(2) INTERNATIONAL APPLICATION PUBLISHED USDER THE PATENT COOPERATION TREATY(FCT)
`(1) Werld tnteHectual Property
`~
`Organization
`international Berea
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` AUEREECEEEn
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`
`
`(43) International Publication Date
`iS September 2614 (£8.09.2014)
`
`WiFOQIPCT
`
`(18) International Publication Number
`WO 2014/138939 Al
`
`(51) Luternational Patent Classification:
`GIS PASI LORY
`GOI L122 (20061119
`BIIK26403 (006.073
`GOINISA8 (2006.07)
`GLB FANE (20NI6.O1)
`GOIN 2EAT3 (2053884
`GRIB TAA? 2006.81)
`GOIN 21/03 (2006.01)
`GER 2 LABR CO006 OF)
`
`(21)
`
`iaternational Application Neuiber:
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`PTTAC ARON AAOPS
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`(22) Tuternafianal Ping Dats:
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`13 Marek 2014 (13.03.2014)
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`(25) Pilhag Language:
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`{26) Publication Language:
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`{Ai} Poiority Tata:
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`GLPFR AGE
`
`13 March 213 (13.05.2013)
`
`Raghsh
`
`Eaplish
`
`US
`
`(7) Applicant: QUEEN'S UNIVERSITY AT RESGSTON
`LCACOCAR Kingsten, ON, RPE 3N6 (CAL
`
`(72) daeenter: WEBSTER, Paul, $.. 1.
`Street, Kingston, ON, KID OAS CA}
`
`SMB-138 Ontario
`
`ci)
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`igh)
`
`(34)
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`Agent: SCRIBNER, Stephen, d: Rm 1625, Bosciences
`Complex, Chicen'’s University, Kirngsion, Gniatio RTL 3NG
`(CA).
`
`
`Designated States funlesyS cnherwise indicated, for every
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`wo2014/138939AtTAMIAMIceth
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`(34) ‘Bide: METHODS ANT SYSTEMS FOR CHARACTERIZING LASER MACHINING PROPERTIES TY MEASURING
`KEYHOLE DYNAMICS USING INTERFEROMETRY
`
`system are provided ty
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`{&7) Abstract: A method.
`
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`mentor and ohare
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`

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`Wo 2014/138939 ATTA EEE TETI
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`TR), OAPT ERP, BY, CEL OG, COM, GA, GN, (0), GW, Pabtished:
`KM, ML, ME, NELSN, TD, TO).
`— sath buernational search rene (tee 2031
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`

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`WO 2014/138939
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`PCT/CA2014/000273
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`Metheds and Systems for Characterizing Laser Machining Properties by Measuring
`
`Keyhole Dynamics Using Interferometry
`
`Field
`
`This invention relates to Imaging using interferometry, including low-eaherence
`
`interferometry, and to optical modification or measurement of materials, such as through the
`
`use oflasers in processes such as machining and welding.
`
`Background
`
`10
`
`Lasers are knownto be important tools for processing a wide range of materials. In
`particular, lasers are very well suited to and see wide application for processing of metals,
`polymers, cerarnics, semiconductors, composites and biological tissue. By focusing a laser
`beam, it can be possible to achieve improved precisionof the laser's action in a direction
`transverse to the beam axis. However, localizing the laser's action in the axial direction of
`
`Ge
`
`the beam can be difficult. During processes such as laser welding, a phase change region
`
`(PCR)is created where the material localized to the bonding region changes dynamically
`from solid to a Hquid and/or a gas state and back to a solid again at the completion ofthe
`weld.
`In some cases the material may change multiple times between the various stafes and
`
`20
`
`also interact with other substances present in the weld zone including other solids, liquids and
`gasses. Controlling this phase change region (PCR)is important fo contro! the quality of the
`weld and theoverall productivity of the welding system. The high spatial coherence oflaser
`light allows pood transverse control ofthe welding energy deposition, but thermaldiffusion
`limits the achievable aspect ratio af welded features when the energy is transmitted through
`the material with conduction alone. For higher aspect ratio features, the more dynamic and
`
`25
`
`unstable process of keyhole welding is used to allowthe conversion of aptical to thermal
`
`energyto occur deeper in the material. Here, axial control {depth of the PCR) is even more
`problematic.
