dc.description.abstract | Laser welding is the logical processing solution to accomplish different needs.
Improvements at the design stage are actually aimed to remove any mechanical
fastening, thus moving towards a technology which would not increase the joint
thickness; moreover, a number of benefits in comparison with conventional welding
methods are provided when considering laser beams, since deep penetration is
achieved and the energy is effectively used where needed, thus melting the
interface to be joined rather than excessively heating up the base metal, which
would suffer from thermal distortion and degradation of metallurgical properties
otherwise.
Further advantages are achieved in laser welding with thin disk sources, since high
output power, high efficiency and good beam quality are simultaneously delivered,
unlike traditional laser systems; costs are significantly reduced in comparison with
lamp-pumped laser systems. As a consequence, specific interest is shown in
aerospace where strict specifications apply.
Nevertheless, a number of issues must be addressed, depending on the material to
be welded, as many variables and sub processes concerning fusion and vaporization
are involved in laser welding and a delicate balance between heating and cooling is
in place within a spatially localized volume. Therefore, extensive studies are
required to manage both the stability and the reproducibility of the overall process,
before introducing any change in industrial environments. Methods, experimental
results and discussions concerning laser welding of common metal alloys for
aerospace are provided in this Ph.D. thesis.
A general view of applications and basic advantages of laser welding is first given,
with mention to diagnostics and safety. Hence, the principles of laser emission are
examined, with respect to the architecture of the sources, beam geometry, quality
and efficiency, in order to better portray the benefits of a thin disk laser concept.
Processing dynamics of laser welding are explained afterward, referring to
conduction and key-hole mode, instability, gas supply and leading governing
parameters such as laser power, welding speed, defocusing and beam angle to be
considered in the experimental work. Procedures are provided for proper bead
characterization, from preliminary examinations including non destructive tests such
as fluorescent penetrant inspections and radiographic tests, to sample preparation
and eventual mechanical assessment in terms of tensile strength and Vickers micro
hardness in the fused zone.
A straightforward description of the design of experiment approach and the
response surface methodology is given, so to introduce the testing method to be
taken, as well as the steps for data elaboration via statistical tools.
Hence, four case studies about metal aerospace alloys are presented and discussed
in their common seam configuration: autogenous butt and overlapping welding of
aluminum alloy 2024; autogenous butt welding of titanium alloy Ti-6Al-4V; dissimilar
butt welding of Haynes 188 and Inconel 718; dissimilar overlapping welding of
Hastelloy X and René 80. All of the welding tests were conducted at the Department
of Industrial Engineering at the University of Salerno; a Trumpf Tru-Disk 2002
Yb:YAG disk-laser source with a BEO D70 focusing optics, moved by an ABB IRB
2004/16 robot was employed. When needed, additional tests for the purpose of
specific bead characterization were conducted by Avio and Europea Microfusioni
Aerospaziali.
As general procedure for each topic, the operating ranges to be examined are found
via preliminary trials in combination with the existing literature on the subject. Then,
special consideration is given to the processing set-up, the resulting bead profile,
possible imperfections, defects and overall features; consistent constraint criteria
for optimization of the responses are chosen on a case-by-case basis depending on
materials and seam geometry and referring to international standards as well as
customer specifications for quality compliance. Optimal combinations of the input
welding parameters for actual industrial applications are eventually suggested,
based on statistical tools of analysis. Convincing reasons are provided to give
grounds to improvements in real applications. Moreover, based on the results, a
proper device for bead shielding, to be conveniently adjusted depending on both
geometry and materials to be welded has been designed, produced and patented
(SA2012A000016).
As concerning aluminum welding, a comprehensive description is given for laserrelated
issues: reflectivity and thermal conductivity influence on the material
response is illustrated; the porosity evolution is discussed with respect to thermal
input and defocusing; a theory for softening in the fused zone is provided through
energy dispersive spectrometry and estimations of magnesium content in the crosssection.
Optimization is performed for butt configuration of 1.25 mm thick sheets;
the discussion about the interactions among the governing factors is deepen with
reference to overlapping welding.
With respect to titanium welding, optimization is performed for 3 mm thick butt
welding; the resulting micro structure in the weld is discussed since it is thought to
be closely related to the mechanical properties. In particular, special care is taken of
the grain size as a function of the governing factors.
Dissimilar welding of super alloys is considered for gas turbine components; for this
specific purpose, laser welding is expected to offer a valid alternative to arc and
electron beam welding, whose weaknesses are pointed out. Given their actual
application in the engine, Haynes 188 and Inconel 718 are examined in butt welding
configuration, whilst an overlapping geometry is preferred for Hastelloy X and René
80. Considerable tolerances are matched, thus promoting the suggested range of
the operating variables. [edited by author] | en_US |