Method for high accuracy measurements of energy coupling and melting efficiency under welding conditions

Abstract

The demands on production processes vastly increased in the last decade. Beside the fulfillment of the fabrication task, a process has to be energy efficient and resource saving to be in line for mass production. For the evaluation of competing technologies or for the optimization of a process regarding these requests the knowledge about the specific process efficiency is crucial. However, the value strongly depends on the chosen process parameters and the environmental conditions, wherefore documented values in literature are inapplicable. Hence, an experimental determination for each individual case is inevitable. Existing methods for the estimation of the process efficiency are either inaccurate or time and cost intensive. Therefore a new method for determining process efficiencies for almost arbitrary materials and process conditions is presented. The method bases on process observation using thermographic imaging and a subsequent adjusted numerical computation of temperature fields. The result of the numerical calculation in combination with the evaluation of the weld seam cross section is the value for the energy coupling efficiency, melting efficiency as well as the overall thermal efficiency. The features of the proposed method are evaluated with a Design of Experiments (DoE) approach. The technique is applied to a conventional laser and plasma welding process as well as to laser-assisted plasma arc welding. In comparison to the individual processes, the laser-assisted plasma arc welding shows a more than doubled overall thermal efficiency, which can be ascribed to a drastic increase in melting efficiency and a moderate increase in energy coupling efficiency.

Thermal Efficiency Analysis for Laser-Assisted Plasma Arc Welding of AISI 304 Stainless Steel

“…This finding obviously indicates that the increased weld cross section area must be ascribed to the more beneficial usage of energy inside the probe. These results are in line with the results of an efficiency analysis with 1 mm standard type 304 stainless steel presented by Hipp et al [19], despite the increase being less pronounced. This leads to the assumption that the heat flow inside the probe—driven either by conductive and/or convective transfer mechanisms—is beneficially changed to generate the resultant weld seam cross-section with increased penetration by more favorable thermal and/or fluiddynamical boundary conditions.…”

“…The method for determining the energy coupling, melting and thermal efficiency is described in detail by Hipp et al [19] and consists of a contactless thermographic measurement of surface temperatures inside defined measuring areas during the process and the cooling regime. Instead of measuring the maximum temperature inside the molten pool, which is highly sensitive to errors [29], this technique measures the temperatures in areas located outside the processing zone.…”

“…Using these data, the heat flux to the workpiece and therefore the energy coupling efficiency η c can be inversely computed. Then, for the determination of the thermal efficiency value η T , the evaluated weld seam cross sections are used in combination with Equation (2) [19]:ηT= PUPA=vx·AS·ρ·(cp·(ϑs-ϑ∞) + hs)UArc·IArc+PL where v x is the welding speed, A S the weld seam area, ρ the density of the probe, c p the specific heat capacity, ϑ S and ϑ ∞ the melting and the ambient temperature, h s the enthalpy of fusion, P L the laser power, U Arc and I Arc the arc voltage and current, respectively. The melting efficiency η M then results from applying Equation (1).…”

“…The melting efficiency η M then results from applying Equation (1). The efficiency determination method and the used computational model were evaluated regarding their accuracy by Hipp et al [19].…”

“…In this context the thermal efficiency or overall process efficiency η T corresponds to the ratio of power P U which is needed to melt the weld material per unit time (without losses) to the total applied power P A . This quantity can be split according to Equation (1) in the melting efficiency η M (energy usage inside the base material) and the energy coupling efficiency η C (energy input from the heat sources) by using the power P T which is transferred from the heat sources to the workpiece [19]. ηT=PUPA=ηM× ηC=PUPT × PTPA…”