Helmut Klöcker

Nitrogen strengthening of a martensitic steel : Relation between microstructure and mechanical behaviour

Abstract Four synthetic martensitic steels containing different nitrogen levels were prepared by powder technique and HIP. The base composition of the four alloys corresponds to H13 steel except for the carbon content. The nitrogen contents of the different specimens are, respectively, 0.2, 0.5, 1.0 and 1.5 wt%. The flow stresses and Youngs moduli of the four alloys were determined between room temperature and 600°C. The volume fractions of micron-size and manometric second-phase particles were determined as a function of the total nitrogen content. The mechanical and microstructural analyses show that nitrogen strengthening of martensitic tool steel is suitably described by Orowans mechanism.

An Investigation of Failure Types in High-Strength Steel Resistance Spot Welds

On the shop floor, as in laboratories, destructive testing remains the main means of quality control of spot welds. After hand or mechanized weld destruction, the so-called “plug diameter” is measured and is usually considered a good indicator of the weld quality. However, it turns out that the “plug” failure of spot welds is far from being the rule. Moreover, fracture may occur in different zones of the weld, leading to very different meanings of the plug diameter. Therefore, the plug diameter is most of the time not well-correlated with the weld strength, which is the main value of the spot weld. Partial or full interfacial failures (through the weld nugget) exhibit equivalent mechanical strengths and therefore should not be rejected. Through this work, it is shown that the failure type depends on various parameters (nugget diameter, sheet thickness, loading mode, …), and consequently, it is not an intrinsic property of the steel grade. In this way, recommended quality criteria are based on weld strength, weld diameter (including the interfacial fracture area if any), or absorbed energy independently of failure occurrence. In addition, non-destructive techniques might be key investigation methods and have to be developed further.

Spot weld strength determination using the wedge test: in-situ observations and coupled simulations

This paper describes an innovative way to characterize the strength of spot welds. A wedge test has been developed to generate interfacial failures in weldments and observe in-situ the crack propagation. An energy analysis quantifies the spot weld crack resistance. Finite Element calculations investigate the stresses and strains along the crack front. A comparison of the local loading state with experimentally observed crack fronts provides the necessary data for a failure criterion in spot weld fusion zones. The method is applied to spot welds of Advanced High Strength steels.

The large strain flow stress behaviour of aluminium alloys as measured by channel-die compression (20-500°C)

Two recent methods for obtaining flow stress-strain relations up to large strains of order 1.5 by channel-die compression are presented: i) for sheet metal formability tests, composite samples have been made of glued sheet layers and deformed at room temperature in a channel-die with the compression axis directed along one of the sheet metal edge directions, i.e. RD or TD. The sheet plane is parallel to the lateral compression die face. It is shown that, using a suitable lubricant, the sample deformation is homogeneous up to strains of 1.5. Tests carried out on 5xxx and 6xxx alloys to evaluate the stress-strain relations show that a generalized Voce law gives a good quantitative fit for the data. ii) for high temperature plate processing, quantitative flow stress data can be obtained up to 500°C with a rapid quench using a hot channel-die set-up. Some new results are presented here for high strain hot PSC tests on Al-Mn and Al-Mg alloys together with microstructure analyses.

Measuring Stress Strain Curves to Large Strains on Sheet Metal

The stress-strain response of aluminium sheet has been determined by a novel plane strain compression test on laminated samples up to equivalent strains of 1.2. The test sample is composed of several sheet layers glued together, machined to shape, and then compressed in a lubricated channel die along the “thin” directions. This simple test has been validated by a comparison of finite element simulations and experimental results. The stress-strain curves can be considered accurate up to strains of 1.2.

Fundamentals of Bending AA6xxx Sheet

This paper describes recent experimental results on the strain distributions developed during bending of AA6xxx sheet for automotive applications, together with a new model for the mechanics and metallurgy of strain localization during bending. A detailed microscopic study (optical and SEM/EBSD) shows that damage development during bending to strains of order unity is controlled by through-thickness shear banding at the grain scale. A new finite element microstructure-based model is introduced to predict this strain localization during practical bending. The sheet metal is modelled as a grain aggregate, each grain having its own flow stress. After validation, the model is applied to the experimental results through an analysis of the critical plastic strain at the outer surface during bending of AA6016 sheet alloys. It correctly describes the respective influences of sheet thickness, grain size and shape, and work hardening. In particular the model brings out the primary importance of large-strain hardening and the spread of the flow stress distribution.

Characterization of Damage Mechanisms during Bending of 6xxx Aluminium Automotive Sheets

Bendability is a key property of aluminum automotive panels. Previous work showed that the bendability may not be characterized by macroscopic parameters. In the present work, the kinetics of damage development during bending of 6016 sheet was first characterized experimentally. Then, a mechanical model analyzing independently the influence of the microstructure, the flow stress distribution, the hardening behavior of the material and the sheet thickness on the bendability was developed.