`In keyhole welding, the depth of the PCR and the absorption of the laser may
`extend deep into the material (for example, depths from 10 micrometers to tens of
`
`30
`
`millimeters), Here, the beam intensityis sufficient to melt the surface to open a vapor
`channel (also knownas a capillaryor “the keyhole”) which allows the optical beam to
`penetrate deep into the material, Depending on the specific application, the keyhole may be
`narrow (e.g., less than | mm)but several millimeters deep and sustained with the application
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`of aptical power {for example in the range from 1-2 W to 20,000 W or more}. As a resull, the
`light-matter interaction region inside the PCR can be turbulent, unstable and highly
`stochastic. Unfortunately, instability of keyhole formation can lead to internal voids and high
`weld porosityresulting in weld failure, with potential catastrophic consequences. Similarly,
`keyhole instability can result in spatter that contaminates nearby system components,
`complicating the application oflaser welding in systems such as vehiculartransmissions.
`
`Weld qualityverification is usually required, often using expensive ex-situ and destructive
`
`testing. Welding imaging solutions are offered but are limited in their capabilities and
`usually monitor regions either before or after the PCR, to track the weldjoint, or record the
`
`10
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`top surface ofthe cooled weld joint.
`
`Summary
`
`45
`
`20
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`25
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`Some embodiments of the invention involve characterization of morphology, for
`
`example, including one or more of length, width, depth, size, shape, and aspect ratio ofthe
`keyhole and surrounding material over time bydirecting an interferometry measurement
`beam (including, for exarnple, a low-coherence interferometry measurement beam) into the
`PCR and surrounding area. The beam may be moved along an x- or y-axis and/or 0/¢ (Le.,
`thetaphi, angle may change from normal).
`
`According ta one aspect of the invention, there is provided an apparatus comprising:
`an imaging aptical source that produces imaging light that is applied to a material processing
`system, wherein the material processing system implements a material modification process
`and creates a phase change region (PCR) in a material: at least one element that directs the
`
`imaging light at a plurality of imaging beam positions proximate the PCR; at least one input-
`output port that outputs a first component of the imaging light to an optical access port of the
`material processing system and that receives a reflection component of the imaging light; an
`optical combiner that combines the reflection component and at least another component of
`the imaging light to produce an interferometry output, the interferometry output based on a
`path length taken by the first component andthe reflection component compared to a path
`length taken bythe at least another component of the imaging light; and an interferometry
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`30
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`output processor that processes the interferometry output to determine at least one
`
`characteristic of the PCR.
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`In sarne embodiments the apparatus may further comprise a material processing beam
`
`source that produces a material processing beam that is applied to the material in the maternal
`
`modification process, wherein the material processing bearn creates the PCR in the material.
`
`According to another aspect ofthe invention, there is provided an apparatus for
`
`modifying a sample, the apparatus comprising: a material processing beam source that
`
`produces a material processing beam that is applied to the sample at a sample location in a
`
`material modification process wherein the material processing beamcreates a phase change
`
`region (PCR)in the sample; an imaging optical source that produces imaging lightthat is
`
`applied at a plurality of imaging beam positions proximate the PCR G.c., m the vicinity ofthe
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`19
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`PCR and/or within the PCR); an opheal interferometer that produces an interferometry output
`
`for each imaging bearpasition using at least a component of the imaging light that is
`
`delivered to the sample, the interferometry output based on at least one optical path length to
`
`the sample compared to another optical path length; and an interferometry output processor
`
`that processes the interferometry outputs to determine at least one characteristic of the PCR.
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`35
`
`According ta another aspect of the invention,there is provided an apparatus for use
`
`with a material processing system that implements a material modification process and
`
`creates a phase change region (PCR) in a material, the material processing system having an
`
`optical access port, the apparatus comprising: an imaging optical source that produces
`
`imaging light that is applied at a plurality of imaging beam positions proximate the PCR;at
`
`20
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`least one input-output part that outputs a frst component of the imaging light to the optical
`
`access port af the material processing system and that receives a reflection component of the
`
`imaging light an optical combiner that combinesthe reflection component andat least
`
`gnother component of the imaging light to produce an interferometry output, the
`
`interferometry output based on a path length taken by the first component and the reflection
`
`28
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`component compared to a path length taken by the at least another component of the imaging
`
`light; and an interferometry output processor that processes the interferometryoutputs to
`
`determine at least one characteristic of the PCR.
