E. Ukar

The milling of airframe components with low rigidity: A general approach to avoid static and dynamic problems

Abstract At present, airframes are mainly composed of monolithic components, instead of small parts joined using welding or riveting. Ribs, stringers, spars, and bulkheads can be included in this category. After milling, they are assembled and joined to the aircraft skins, which have also been milled. The aim of these parts is to obtain a good strength-weight ratio, owing to their homogeneity. The milling of a monolithic structural part implies removing up to 95 per cent of the weight from the raw block material. Therefore, the main objective is to achieve the highest removal rate possible. However, conditions required to achieve this (high feed, large depth of cut) in milling imply high cutting forces, which in turn induce part deflection or vibrations in those zones (thin walls and floors) where stiffness is not sufficiently high. These static and dynamic problems often lead to inaccuracy of geometry, roughness, and possible damage to the machine spindle. This paper proposes a working methodology for efficient process planning, based on previous analysis of the static and dynamic phenomena that can occur during high-speed cutting. This methodology provides several steps that can be taken in order to minimize the bending and vibration effects; suggests optimal monitoring methods to detect process instability; and describes the best way to tune the cutting conditions and chip load, by means of simulation at different machining stages. In this way, the reliability of aeronautical production significantly increases. The global approach presented in this paper has been applied to two test pieces and two real parts, which were milled without suffering either static or dynamic problems.

Modelling of process parameters in laser polishing of steel components using ensembles of regression trees

Laser polishing of steel components is an emergent process in the automation of finishing operations in the industry. The aim of this work is to develop a soft computing tool for surface roughness prediction of laser polished components. The laser polishing process depends primarily on three factors: surface material, initial topography and energy density. Although the first two factors can be reasonably estimated, the third one is often unknown under real industrial conditions. The modelling tool developed solves this limitation. The application is composed of four stages: a data-acquisition system, a data set generated from the inputs, a soft computing model trained and validated with the data set. Finally, the model obtained is used to generate different plots of industrial interest. Different prediction models are tested until the most accurate one is selected, in order to generate the soft computing model, and due to the highly complex phenomena that influence surface roughness generation in laser polishing. Ensembles of regression trees yield the best results for the methods under consideration (multilayer perceptrons, radial basis function networks and support vector machines). It has been proven that the results of an ensemble, which is a combination of several models, are better than single methods in many applications.

Laser polishing parameter optimisation on selective laser sintered parts

This paper presents the effect of the laser-polishing process applied to parts built-up by selective laser sintering (SLS). One of the main drawbacks of the SLS technique is the high surface roughness of resulting parts. Therefore, a final polishing process is necessary to be valid for operation conditions. Polishing processes are usually carried out by manual abrasive techniques. In the present paper, a surface polishing method based on laser irradiation is presented to automate the polishing operation. The laser polishing process is based on a surface microscopic layer melting, which re-solidifies under shielding gas protective conditions, resulting on a smoother surface. Laser-polishing tests for lines, horizontal areas, inclined planes and a sample real part were performed, with satisfactory results in all the cases. The experimental tests were carried out on LaserForm ST-100© sintered test parts with an initial roughness of 7.5 to 7.8 μm Ra. Experimental results present final surface roughness below to 1.49 μm Ra, which represent a reduction of the mean roughness over 80%.

Laser Polishing Operation for Die and Moulds Finishing

The presented article proposes a laser based polishing operation which consists in the application of a laser beam in a very controlled way. The work presents several polishing tests carried out with CO2 and High Power Diode Lasers, for a DIN 1.2379 tool steel tempered up to 62HRC. This material is commonly used for injection moulds and inserts for stamping dies. The Laser Polishing tests results have been used as the input data for a design of experiments (DoE), therefore the optimum operation parameters for the process as well as its degree of influence in the melted surface have been defined. Experimental tests have been performed from different initial roughness values obtained by High Speed Milling operations. Reductions of final roughness higher than 80% have been obtained with respect to the initial roughness, with values below 0.8μm Ra.

Laser Polishing Parameter Optimization for Die and Moulds Surface Finishing

Final polishing operation for die and mould manufacturing represents up to 30% of the total manufacturing cost and it is a high added value operation carried out manually by qualified personnel. The work presented in this paper proposes an automated solution for this task by the process known as Laser Polishing. This process is based on the application of a laser beam melting a microscopic layer of material, which lately solidifies filling the gaps, and smoothing the overall topography. Several Laser Polishing tests have been done with CO2 and High Power Diode Lasers (HPDL) on two different materials commonly used in die and mould industry: a DIN 1,2379 Tool Steel tempered up to 62HRC, used for injection moulds inserts, and a spheroidal graphite Cast Iron DIN GGG70L used typically on large stamping dies manufacturing. By means of the tests and Design of Experiments (DoE) technique, the operation parameters for the Laser Polishing process as well as its degree of influence in the melted surface have been defined. Starting off from an initial surface obtained by means of High Speed Milling operation, it has been possible to obtain satisfactory results with final roughness reductions higher than 80% with respect to the initial values, and mean roughness values below 0.8μm Ra.Copyright

