U.S. patent application number 12/953726 was filed with the patent office on 2011-06-02 for method for manufacturing massive components made of intermetallic materials.
This patent application is currently assigned to AVIO S.p.A.. Invention is credited to Diego Basset, Silvano Rech, Andrea Trentin, Simone Vezzu', Giovanni Paolo Zanon.
Application Number | 20110129379 12/953726 |
Document ID | / |
Family ID | 42557282 |
Filed Date | 2011-06-02 |
United States Patent
Application |
20110129379 |
Kind Code |
A1 |
Zanon; Giovanni Paolo ; et
al. |
June 2, 2011 |
METHOD FOR MANUFACTURING MASSIVE COMPONENTS MADE OF INTERMETALLIC
MATERIALS
Abstract
The invention relates to a method for manufacturing a massive
component substantially wholly made of intermetallic material,
comprising the steps of: a) preparing a mixture of powders of at,
least two metallic elements, the powders being present in the
mixture in a proportion by weight corresponding to the atomic
percentage in which the at least two metallic elements are present
in a corresponding intermetallic compound which may be formed from
these metallic elements; b) applying a plurality of layers of such
a mixture of powders by cold spraying on a substrate so as to
obtain, on the substrate, a preform of metallic mixture of
predetermined thickness; c) thermally treating at least the preform
of metallic mixture so as to cause the reaction between the at
least two metallic elements to form the corresponding intermetallic
compound; and d) removing the substrate, thus obtaining the massive
component made of intermetallic material.
Inventors: |
Zanon; Giovanni Paolo;
(Rivalta Di Torino, IT) ; Vezzu'; Simone; (Padova,
IT) ; Rech; Silvano; (Seren Del Grappa, IT) ;
Trentin; Andrea; (Valdagno, IT) ; Basset; Diego;
(Mareno Di Piave, IT) |
Assignee: |
AVIO S.p.A.
|
Family ID: |
42557282 |
Appl. No.: |
12/953726 |
Filed: |
November 24, 2010 |
Current U.S.
Class: |
419/23 ; 419/26;
419/38; 420/588 |
Current CPC
Class: |
C23C 24/04 20130101 |
Class at
Publication: |
419/23 ; 419/38;
419/26; 420/588 |
International
Class: |
C22C 1/04 20060101
C22C001/04; B22F 3/24 20060101 B22F003/24; C22C 30/00 20060101
C22C030/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 24, 2009 |
IT |
TO2009A000908 |
Claims
1. A method for manufacturing a massive component substantially
wholly made of intermetallic material, comprising the steps of: a)
preparing a mixture of powders of at least two metallic elements,
the powders being present in the mixture in proportion by weight
corresponding to the atomic percentage in which said at least two
metallic elements are present in a corresponding intermetallic
compound which may be formed from these metallic elements; b)
applying a plurality of layers of such a mixture of powders by cold
spraying on a substrate (S) so as to obtain, on the substrate (S),
a preform of metallic mixture of predetermined size; c) thermally
treating at least the preform of metallic mixture so as to cause
the reaction between said metallic elements to form said
intermetallic compound; and d) removing said substrate (S), thus
obtaining the massive component made of intermetallic material.
2. A method according to claim 1, characterized in that said step
d) of removing is carried out before said step c) of thermal
treating and by separating said preform of metallic mixture from
said substrate (S), said massive component made of intermetallic
material being obtained in step c) from said preform of metallic
mixture.
3. A method according to claim 1, characterized in that said step
d) of removing is carried out after said step c) of thermal
treating, in step c) thus obtaining a massive preform of
intermetallic material, said massive component made of
intermetallic material being obtained in step d) from said massive
preform of intermetallic material.
4. A method according to claim 1, characterized in that said
substrate is made of a material which does not react with said at
least two metallic elements.
5. A method according to claim 1, characterized in that said
substrate is at least partially coated with an insulating release
layer.
6. A method according to claim 1, characterized in that said step
of thermal treating is carried out at a temperature essentially
close to the eutectic or peritectic temperature related to said
intermetallic compound.
7. A method according to claim 1, characterized in that said step
b) of applying is carried out at a temperature of the particles
which is lower than half the melting temperature of the
lowest-melting metallic element of said mixture of powders.