`
`According te another aspect of the invention,there is provided a method comprising:
`
`applying an imaging light to a material processing system, wherein the material processing
`
`30
`
`system implements a material modification process and creates a phase change region (PCR)
`
`in a material; using at least one elementto direct the imaging lightat a plurality of imaging
`
`beam positions proximate the PCR; outputting a first component of the imaging hght to an
`
`optical access port ofthe material processing system and receiving a reflection component of
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`the imaging light; combining the reflection component and at least another component ofthe
`
`imaging light to produce an interferometry output, the interferometry output based on a path
`
`length taken bythe first component andthe reflection component comparedto a path length
`
`taken by the at least another component of the imaginglight; and processing the
`
`interferometry output to determine at least one characteristic of the PCR.
`
`in some embodiments, the method mayfurther comprise applying a material
`
`processing beam to the material in the material modification process, wherein the material
`
`processing beam creates the PCR in the material,
`
`According to another aspect of the invention, there is provided a method for
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`10
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`modifying a sample, the apparatus comprising: producing a material processing beam thatis
`
`applied to a sample at a sample location in a material modification process wherein the
`
`material processing beam creates a phase change region (PCR)in the sample; producing
`
`imaging light that is applied at a plurality of imaging beam positions proximate the PCR:
`
`producing an interferometry output for each imaging beam positionusing at least a
`
`a5
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`component of the imaging light that is delivered to the sample, the interferometry output
`
`based on at least one optical path length to the sample compared to another optical path
`>
`length; and processing the interferometry outputs to determine at least one characteristic of
`
`the PCR,
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`20
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`Brief Description of the Drawings
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`Embodiments will be described below, by wayof example, with reference te the
`
`accompanying drawitigs, wherein:
`
`Fig. I is a cross section diagram of a material welding process featuring keyhole
`
`imaging in accordance with an embodimentof the javention;
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`25
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`Fig. 2 is a schematic diagram of an apparatus that implements keyhole imaging in a
`
`material welding process, according to one embodiment;
`
`Fig. 3 18 a schematic diagram of another apparatus that implements keyhole imaging
`
`in a material welding process, according to another ermbodiment, similar to the apparatus
`
`used to generate the imagesin Figs. 4A-4E:
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`30
`
`Figs. 4A-4Edepict experimental keyhole imaging image data obtained during
`
`welding using a 1.1 kWlaser on the sample, a 200 um welding spot, a ~70 am imaging spot,
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`at 20 mins, and imaging sample rate of 100 kHz, wherein the imaging beam was leading
`(Fig. 4A), aligned with (Fig. 4B), or trailing (Figs. 4C, 4D, 45) the processing beam:
`
`Fig. 5 shows an example of an image ofa laser spot weld;
`
`Fig. 6 shows an example of a system with an adjustable delay line in the reference
`
`ann;
`
`Pig. 7 shows an example of a system with separate objectives for the processing beam
`
`and the image beam:
`
`Figs. 8 and 9 showtwo example interferometry systems;
`
`Fig. 10 showstwo images of lap welding with digitally tracked keyhole floors, further
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`16
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`showing examples of keyhole instability;
`
`Fig. 1] shows experimental interferometry data from the PCR ofa laser weld at a
`
`plurality ofpositions ranging from in front of the processing beam to behind the processing
`
`beam
`
`Fig, 12 shows experimental interferometry data from the PCR of a laser weld ata
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`15
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`plurality of positions ranging from the left to the right of the processing beam;
`
`Figs. 13A-13Dare coherent images of keyhole laser welding with the imaging beam
`
`aligned ahead of or behind the processing bearn: and
`
`Figs. 14.4 and 14B are schematic diagrams of further embodiments of an apparatus
`
`that implements keyhole imaging in a material welding process, using a pre-objective scanner
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`20
`
`(Fig. 14A) or a past-objective scanner (Fig. 14B).
`
`Detailed Description
`
`in all embodiments described herein, a material modification bear, also referred fo as
`
`a material processing beam, is used. Examples ofa material processing beam include a laser
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`25
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`beam, an electron (or ather particle) beam, plasma beam, electric arc, or water jet. Auxiliary
`
`laser beamis and combinations of these {e.g., a laser beam guided by a water jet, hybrid laser
`
`are welding) are also encompassed. Thus, whereas most embodiments are described as using
`
`a laser beam, it will be understood that the invention is not linuted thereto.
`a5
`en
`As used herein, the terms “keyhole”,
`
`“capillary”, and “vapour channel”are considered
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`30
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`to be equivalent and are mtended to refer to the gaseous cavitythai exists in a phase change
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`region of a material during a material modification process using a material modification
`
`beam.
