for high strength aluminum threadforming fasteners for usage in magnesium powertrain components in service. For this purpose a concept for evaluation of the contact corrosion behavior of bolted magnesium components under practical application conditions was created. Accordingly to the high requirements for corrosion protection of powertrain components due to the recommended 720 hours salt spray test with reference to DIN 50021, the corrosions test results indicate a better galvanic corrosion performance of EN AW 7075 screws with special top coat compared to the in the automotive industry established EN AW 6056 screws. In addition to the aluminium bolts steel bolts with different surface systems and protection systems were investigated. Furthermore a testing method for detection of stress corrosion cracking (SCC) was developed. Overelastic tightened high strength aluminium fasteners type EN AW 7075 (T6, T7x) in test specimens consisting of aluminium, magnesium and stainless steel with removal in salt spray tests according to DIN 50021 and alternating climate tests according VDA 621- 415, indicated that SCC mostly appears with the bolted stainless steel test specimens. This indicates that SCC under these conditions will be activated primary due to an anodic dissolution near to the grain boundaries of the EN AW 7075 bolt material. Bolted magnesium test specimens show no cracks of the EN AW 7075 T6 bolts under SCC testing conditions because EN AW 7075 bolts are cathodic protected due to the ignoble magnesium. Therefore the risk for SCC is on a low level when using EN AW 7075 bolts for fastening of magnesium components. This study also shows a significant influence of the thread rolling process after heat treatment on SCC sensitivity as compressive residual stresses and the influence of relaxation when thread rolling happen before heat treatment. Thread forming screws EN AW 7075 T7x with final thread rolling condition and with adapted top coats offer a big potential for a threadforming process into magnesium alloys with process security, low galvanic corrosion attack to magnesium and a low risk for SCC. A final verification for these conditions with adapted SCC tests is recommended.
Translated title of the contributionCorrosion tests of bolted magnesium components
Original languageGerman
QualificationDipl.-Ing.
Supervisors/Advisors
  • Grün, Florian, Supervisor (internal)
Award date27 Mar 2015
Publication statusPublished - 2015

Bibliographical note

embargoed until 11-02-2020

Keywords

  • Aluminiumscrews
  • EN AW 7075
  • EN AW 6056
  • galvanic corrosion
  • stress corrosion cracking
  • bolted magnesium componets
  • thread forming screws

Cite this