8. A method according to claim 1, characterized in that said
mixture of powders consists of particles having an average size
from 1 to 200 .mu.m.
9. A method according to claim 1, characterized in that said
mixture of powders consists of particles having an average size
from 1 to 50 .mu.m.
10. A method according to claim 1, characterized in that it
comprises, between said step b) of applying and said step c) of
thermal treating, a step e) of stress relieving treatment at a
temperature such as to not trigger the reaction of formation of the
intermetallic compound.
11. A method according to claim 1, characterized in that said step
a) of preparing a mixture of powders of at least two metallic
elements is carried out by mixing the at least two elements either
pure or alloyed or in the form of coated powders.
12. A method according to claim 1, characterized in that said step
b) of applying by cold spraying is carried out using a carrier gas
selected from the group consisting of helium, neon, argon,
nitrogen, oxygen, air or any mixture thereof in any proportion.
13. A method according to claim 1, characterized in that said step
c) of thermal treating is carried out in an oven or by means of
other means of delivering thermal energy.
14. A massive component made of intermetallic material obtainable
by the method according to claim 1.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the priority of Italian Patent
Application No. TO2009A 000908, filed on Nov. 24, 2009, the
entirety of which is hereby incorporated by reference.
[0002] The present invention relates to a method for manufacturing
massive components made of intermetallic materials, particularly
for aerospace applications.
BACKGROUND OF THE INVENTION
[0003] The materials used in aeronautic propulsion systems should
have features of high mechanical strength and temperature creep
resistance, fatigue resistance, oxidation and corrosion resistance,
lightness and structural stability.
[0004] Traditionally, many materials are already used, and are
differentiated according to composition and use requirements, but
research and development activities continue for overcoming the
limits posed by current technologies and for identifying new
materials with increasingly better features.
[0005] A new class of materials having the features required for
use in aeronautic engines is the intermetallic materials.
[0006] In recent years, the interest towards intermetallic
materials has increased, especially due to the high weight saving
potential which may be obtained by virtue of low density and high
specific strength. The intrinsic fragility of this type of
materials requires, in all cases, to set up particularly
sophisticated manufacturing and inspection techniques and to
implement important study and characterization campaigns to learn
about the properties on statistically significant bases.
[0007] The most interesting intermetallic materials belong to the
TiAl (titanium aluminides) family, useable instead of Nickel- or
Cobalt-based superalloys up to temperatures close to 700.degree.
C., but other interesting intermetallic materials for the
aeronautic field are those based on NiAl and Ni3Al, or FeAl
systems.
[0008] Several uses of the intermetallic materials are known:
intermetallic materials as precipitating phases, which are the
reinforcing element of metallic alloys, and which allow to improve
them by means of thermal treatments, or intermetallic materials as
anti-wear, anti-oxidation or anti-corrosion coating systems.
[0009] Methods are known, e.g. from WO2006/109956, for the
preparation of a metal matrix composite material wherein
intermetallic compounds and ceramic powders are dispersed. In
particular, there is disclosed the case where one metal element,
which is intended to form the metal matrix of the composite--be it
a single component or an element of a metal alloy--is made to react
with other metallic particles via cold spray techniques. A much
desirable dispersion reinforcement effect is thereby obtained
through precipitation of the intermetallic compounds thus formed.
However, while metal-matrix ceramic composites where intermetallic
compounds are finely dispersed within the metal-matrix phase can be
obtained by the method of WO2006/109956, no indication is provided
as to how to prepare a massive component substantially wholly made
of an intermetallic material, i.e. one having homogeneous chemical
composition and mechanical properties throughout.
[0010] Known intermetallic mechanical components of massive type
are typically obtained by solidification of mixtures of one or more
metallic elements in molten state. These mixtures contain the
metallic elements in a given proportion by weight, which
characterizes a particular alloy, or corresponds to the
characteristic atomic percentage composition of a given
intermetallic compound, which may be formed from these elements.
The intermetallic compounds are, in general, intermediate phases of
systems with two or more metallic elements (see, for example, the
state diagrams of bi-metallic systems in Figures from 1 to 4) and
are characterized by a crystal lattice, in which points are tidily
occupied by atoms of different metals. Thereby, the aforesaid
metallic mixtures may be cast to form ingots intended to be then
mechanically machined, or directly cast in moulds (ceramic shells)
with investment casting techniques or the like, or even atomized to
obtain powders to be consolidated, in a subsequent step, by massive
sintering or rapid manufacturing techniques (such as Electron Beam
Melting or Laser Sintering), or even by hot spraying.