`
`Fig. 1 is a cross section diagram of a typical material welding process featuring
`
`coherent imaging in accordance with an embodiment of the invention. Two metal samples 12
`
`and 14 are to be joined together in a continuous welding (CW), keyhole welding laser
`
`process. The laser beam [6 is moved across the surface in the direction indicated by arrow
`
`V7.
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`The PCR (phase change region} comprises a liquid region 32, a gas or keyhole region
`
`30, and a bonded solid region 34, the solid having been reformed from the ather two states.
`
`10
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`In general, if keyhole welding is occurring successtully, there will be three phases, as
`
`depicted mm Fig. 1. However, in some embodiments, the apparatus and methodare used to
`
`detect the lack of keyhole formation, in which case there maybe only liquid and solid states,
`
`or onlya solid state.
`
`A plurality of imaging beams 20(herein depicted as 20a through 201) are introduced
`
`IS
`
`at multiple points and/or at multiple incident angles in, and optionally near, the PCR. In the
`
`specific example depicted, there are seven beams 20a, 20b, 20c, 20d, 20e, 20f, 20g that are
`
`substantially normal to samples, and two beams 201, 20h that have incident angles that are not
`
`normal to the samples. The imaging beams 20 are used to generate measurements using low-
`
`coherence interferometry at each ofthe multiple points and/or nnultiple incident angles.
`
`20
`
`While Fig. 1 shows a specific plurality of maging beams 20 introducedat a specific set of
`
`points and incident angles, more generally, a plurality of measurements at some set of
`
`imaging beam positions are taken. The multiple imaging beam positions may involve one or
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`9 combination of
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`ene er more static beams;
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`25
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`ane or more beams that are moved:
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`ene or more beams normal to the sample Incation:
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`one or more beams whose angle is changed;
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`one or more beams that are moved and whase angles are changed: and
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`beams that originate from one ar multiple light sources, including a light
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`30
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`source that is multiplexed to produce multiple outputs.
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`In some embodiments, ane of the plurality of maging beam positions 1s created by
`
`the multiple interna! reflection of an imaging beam inside of an optical element (which may
`
`also be referred to herein simply as an “optic”} that the imaging light interacts with inside of
`
`the beam delivery system. This multiple reflection introduces additional optical path length
`
`(thus shifting the location ofthe reflection to a depth in the image and allowing it tobe
`
`distinguished framthe another beam measuring something else such as the keyhole depth)
`
`and a transverse shift of the focus of the beam. This allows for convenient simultaneous
`
`measurement of the top surface reference point(s) (TSRP} and weld depth. Top surface
`
`reference points are discussed tn further detail below. An image showing such sinvaltancous
`
`id
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`measurement capability is shown in Fig. 4B. The reflection showing the TSRPis located at
`
`an indicated depth of approximately 630 pm.
`
`Interferometry/Coherent Imaging Implementation
`
`Each ofa plurality of imaging beams (e.g., beams 20a-201) originates from a sermi-
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`15
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`coherentlight source, although as described above multiple beams may originate from a
`
`single light saurce. A very specific example of this type of light source is a superluminescent
`
`diode with a spectrum ranging from $20-860 nm and outpet power of 20 mW coupled into a
`
`single mode optical fiber such as Corning HI780. Light sources meeting these criteria are
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`commercially available and manufactured by Saperlam Diodes Inc. (Ireland) and other
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`20
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`manufacturers. The bearn from the light source is carried, directed and manipulated through
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`various media and components that might include fiber optic cables, air (or other gases),
`
`mirrors (or semi-reflective mirrors), lenses, or other optics. The fiber optic cables can be of
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`the single-mode, multimode, and/or polarization maintaining types. The light source beam is
`
`split into two or more beams, for example using a semi-reflective mirror. One beam known
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`25
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`as the imaging beam ar sample beam is directed towards the sample; each ofthe beams
`
`depicted in the figure as 20a to 201 is such a sample beam. Another beam knownas the
`
`reference beam is reflected offa reference surface (e.g., a murar). The sample beam and the
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`reference beam are then aptically recombined, for example bythe same semi-reflective
`
`mirror, so that they create and interference pattern, While a Michelson-style interferometer
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`30
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`was just desertbed, other interferometer configurations such as Mach-Zehnder (including the
`
`use of aptical circulators), Sagnac, and common-path may also be applied in some
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`embodiments. The interference pattern,7A), will vary depending on the path length of the
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`reflected imaging beamrelative to the path length of the reference beam, Ac, according to the
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`relationship 1(k)= AU®)enne +!aan+ TooneToe
`interferometry patterns are then captered and digitized asing a commercially avaiable
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`Joos(2kAs)} . These
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`spectrometer and camera such as the DeepView™ NIRspectrometer (BaySpec, Inc. San
`
`Jose, USA). Additional established optical coherence tomagraphy techniques and those from
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`inline coherent imaging are then used to calculate depth relative to a knownreference
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`position. Specific examples of interferometry systems will be described belowwith reference
`
`to Figs. 8 and 9,
`
`Caloulation of Keyhole/PCR Characteristics and/or Parameters
`
`40
`
`The following are examples ofmethods that may be used calculate keyhole PCR
`
`characteristics and/or parameters. A reference position(s) is established using pomts on the
`
`sample surface identified to be TSRPs.
