The stability of a cylindrical keyhole is investigated using the energy and pressure balance. Non-equilibrium evaporation from the keyhole surface, surface tension, hydrostatic and hydrodynamic pressure in the melt as well as heat conduction into the workplace are considered. In contrast to former investigations, the temperature at the keyhole wall and the radius of the keyhole adjust themselves self-consistently. A threshold for the laser power per thickness of the workpiece is found above which the formation of a stable keyhole commences. For iron, this threshold is 790 W mm-1. The temperature at the keyhole wall exceeds evaporation temperature by approximately 100 K. The keyhole radius exceeds the radius of the laser beam and is at least 1.7 times the laser radius.

Free oscillations of the keyhole in penetration laser beam welding are studied theoretically with regard to characteristic frequencies, damping rates and stability at large amplitudes. The normal modes form a discrete set which may be characterized by axial and azimuthal numbers. Due to viscous damping, only the lowest modes survive many oscillation periods, which yields a limited range of frequencies for the dynamic response of the keyhole to fluctuations of external welding parameters.

The dynamic behaviour of a keyhole in laser welding is studied theoretically. Starting from the stationary state, where the recoil pressure from ablating particles is in equilibrium with the surface tension at the keyhole wall, the collapse time due to a sudden laser shut-down is calculated. The characteristic time constant (r0 3 rho / gamma )1/2 of the system (r0 is the initial keyhole radius, rho is the density of the melt, gamma is the gamma coefficient of surface tension) which is approximately 0.1 ms for Al, Fe and Cu turns out to be a lower limit of the keyhole closing time. Linear stability analysis of the stationary state reveals that under conditions relevant in practice, the keyhole is expected to perform oscillations with frequencies of several hundred Hertz. The results of this investigation are particularly important for pulsed laser applications.

The conjecture which explains the humping phenomenon in terms of Marangoni convection is discussed and rejected. Instead, Rayleigh's theory of the instability of a free liquid cylinder due to surface tension is applied. The width-to-length ratio of the weld pool has to exceed 1/2 pi to avoid humping. The growth time of a disturbance is found to be approximately the same as the growth time of a hump. The analysis of a bounded cylinder provides a new stability criterion which allows the introduction of a bounding function to distinguish between arc and laser welding. The weld pool dimensions are estimated in terms of a simple heat conduction model. The threshold value predicted theoretically for the travel speed above which humping commences agrees well with the experimental value. It decreases with increasing power, which is in qualitative agreement with experimental results.

With two pilot plants in operation, the Direct Strip Casting (DSC) technology has reached a state from which it can be concluded that a DSC production process is feasible. The core of the process consists of a caster in which liquid steel is fed on an intensively cooled revolving belt. After solidification in a protective atmosphere, the yielded strip of about 10 mm in thickness is directly hot rolled without intermediate reheating. Thus, due to the reduced expenditures for hot rolling and reheating, substantial energy savings compared to conventional slab casting can be achieved. Moreover, the production of new high‐strength, light‐weight steels with an increased content of manganese, aluminium and/or silicon is enabled by the special features of the DSC process. The use of these steels in automotive applications would lead to further energy savings induced by significant weight reductions and an enhanced life cycle of the car body. Furthermore, also a higher share of scrap based strip steel production, requiring less than half of the energy needed for the blast furnace route, becomes conceivable for quality steel grades, as a higher content of tramp elements, e.g. copper and tin, is tolerable without quality losses (surface cracks). Finally, the compact design and the high productivity of the DSC process save capital and processing costs. In the paper, process development steps, material properties and energy saving potentials are outlined.