[0011] The above technologies commonly used for manufacturing
massive components made of intermetallic materials have sensitive
critical states related to the high hardness, fragility and
reactivity of these materials, which significantly limit machining
possibilities by means of plastic deformation, such as forging,
extrusion, rolling, etc., or lost-wax casting as well, which has a
further drawback in the reaction of the intermetallic material with
the ceramic shell.
[0012] Specifically, the techniques based on rapid manufacturing
powder consolidation allow, to a certain extent, to obtain massive
components characterized by high mechanical properties, and low
faults and presence of machining allowances; these manufacturing
procedures are very limited from the point of view of production
capacities and require great system investments, accompanied by
equally significant management and maintenance costs.
[0013] On the other hand, massive components obtained by the
aforesaid direct hot spraying of intermetallic material powders
generally have high internal stresses due to the contraction of the
sprayed material and a high number of faults, both in the form of
diffused porosity and of micro-cracks. These drawbacks decrease the
reliability of this technique and its applicability from the
industrial point of view.
[0014] In the art, there is a need to provide a method for
manufacturing massive components made of intermetallic material
which requires low system investments and low management and
maintenance costs and which is able to ensure a high
productivity.
[0015] Furthermore, in the aeronautic field, there is a need to
provide a method for manufacturing massive components made of
intermetallic material which have micro-structural and mechanical
features such to satisfy the requirements imposed by particular
conditions of use.
SUMMARY OF THE INVENTION
[0016] It is thus the object of the present invention to provide a
method for manufacturing massive components made of intermetallic
material, which allows to simply and cost-effectively satisfy at
least one of the aforesaid needs.
[0017] The aforesaid object is achieved by the present invention as
it relates to a method comprising the steps of:
[0018] a) preparing a mixture of powders of at least two metallic
elements, the powders being present in the mixture in a proportion
by weight corresponding to the atomic percentage in which the at
least two metallic elements are present in a corresponding
intermetallic compound which may be formed from these at least two
metallic elements;
[0019] b) applying a plurality of layers of such a mixture of
powders by cold spraying on a substrate (S) so as to obtain, on the
substrate (S), a preform of metallic mixture of predetermined
size;
[0020] c) thermally treating at least the preform of metallic
mixture so as to cause the reaction between the metallic elements
to form such an intermetallic compound; and
[0021] d) removing the substrate (S), thus obtaining the massive
component made of intermetallic material.
[0022] Furthermore, the present invention relates to a massive
component made of intermetallic material obtainable by the
method.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] For a better understanding of the present invention, a
preferred embodiment will now be described below only by way of
non-limitative example, and with reference to the accompanying
drawings, in which:
[0024] FIG. 1 illustrates the Ti--Al phase diagram;
[0025] FIG. 2 illustrates the Al--Cr phase diagram;
[0026] FIG. 3 illustrates the Fe--Al phase diagram;
[0027] FIG. 4 illustrates the Al--Ni phase diagram;
[0028] FIG. 5 shows a diagrammatic view of a cold spraying
apparatus;
[0029] FIG. 6 diagrammatically shows the steps of the method
according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0030] Figures from 1 to 4 show, by way of non-limitative example,
the phase diagrams of some two-component systems characterized by
the possibility of forming an intermetallic compound.
[0031] It is worth noting that, in all cases, for each
intermetallic compound the formation of which is thermo-dynamically
possible, there is a specific stoichiometric ratio between the
metallic elements which compose the same, which corresponds to a
very precise percentage composition by weight, obtainable by simple
stoichiometric calculations. For example, the Ti--Al intermetallic
compound (see FIG. 1) corresponds to an atomic percentage
composition of 50 atoms of Ti per 50 atoms of Al, which correspond
to 64% by weight of Ti and 36% by weight of Al. Similarly, the
following percentage composition by weight is obtained in the case
of Ti-48Al-2Cr-2Nb (not shown): 32-33.5% by weight Al; 2.4-2.7% by
weight Cr; 4.5-5.1% by weight Nb; the rest (at the most 60% by
weight) Ti.