Polishing of Ductile Cast Iron with Scan-Head Guided Fiber Laser

Present work deals with laser polishing process on GGG70L spheroidal cast iron, which is widely used in die making for the automotive industry. Free graphite in the structure of GGG70L makes difficult the polishing and surface roughness reduction because the high melting temperature. In this work a complete study is presented, where main process parameters are identified and free graphite is eliminated from surface. The quantification of surface improvement is presented in terms of resulting surface roughness, hardness, and heat affected layer thickness. Thus, using optimal parameters, laser polishing with 2D scan head gives satisfactory results on GGG70L cast iron with roughness reduction rates up to 80% and minimum mean roughness Ra of 0.5 μm.

Laser hardening model development based on a semi-empirical approach

The present research paper deals with laser surface hardening applied to medium carbon alloy steel AISI 1045. In order to control as much as possible the process, two different models were developed: a semi-empirical and a numerical model. To validate the results several experimental tests were carried out with a high power diode laser, measuring surface temperature with a two colour pyrometer. The process parameters considered in the study have been the laser power, from 1,700 to 1,900 Watt, and the interaction time, from 1 to 4 seconds, for a beam spot diameter of 10.2 mm. The results obtained in hardness value and heat affected zone depth show that the methodology explained is acceptable to evaluate laser hardening effects in an industrial application.

Influence Of The Laser Cladding Strategies On The Mechanical Properties Of Inconel 718

This work presents different experimental results of the mechanical properties of Inconel® 718 test parts built‐up by laser cladding. Recently, turbine manufacturers for aeronautical sector have presented high interest on laser cladding processes. This process allows building fully functional structures on superalloys, such as Inconel® 718, with high flexibility on complex shapes. However, there is limited data on mechanical properties of the laser cladding structures. Moreover, the available data do not include the influence of process parameters and laser cladding strategies. Therefore, a complete study of the influence of the laser cladding parameters and mainly, the variation of the tensile strength with the laser cladding strategy is presented. The results show that there is a high directionality of mechanical properties, depending on the strategies of laser cladding process. In other words, the test parts show a fiber ‐like structure that should be considered on the laser cladding strategy selection.

An Approach to Thermal Modeling of Laser Polishing Process

Polishing operation of die and molds represents up to 30% of the total manufacturing cost, since it is a high added value operation that is necessary carried out manually by qualified personnel. There are several alternatives to hand operations, such as abrasive automated methods operated by robots, ball burnishing or laser polishing process in order to reduce operation time and costs. The presented article proposes a laser based polishing operation which consists in the application of a laser beam in a very controlled way. The radiated energy melts a microscopic layer which flows and re‐solidifies smoothing the topographic irregularities of the surface resulting in a reduction of the initial roughness. The article presents, in addition to some experimental results of laser polishing process, a thermal model developed based on the finite difference method. The model is able to take into account different types of lasers; in particular, simulations for a CO2 laser and a high power diode laser have been carried out. As the model gets the full map of temperatures in 3D, it is possible to predict the melted material layer thickness, which is a crucial parameter in the laser polishing process. Finally, two types of validation tests are presented: First, some tests measured by pyrometers on the test part surface. Secondly, the comparison of a series of metallographic analysis on W.‐Nr.1.2379 tool steel, where the metallurgical changes caused by the laser radiation can be observed with the thermal field estimated by the model.

DETERMINATION OF CUTTING CONDITIONS FOR THE STABLE MILLING OF FLEXIBLE PARTS BY MEANS OF A THREE-DIMENSIONAL DYNAMIC MODEL

In this paper, a three-dimensional dynamic model for the prediction of the stability lobes of high speed milling is presented considering the combined flexibility of both tool and workpiece. The aim is to avoid chatter vibrations in the finish milling of aeronautical parts that include thin walls and thin floors, taking into account the variation of the dynamic properties of the workpiece during machining. Hence, the accurate selection of both axial depth of cut and spindle speed can be accomplished. The model has been validated by means of a test device that simulates the behaviour of a thin floor. The following methodology is applied: first, a modal analysis of the test device is performed, second, stability lobes are calculated, and finally, a milling test validate the approach. Vibration signals from machining have been analysed to detect the chatter vibrations. The lobes diagrams obtained considering only the machine or only the tool are not in touch with reality; only through consideration of the relative frequency response function, the real borderline of stability can be obtained.Copyright