`
`In the case where the sample is substantially flat, at
`
`least one TSRP can be used to define a top surface reference plane. Additional top surface
`
`reference points can he determinedbased an the top surface reference plane without taking
`
`45
`
`corresponding additional measurements. Alternatively, mvultiple top surface reference points
`
`are used to calculate depth of the process.
`
`The reference position, such as the TSRP, may be set, measured, or calibrated before,
`
`during, or after the welding process. This may be achieved by taking a baseline depth
`
`measurement or measurementsat locations on the sample unaffected by the welding process,
`
`20
`
`such as the location illuminated by beam 20¢ in Fig. 1. The TSRP can also be defined in real
`
`time bysimaltancously imaging the top surface and keyhole bottom either through the use of
`
`multiple imaging channels or by enlarging the imaging spot to simultaneously or dynamically
`
`(.c., sometimes the top, sometimes the bottom based on keyhole oscillations) cover both
`
`locations.
`
`In the simplest case, the TSRP can be determined by taking one er more
`
`measurements of the material immediately before the weld begins. [f the material is
`
`sufficiently flat relative to the weld motion, then this initial measurement can define the
`
`TSRE for the rest of the weld. In other cases the TSRP is mechanically fixed at a spectfic
`
`distance or may be measured using other standard electrical, mechanical, or aptical means.
`
`An example of this would be a beam delivery system that rolls across the workpiece(s).
`
`In
`
`30
`
`this case, the virgin material surface would be a known distance away from the welding
`
`optics that is directlyrelated to the distance between the unit’s wheel{s) and the optics.
`
`Another example would be a welding system that utilizes a fixture or clarnpto hold the
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`workprece(s). Again, since the distance between the optics and the fixture is known, the
`
`distance between the optics and the virgin surface of the material is known.
`
`The imaging beams, such as beams 20 of Fig. |, are used to measure, instantancously
`
`and/or over a period of time, one or a combination of two or more of keyhole length, width,
`
`depth, surface shape, sub-surface profile, wall slope, collapse, instability, undercut, and other
`
`physical parameters of the PCR. Specific example methods of calculating each of these
`
`values will now be described. More generally, what constitutes length, width, depth, surface
`
`shape, sub-surface profile, wall slope, collapse, instability, undercut, or the other physical
`
`parameters of the PCR can be defined on an implementation-specific basis.
`
`10
`
`A single depth measurement is generally defined as the distance below the TSRP
`
`measured by the imaging beam. Note that depth can be a negative value if the measurement
`
`is above the TSRP.
`
`For the following examples, the imaging beams are normal or close to normal to the
`
`sample surface.
`
`15
`
`Keyhole Depth - Keyhole depth for any instant in time is generally defined as the deepest
`
`point of the keyhole. This maybe, for example, by taking multiple depth measuremenis
`
`within the keyhole and taking the maximum ofthese readings, Because the keyhole changes
`
`over tine, In some embodiments, readings are taken in succession to determine how
`
`maxinium depth changes over time. In practice, due to material properties and depth
`
`20
`
`accuracy required, only a limited number of measurements in both position and time may be
`
`necessary.
`
`In other cases, a large number of measurements locations and/or measurement at
`
`high speed may be performed.
`
`Location ofMaximum Keyhole Depth ~ The location of maximum keyhole depth is the
`
`location at anyinstance in time from whichthe keyhole depth value is determined (i-c., the
`
`25
`
`deepest location}.
`
`Average Keyhole Depth - The average keyhole depth is determined by taking the average of
`
`the individual keyhole depth values over some period of time. Otherstatistical techniques
`
`{e.gstandard deviation, median, min/max thresholds, higher order moments} mayalso be
`
`applied. Suchstatistical techniques can be used as direct indicators of weld stability, the
`
`30
`
`probability of defects and therefore quality. Statistical snapshots of weld regions produced
`
`by image processors may also be used by feedback/process cantrollers ta trigger
`
`annunciations and effect changes to weld parameters.