[0032] Therefore, from the analysis of bi- and multi-metallic
system state diagrams, which intermediate compounds is obtainable
and which compositions, expressed in atomic percentages and/or by
weight, of metallic elements correspond thereto may be easily
assessed.
[0033] According to this analysis, a mixture of powders of at least
two metallic elements is thus prepared, according to the method of
the invention, the metallic powders being present in the mixture in
a proportion by weight corresponding to the atomic percentage in
which said at least two metallic elements are present in a
corresponding intermetallic compound which may be formed from these
metallic elements. Such a mixture of powders may be obtained by
simply mixing pure constituents or by using mixtures in which the
constituent elements are alloyed, or yet again by coated powders
obtained by grinding, or by means of other known methods.
[0034] Given the above, it is worth noting that, in the context of
the present invention, by "a mixture of powders of at least two
metallic elements" reference is made to a mixture of powder
metallic elements which is selectively prepared to be
stoichiometrically and thermodynamically bound to react, under the
operating conditions in accordance with step c) of the method as
outlined above, to form substantially only the intermetallic
compound at issue--if not for the possible formation of traces of
other chemical species.
[0035] In other words, the "mixture of powders of at least two
metallic elements" referred to herein is selectively prepared in
order for the method of the invention to yield a massive component
which is substantially wholly made (if not for the possible
presence of traces of other species, impurities and the like) of
the intermetallic compound at issue, i.e. a massive component which
comprises at least 90% by weight of the intermetallic compound at
issue.
[0036] According to the method of the invention, a plurality of
layers L of the powder mixture is applied (see FIG. 6) by cold
spraying on a substrate S so as to obtain, on the substrate, a
preform of metallic mixture SLM of predetermined size. In
particular, the size of the preform of metallic mixture SLM will be
substantially the same as the massive components CM intended to be
machined, with the exception of variations which may be obtained
upon subsequent mechanical machining.
[0037] The cold spraying technique generally comprises the steps of
injecting the powder mixture into a nozzle and applying the mixture
of powders onto the substrate by accelerating the powder mixture in
non-molten state to a speed of the order of 300-1200 m/s by means
of a flow of carrier gas crossing the nozzle. FIG. 5
diagrammatically shows a cold spraying apparatus 100 for applying a
plurality of layers of metallic powder mixture on the surface of a
substrate S.
[0038] For this purpose, apparatus 100 comprises a compressor 110,
a system 120 for feeding the mixture of powders, and a nozzle 130
for spraying the powder itself.
[0039] Furthermore, apparatus 100 may comprise means 140 for
heating the input gaseous flow.
[0040] Being compressed at a pressure higher than about 5 bars, the
gas is put into contact with the constituent particles of the
metallic powder mixture fed by system 120 and draws them by
expelling them through the nozzle 130 at high speed. In order to
generate a subsonic or supersonic flow, as shown in Figure Z, a
converging-diverging nozzle 130 is advantageously used.
[0041] A portion of the gas flow is preferably heated before
reaching nozzle 130.
[0042] The distance between the point where the gas and the
particles come in contact with the base of the nozzle 130 may be
typically varied to modulate the final temperature of the powders
and is comprised in the range from 20 and 200 mm.
[0043] A monatomic inert gas is preferably used as the carrier gas,
such as helium or argon, so as to exclude possible reactions with
the components of the metallic powder mixture and have high gas
speeds by virtue of its y ratio. However, if the contact times
between components of the metallic powder mixture and the carrier
gas are very short, more cost-effective carrier gases may also be
used, such a nitrogen or air. Any proportion of the mixtures of the
previously mentioned gases may be further used, and in particular
nitrogen, air, argon, helium, neon, krypton.
[0044] The temperature at which the gas is heated by the carrier
system 140 is directly related to the final temperature and speed
of the sprayed particles. In cold spraying processes, the gas
temperature is typically between 300 and 1200.degree. C.
[0045] The powders remain in contact with the gas for a very short
time, whereby the temperature of the powders, although not
measurable, never reaches the gas temperature.
[0046] The deposition temperature is typically the lowest possible,
compatibly with the need to obtain a minimum deformation level of
the sprayed powder particles.
