C. Casavola

Preliminary investigation on distribution of residual stress generated by the selective laser melting process

Selective laser melting (SLM) is one of the most interesting technologies used in rapid prototyping processes because of the possibility of building complex three-dimensional metal parts of nearly full density and with mechanical properties similar to those obtained with conventional manufacturing processes. This goal can be achieved using high-power lasers and low values of scan velocity. These conditions, together with an appropriate scanning strategy, allow full melting of the powders used in the process to be obtained. The aim of this paper is to investigate the residual stresses in SLM specimens manufactured from AISI Marage 300 steel. First, the strain gauge hole drilling method is utilized to determine residual stress profiles in a set of test samples of different thicknesses, placed in different positions on the building platform. Statistical analyses are performed in order to study the relationships between sample position on the platform, the distance from the specimen surface, and maximum/minimum principal residual stresses. The experimental results show that the melting/solidification mechanism generates highly variable thermal residual stresses in the SLM parts used in this study.

Analysis and Comparison of Friction Stir Welding and Laser Assisted Friction Stir Welding of Aluminum Alloy

Friction Stir Welding (FSW) is a solid-state joining process; i.e., no melting occurs. The welding process is promoted by the rotation and translation of an axis-symmetric non-consumable tool along the weld centerline. Thus, the FSW process is performed at much lower temperatures than conventional fusion welding, nevertheless it has some disadvantages. Laser Assisted Friction Stir Welding (LAFSW) is a combination in which the FSW is the dominant welding process and the laser pre-heats the weld. In this work FSW and LAFSW tests were conducted on 6 mm thick 5754H111 aluminum alloy plates in butt joint configuration. LAFSW is studied firstly to demonstrate the weldability of aluminum alloy using that technique. Secondly, process parameters, such as laser power and temperature gradient are investigated in order to evaluate changes in microstructure, micro-hardness, residual stress, and tensile properties. Once the possibility to achieve sound weld using LAFSW is demonstrated, it will be possible to explore the benefits for tool wear, higher welding speeds, and lower clamping force.

Mechanical Characterization of SLM Specimens with Speckle Interferometry and Numerical Optimization

This paper describes the process of mechanical characterization of specimens built via Selective Laser Melting (SLM). An hybrid approach based on the combination of phase-shifting electronic speckle pattern interferometry (PS-ESPI) and finite element analysis is utilized. Three-point-bending experimental tests are carried out. The difference between displacement values measured with ESPI and their counterpart predicted by FEM analysis is minimized in order to find the values of the unknown elastic constants.

Drilling Speed Effects on Accuracy of HD Residual Stress Measurements

The effect, on the residual stress measurement accuracy, of the drilling speed of the end-mill during the hole-drilling measurements was evaluated in Ti6Al4V. In spite of the well-known consideration that the highest achievable speed should be used during hole drilling, very few experimental works exist analyzing the effects of using lower velocities. Hole-drilling experiments were performed in this study by measuring the released strain by electronic speckle pattern interferometry. A known stress state was generated by loading the sample in a four point bending frame up to 50 % of the yield strength. Drilling speed ranging in 5,000 ÷ 50,000 rpm was investigated by using an electronically controlled mill. The expected stress field, evaluated by a numerical model in ANSYS®, was compared with the measured one at different drilling speeds.

Hybrid Characterization of Laminated Wood with ESPI and Optimization Methods

In the last decades wood has assumed an increasing relevance in building engineering both due to new technologies in wood production and manufacturing, such as computer-aided optimization of log sawing, automated lumber grading etc., and to the development of the green building. Fibrous structure of the wood makes it an orthotropic material so that it becomes important to perform a measurement, which allows to fully characterize the orthotropic properties of the specimen. Hybrid numerical-experimental techniques can be used in order to accurately determine the Young modulus and the Poisson coefficient of the material.

