# The Prediction of High-Temperature Bulging Deformations in Non-Uniform Welded Tubes and Its Application to Complex-Shaped Tubular Parts

## TL;DR

This paper improves predictions of high-temperature deformation in welded boron steel tubes used for automotive parts.

## Contribution

A modified Johnson–Cook model is proposed to predict deformation behavior in non-uniform welded tubes.

## Key findings

- The weld has 12.8% higher yield strength and 3.9% higher tensile strength than the base metal.
- The modified model achieved correlation coefficients of 0.99 for base metal and 0.982 for the weld.
- Predictions of wall thickness distribution in complex-shaped parts had a maximum deviation of less than 8%.

## Abstract

Boron steel welded tubes show strong potential as blanks in the integrated hot gas forming–quenching process for fabricating complex thin-walled automotive parts. Nonetheless, the non-uniform characteristics of the base metal and the weld in the high-heat welded tube can result in uneven deformation during the bulging process. This inconsistency hampers precise predictions of the deformation behavior of the welded tubes at high temperatures. Accordingly, this research explored the flow characteristics and mechanical properties of PHS1500 boron steel welded tubes. This research was conducted at 850 °C and 900 °C, with strain rates of 0.01 s−1–1 s−1. The Johnson–Cook model was modified for both the base metal and the weld using experimental stress–strain data. Meanwhile, to assess the model precisions, the correlation coefficient r and the average absolute relative error (AARE) were employed. Finally, hot gas forming of PHS1500 boron steel welded tubular parts with complex shapes was predicted through a finite element analysis. This research showed a positive correlation of the strain rate with both the yield and tensile strengths in the base metal and the weld. The average yield strength and tensile strength of the weld were 12.8% and 3.9% higher than those of the base metal, respectively. The r and AARE of the modified Johnson–Cook constitutive model for the base metal’s and the weld’s flow stress were 0.99 and 2.23% and 0.982 and 5.31%, respectively. The maximum deviation in the predictions of the distribution of the wall thickness of a typical cross-section of the formed complex-shaped tubular parts was less than 8%.