[0047] The temperature at which the powders are kept before coming
in contact with the gas at the base of nozzle 130 is sufficiently
low to minimize the possibility, of activating the reaction
mechanisms which could lead to the early formation of the
intermetallic compound; it is worth not exceeding a temperature
equal to half the melting temperature of the lowest-melting
metallic element.
[0048] With regards to the average size of the metallic particles
forming the mixture of powders, this may be advantageously chosen
in the range from 1 to 200 .mu.m, so as to facilitate the
dispersion and mixing in the step of preparing the powder
mixture.
[0049] More preferably, as the powders are subsequently subjected
to a thermal treatment to promote the reaction of forming the
intermetallic compound, the average dimension of the metallic
particles is in the range from 1 to 50 .mu.m, so as to promote a
more uniform, gradual reaction, since such a reaction involves
scattering phenomena at the atomic level.
[0050] Indeed, a yet smaller size would be accompanied by an
excessively light particle weight, and therefore the motion amount
accumulated during the step of cold spraying and discharged when
impacting against the substrate surface, i.e. against a previously
applied layer of metallic powders, would be too small. On the other
hand, if the average size of the particles is too large, the motion
amount being high, the impact frequency and the area concerned by
each impact would be too low.
[0051] The amount of metallic powder mixture deposited on the
substrate will generally be such to form the envelope of the
massive component to be manufactured, taking into account any
possible deformations and volume variations which may be induced
during the thermal treatment and any possible subsequent mechanical
machining operation.
[0052] According to the method of the invention, a substrate made
of a material which allows an easy release of the preform of
metallic mixture at the end of the step of cold spraying may be
advantageously used. For example, a support may be used which does
not react with the metallic elements contained in the powder to be
applyed by cold spraying, such a non-metallic material.
[0053] Alternatively, a (metallic or non-metallic) substrate may be
coated with an insulating release layer. In this context,
"insulating layer" means a layer of material which does not react
with the metallic elements contained in the powder to be applied by
cold spraying, such as for example a layer of an appropriate
releasing agent paint, or even a substrate which is removable by
chemical dissolution, etc.
[0054] Thereby, a step of mechanical releasing (e.g. by abrasion)
is avoided, thus advantageously obtaining a simplification of the
method and a reduction of machining times and costs.
[0055] The substrate typically consists of a material having
sufficient rigidity to withstand the impact of the metallic powder
mixture particles which are sprayed against its surface.
Advantageously, the substrate consists of a mould, substantially
having the complex shape of the massive component to be
manufactured, with the exception of the changes to be obtained upon
subsequent machining operations.
[0056] The method of the invention further comprises a step of
thermally treating the preform of metallic mixture so as to cause
the reaction between the metallic elements to form the
intermetallic compound.
[0057] Such a thermal treatment is advantageously carried out once
the preform of metallic mixture has been released from the
substrate.
[0058] Alternatively, if the substrate is made of a material such
as to withstand the conditions imposed by the thermal treatment and
not react at all, during the thermal treatment itself, with the
deposited metallic elements, the substrate (mould) may be removed
at the end of the whole process of forming the intermetallic
compound.
[0059] The release of the metallic mixture preform (if the release
occurs before the thermal treatment) or of the massive preform made
of intermetallic compound (if the release occurs after the thermal
treatment) from the substrate may occur either manually or
mechanically, by chemical dissolution, etc.
[0060] Alternatively, the release may also be carried out upon
further mechanical machining operations performed before the
thermal treatment, e.g. with the objective of obtaining a specific
complex shape and/or for obtaining a surface finishing.
[0061] The temperature at which the thermal treatment is carried
out depends on the composition of the single intermetallic
compound, and generally depends on the corresponding eutectic or
peritectic temperature.
[0062] In particular, since the metallic particles forming the
powder mixture, upon repeated collisions imposed by the cold
spraying deposition technique, have a high degree of deformation
and, in general, a high concentration of voids and faults, the
reaction of formation of the intermetallic compound may be
thermo-dynamically promoted at temperatures even much lower than
the eutectic or peritectic temperatures shown in the corresponding
phase diagrams.
[0063] However, in order to increase the productivity and reduce
the associated costs, the thermal treatment is carried out at a
temperature substantially close to the eutectic or peritectic
temperature related to the intermetallic compound which is intended
to be formed.