A Comprehensive Numerical Stress—Strain Analysis of Laser Beam Butt-Welded Titanium Compared with Austenitic Steel Joints

Experimental tests on laser beam butt-welded joints made of commercially pure titanium and Ti—6Al—4V alloy showed a fatigue behaviour quite different from that usually observed in the case of AISI 304 joints connected with the same welding technique. In fact, fatigue failures of titanium joints were observed at the level of the base material but not in the notch region, as have been found for austenitic steel. In order to explain this experimental evidence, very detailed finite element models are developed in this paper. The models reproduce the real cord profiles of Ti—6Al—4V and AISI 304 steel welded joints detected by a high-precision coordinate measuring machine. Furthermore, mechanical properties given as input to the finite element models correspond to the real joint microstructure revealed by micro-hardness tests carried out in the different regions of the specimen. Finally, finite element models reproduce the distribution of porosity detected via microscopic inspection of the fractured surfaces. Interactions of weld geometry, material properties, presence/absence of porosity, and level of applied stress are considered in this study, which includes nine different finite element models of titanium and austenitic steel joints. A relevant complication in the analysis of titanium joints is represented by the small thickness and stress concentration effects. Numerical results show that the stress field in titanium joints is sensitive to the combined effect of weld seam geometry and presence of welding defects (pores). The latter produces stress concentration much more severe than that caused by the weld cord. Microstructural support length evaluated near the pores of titanium joints is about the same as that computed near the notch of austenitic steel joints (i.e. near 0). Furthermore, the Topper parameter evaluated for the titanium joints in the presence of pores is much smaller than for the stainless steel joints. Microstructure modifications must be accounted for in order to assess correctly the fatigue behaviour of austenitic steel joints.

Manufacturing and Characterization of 18Ni Marage 300 Lattice Components by Selective Laser Melting

The spreading use of cellular structures brings the need to speed up manufacturing processes without deteriorating mechanical properties. By using Selective Laser Melting (SLM) to produce cellular structures, the designer has total freedom in defining part geometry and manufacturing is simplified. The paper investigates the suitability of Selective Laser Melting for manufacturing steel cellular lattice structures with characteristic dimensions in the micrometer range. Alternative lattice topologies including reinforcing bars in the vertical direction also are considered. The selected lattice structure topology is shown to be superior over other lattice structure designs considered in literature. Compression tests are carried out in order to evaluate mechanical strength of lattice strut specimens made via SLM. Compressive behavior of samples also is simulated by finite element analysis and numerical results are compared with experimental data in order to assess the constitutive behavior of the lattice structure designs considered in this study. Experimental data show that it is possible to build samples of relative density in the 0.2456–0.4367 range. Compressive strength changes almost linearly with respect to relative density, which in turns depends linearly on the number of vertical reinforces. Specific strength increases with cell and strut edge size. Numerical simulations confirm the plastic nature of the instability phenomena that leads the cellular structures to collapse under compression loading.

Hybrid thermography and acoustic emission testing of fatigue crack propagation in Aluminum Samples

In this paper the crack propagation process was monitored by two different experimental techniques. Acoustic emissions sensors were placed on the sample in order to monitor the evolution of the acoustical events during the test; at the same time the change in temperature was monitored by thermography. Test were run on aluminum samples (Al 5068). Specimens were previously cracked, by cutting notches having known sizes and geometry. Successively, X-Ray diffractometry analysis were performed in order to establish the given initial stress state of each sample. Specimen were then subjected to mechanical tests. During these tests the crack propagation was continuously monitored and recorded by both techniques. Data obtained, in terms of number of hits, amplitude signals and maps’ temperature, were critically compared in order to assess the capability of each technique in following the evolution of the damage process.

Remarks on Residual Stress Measurement by Hole-Drilling and Electronic Speckle Pattern Interferometry

Hole drilling is the most widespread method for measuring residual stress. It is based on the principle that drilling a hole in the material causes a local stress relaxation; the initial residual stress can be calculated by measuring strain in correspondence with each drill depth. Recently optical techniques were introduced to measure strain; in this case, the accuracy of the final results depends, among other factors, on the proper choice of the area of analysis. Deformations are in fact analyzed within an annulus determined by two parameters: the internal and the external radius. In this paper, the influence of the choice of the area of analysis was analysed. A known stress field was introduced on a Ti grade 5 sample and then the stress was measured in correspondence with different values of the internal and the external radius of analysis; results were finally compared with the expected theoretical value.

Calibration of Barkhausen Noise for Residual Stress Measurement

Industry has been searching for ways to measure residual stresses accurately, quickly and easily without damage to the material being tested. Several methods have been developed; however, these are either destructive or of limited capability. A less conventional approach, the magnetic Barkhausen noise (MBN) method, is of particular interest because of its potential as a non-destructive industrial tool to measure residual stress and other microstructural parameters. The purpose of this study is evaluation of the relation between residual stress and magnetic Barkhausen noise in Fe360 Steel specimens. A MBN-stress calibration set-up was developed. To control the accuracy and the effectiveness of the developed system and procedure, various MBN measurements were carried out; hardness measurements were also conducted.