A simplified analytical model is presented that predicts the depth progress during and the final hole depth obtained by laser percussion drilling in metals with ultrashort laser pulses. The model is based on the assumption that drilled microholes exhibit a conical shape and that the absorbed fluence linearly increases with the depth of the hole. The depth progress is calculated recursively based on the depth changes induced by the successive pulses. The experimental validation confirms the model and its assumptions for percussion drilling in stainless steel with picosecond pulses and different pulse energies.
We present a model to predict the final depth of percussion-drilled holes that are produced with picosecond laser pulses in metals. It is based on the assumption that boreholes always have conical geometries when the drilling process terminates. We show that the model is valid for various process parameters when drilling in stainless steel. This was even confirmed by drilling with 3 mJ pulses, which resulted in a 10 mm deep borehole without thermal damage.
We report on high-quality high-throughput laser milling of silicon with a sub-ps laser delivering more than 1 kW of average laser power on the workpiece. In order to avoid heat accumulation effects, the processing strategy for high-quality laser milling was adapted to the available average power by using five-pulse bursts, a large beam diameter of 372 µm to limit the peak fluence per pulse to approximately
0.7
J
/
c
m
2
, and a high feed rate of 24 m/s. As a result, smooth surfaces with a low roughness of
S
a
≤
0.6
µ
m
were achieved up to the investigated milling depth of 313 µm while maintaining a high material removal rate of
230
m
m
3
/
min
.
The availability of commercial ultrafast lasers reaching into the kW power level offers promising potential for high-volume manufacturing applications. Exploiting the available average power is challenging due to process limits imposed by particle shielding, ambient atmosphere breakdown, and heat accumulation effects. We experimentally confirm the validity of a simple thermal model, which can be used for the estimation of a critical heat accumulation threshold for percussion drilling of AISI 304 steel. The limits are summarized in a processing map, which provides selection criteria for process parameters and suitable lasers. The results emphasize the need for process parallelization.
Stormont House Agreement (2014) Paragraph 21. 'As part of the transition to long-term peace and stability the participants agree that an approach to dealing with the past is necessary which respects the following principles: promoting reconciliation; upholding the rule of law; acknowledging and addressing the suffering of victims and survivors; facilitating the pursuit of justice and information recovery; is human rights compliant; and is balanced, proportionate, transparent, fair and equitable.' 6 NOTE ON TERMINOLOGY As ever in the Northern Ireland context, disputes on terminology can arise. Our focus herein is on providing accessible legal and policy analysis, using clear and unambiguous terms to help readers make up their own mind on these complex and sensitive matters. The conflict, sometimes referred to as the Troubles, refers to actions concerning the constitutional status of Northern Ireland by republican paramilitaries, loyalist paramilitaries, the security forces (in Northern Ireland and the Republic of Ireland) and others from 1966 onwards.
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