Warm Forming of Light Metals


Introduction


Aluminium and magnesium sheet metal alloys are prime candidates for use in the automotive industry because of their good strength and stiffness-to-weight ratios. The drawback to their widespread use is their room temperature formability. Aluminium alloys possess moderate formability at room temperature, while magnesium alloys have very little formability. It is widely known that warm forming of these alloys greatly improves their formability and this is the main focus of the warm forming research at the University of Waterloo. A large component of this work is on the development of finite element constitutive models that are able to capture the anisotropic behaviour of these metals at elevated temperatures and strain rates.


Warm Forming of Aluminium


Warm forming experiments on a clad aluminium brazing sheet are conducted to characterize the material’s behaviour at elevated temperatures and strain rates, develop material constitutive models, determine the effect of warm forming on the deep drawing process and validate finite element models. A schematic of the warm forming apparatus at the University of Waterloo is shown below for the deep drawing configuration. Warm stretch-forming tooling is also available.


A schematic and photograph of the warm deep drawing apparatus

For this work, non-isothermal warm forming (where the punch is cooled, while the blank holder and die are heated) was conducted. Non-isothermal warm forming allowed high material strength to be maintained near the punch radius where fracture typically occurs. In isothermal deep drawing, the punch is heated. The heated punch and die allows the flange area to flow more easily at elevated temperatures, which reduces the magnitude of stress in the cup sidewall and reduces tooling loads. Deep drawing experiments showed that a 9" diameter blank could not be formed at room temperature, but at a temperature of 250°C, the blank was fully drawn as shown below.


A deep drawn cup at room temperature and 250°C

The material behaviour at various temperatures and strain rates was measured through elevated temperature uniaxial tension tests. The figure below demonstrates the strong dependence of the stress-strain response on temperature and especially strain rate at the elevated temperature tests. Phenomenological and physically based constitutive models were fit to the experimental data and it was shown that the Bergstrom model (physically based) best captured the hardening response at various temperatures and strain rates as shown below.


Effect of temperature and strain rate on the stress-strain response of aluminium

The predicted engineering stress-strain response from uniaxial tension test simulations using the Bergstrom constitutive model

Using the Bergstrom constitutive model, coupled thermo-mechanical finite element simulations of the deep drawing experiments accurately predicted the strain and thickness distributions, as shown in the major strain comparison below. The punch loads were also predicted accurately for the Teflon lubricant used in the study.


Temperature (°C) distribution during the coupled thermo-mechanical FE simulation of deep drawing at 250°C

Predicted and measured strain distribution for a 250°C deep draw

Warm Forming of Magnesium


An increasing interest in making cars lighter and more fuel efficient has encouraged the automotive industry to use magnesium sheet metal alloys. Magnesium has a hexagonal closed packed crystal structure, which inherently has a low number of active slip systems at room temperature. Twining provides an additional deformation mechanism but can only accommodate a limited amount of strain. The mechanical response of magnesium to different load paths exhibits tension/compression asymmetry and is highly anisotropic. In order to successfully form magnesium sheet metal alloys, additional slip systems must be activated. At elevated temperatures, the necessary additional slip systems become activated and provide sufficient formability to conduct deep drawing experiments as shown below.


A deep drawn cup at room temperature

A deep drawn cup at an elevated temperature

The current research on magnesium alloys at the University of Waterloo, funded by the Magnesium Network Canada (MAGNET), includes the following activities:

  • Warm forming experiments (deep drawing and stretch forming) of magnesium sheet metals at different temperatures and strain rates
  • Material characterization at different load paths for a wide range of strain rates (0.001-1000s-1) and temperatures (23-250°C) see the High Strain Rate Material Behaviour research area
  • Flow stress and R-value measurements from uniaxial tension tests conducted at different temperature and strain rate ranges (see below)
  • Material modeling using different types of yield surfaces
  • Finite element modeling of sheet metal forming operations using Ls-Dyna

Effect of temperature on the uniaxial true stress vs. strain behaviour of AZ31 sheet metal alloy (temperature ranges from 23°C to 200°C)

R-value extensometer used in elevated temperature tests



Contacts


Dariush Ghaffari Tari, PhD Candidate
dariushtari@lagavulin.uwaterloo.ca

Michael Worswick, Professor
worswick@lagavulin.uwaterloo.ca

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