[0064] As previously disclosed, the reason why the thermal
treatment of the invention is preferably carried out close to the
eutectic or peritectic temperature, which characterizes the
specific intermetallic compound, is that, in participle, it is
involved in no liquid phase thermo-dynamically balanced under such
a temperature, and therefore it is adapted to promote obtaining the
intermetallic compound in a highly dispersed form.
[0065] However, in real systems, as the involvement of the liquid
phase at temperatures lightly higher than the eutectic or
peritectic temperature is negligible, the formation of the
intermetallic compound is not substantially influenced as well.
Therefore, the expression "at a temperature substantially close to
eutectic or peritectic temperature" used herein takes into account
this phenomenon, so as to include the whole temperature range
within which the method of the invention does not loose its
efficiency.
[0066] The temperature stay times will generally depend on the
selected temperature. More in particular, the temperature stay
times will be such to allow the inter-scattering of the metallic
elements forming the mixture, therefore they will depend on the
degree of mixing the powders in the mixture.
[0067] Some hours of temperature stay are generally sufficient to
obtain a complete conversion of the metallic mixture into
intermetallic compound.
[0068] From the practical point of view, the treatment times may be
optimized by experimentally evaluating the degree of progress of
the formation of the intermetallic phases by means of
diffractometry.
[0069] Furthermore, the thermal treatment aimed at forming the
intermetallic compound may be advantageously combined with a
thermal treatment aimed at conferring particular micro-structural
features and mechanical properties to the intermetallic compound.
For example, the thermal treatment for forming the intermetallic
material Ti-48Al-2Cr-2Nb may be combined with the typical thermal
treatment of this material, which is carried out at a temperature
of 1205.degree. C. for at least 2 hours, with subsequent cooling at
controlled speed, in order to have an adequate, gamma-phase,
lamellar microstructure.
[0070] The thermal treatment according to the invention may be
carried out in an oven, i.e. using alternative thermal energy
supply means, such as radiation, laser, etc.
[0071] After the step of cold spraying deposition and upstream of
the above-described thermal treatment, the preform of metallic
mixture may be subjected to a thermal, stress-relieving treatment
in advance, having the objective of reducing the entity of internal
stresses introduced by the cold spraying process itself and of
improving the preform ductility, thus facilitating the mechanical
stock removal and finishing and minimizing the risks of breakage.
However, such a stress-relieving treatment should be carried out at
a relatively low temperature in order to avoid the early
intermetallic compound formation reaction.
[0072] The components produced by the method of the invention may
then be subjected to a hot isostatic pressing treatment in order to
reduce the porosity while increasing the material density.
[0073] As compared to the known methods for manufacturing massive
compounds made of intermetallic material, the method of the
invention results in a series of interesting advantages.
[0074] It is indeed known that the most critical aspects related to
the prior art concern high hardness, fragility and reactivity of
the intermetallic materials.
[0075] Despite their fragility, the intermetallic compounds offer
an advantageous compromise between the properties of the ceramic
materials and those of the metallic materials, in particular for
conditions in which high hardness and high temperature resistance
are particularly important. Moreover, some have particular magnetic
and chemical properties which derive from their very tidy structure
and from the nature of the involved bonds.
[0076] With respect to the known machining procedures, which
involve plastic deformation operations, such as forging, extrusion,
rolling, etc., the method of the invention advantageously
suppresses the need to subject the intermetallic material to
plastic deformation because the formation of the intermetallic
compound occurs only by means of the reaction between its
elementary components and, in essence, only once the component has
been definitively processed with regards to forming and surface
finishing.
[0077] Furthermore, as compared to the known solutions based on
using lost-wax casting processes, the possibility of critical
reaction interaction situations at high temperature between the
molten intermetallic material and the ceramic shell is avoided by
limiting the entity of machining allowances needed for removing the
contaminated surface layer. The generation of faults is also
advantageously minimized during the step of solidifying the
intermetallic compound (e.g. micro-shrinkage).
[0078] Finally, as compared to the solutions based on rapid
manufacturing techniques, such as Electron Beam Melting process,
the method of the invention implies very low investment, management
and maintenance costs and allows a significant increase of
productivity.
[0079] It is finally apparent that changes and variations may be
made in the system described and shown, without departing from the
scope of protection of the independent claims.
* * * * *