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A set of temperature-dependent friction coefficients was developed to increase the accuracy of finite element (FE) simulations of cold bolt forging. The initially attained friction coefficients at different temperatures were calibrated with the iterations between the experimental and thermomechanical model extrusion test loads. The constant friction coefficient and the determined set of friction coefficients as function of temperature were then implemented to the simulations of the cold bolt-forging processes. Further calibrations and model validations were made based on the temperature measurements of the workpiece in the actual bolt-forging processes. To show the advantages of developed temperature-dependent friction coefficients, the loads of four different bolt-forging processes were compared with the thermomechanical model loads calculated using the constant friction and temperature-dependent friction coefficients. The modeling results indicated that the use of temperature-dependent friction coefficients in the FE simulations resulted in nearer temperature distributions and the loads of the workpiece during forging as compared with the use of a constant friction coefficient.

In this study, fatigue behavior of Zn-Cr+3 and zinc flake coated bolts was investigated experimentally by considering corrosion effects. Fatigue tests were conducted on M8x50x1.25 8.8 DIN 933 bolts at normal and corroded conditions at force amplitudes between 2.35 and 5.36 kN at test frequency of 75 Hz. A part of bolts was subjected to salt atmosphere for corrosion. According to the Wöhler curves obtained from fatigue tests before corrosion, zinc flake coated bolts showed 1.55 times higher fatigue life compared to Zn-Cr+3. After corrosion, fatigue tests showed that the fatigue life difference increased to 2.10 times for the zinc flake coating. Moreover, corrosion was found to be ineffective on crack initiation region. For both coatings and corrosion conditions, fatigue cracks were seen to initiate from the thread roots.

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