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`Kevhale Location ~ the relative positions of the leading edge, trailing edge, left side and night
`
`side ofthe keyhole relative to the processing (e.g., laser} beam.
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`Kevhale Length - Keyhole length is determined, for example, by calculating the furthest
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`distance between two measurement bearn readings that are below theTSRP and aligned with
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`the axis of laser motion. For example, in Fig. 1, the keyhole length might be defined bythe
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`distance between measurement bears 70a and 20d.
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`Keyhole Width - Keyhole width maybe similarly defined but with readings aligned
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`perpendicularto theaxis of laser motion,
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`Kevhole Surface Shape - The left and right side widths of the keyhole as measured relative to
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`the processing laser at various points along the length ofthe keyhole.
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`Subsurface Keyhole Length and Widrh- Subsurface keyhole length and/or width can also be
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`determined by calculating the length and/or width values relative to a plane af a
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`predetermined distance below the TSRP.
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`Sevhole Prafile ~ The depth of the keyhole measured at various points along the length ofthe
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`15
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`keyhole,
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`Keyhaie Wail Slape - Wail slope may be determined by calculating the slope ofa linc that fits
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`two or more points on the wall ofthe keyhole. For example a line joining depth points 20a
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`and 20b will give the slope ofthe front wall of the key hole. Similarly back and side wall
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`slopes can be calculated.
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`Kevkole Collapse - Keyhole collapse can be determined if successive readings of keyhole
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`depth temporarily or intermittently fail io meet or exceed some specified value.
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`Keyhole Instability ~ Keyhole instability can be determinedfrom the variability of successive
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`keyhole depth readings.
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`Other calculations using coherent imaging mayalso be performed.
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`25
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`All ofthe examples above rely on imaging beams that are normalor substantially
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`normal to the plane of the sample surface.
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`In some cases it may be advantageousto take
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`coherent imaging readings at angles that are not normal to the planeof the sample surface.
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`Readings from imaging beams 20h and 20) of Fig.
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`| would be examples of this. For
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`example, in some embodiments, these off-normal imaging beams are used to determine or
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`ao
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`contribute to the determination of one or more of wall instability, partial keyhole collapse or
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`situations that could Jead to voids or porosity in a welding process. Particularly at high
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`welding speeds at deeper depths, the keyhole vapour channel may underext some ofthe
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`liquid (towards the trailing edge of the weld) such that there is not a direct optical path to the
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`bottomof the keyhole that is also normal te the material surface. This situation is particularly
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`vulnerable to unstable pathological behaviour and may be detected by camparing signals
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`from imaging bears 20b and 20h (or one similarly angled to reach the bottom ofa undercut
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`keyhole).
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`The dynamics oftheliquid region of the PCR are examined. This can be done, for
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`example, by taking multiple maging beam measurements in and around where the Hquid
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`phase region is expected to be located. For the example of Fig. 1, multiple imaging beam
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`10
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`measurements near imaging beam 20 may be used to look at the slope, changes, waves, or
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`other characteristics of the liquid.
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`in some embodiments,the interface between the liquid/solid region of the PCR is
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`located using the measurements.
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`in a specific example, multiple imaging beam
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`measurements are taken in and around where the interface is expected to be located (for
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`example in and around the location of beams 20e, 20f of Fig. 1). The liquid will oscillate and
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`detectably change its position whereas the solid region will be static, thas producing
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`measurable contrast between the two phases,
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`In some embodiments, waves are excited and generated in the liquid region ofthe
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`PCR using acoustic and/or optical energy source techniques to assist with generating imaging
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`contrast and understanding PCR geometry, dynamics, and characteristics such as viscosity.
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`For example, an acoustic vibration may be excited in the liquid at a frequency that is smaller
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`than the imaging sample rate. An imaging beam observing such a liquid region would be
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`able to measure the phase and amplitude ofthe geometric distortion that follows the acoustic
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`vibration, thereby confirming the liquid state of the point being imaged.
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`25
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`in some embodiments of the invention, at least one of the plurality of imaging beams
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`positions is outside the PCR. Beams 20f and 20g are examples of this in Fig. 1.
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`ia some embodiments of the invention, light is applied to at least two ofthe plurality
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`of imaging beams positions simultaneously, The multiple imaging beams can be generated in
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`this case using multiple beam sources, or by using a single beam source and one or more
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`splitters.
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`In some embodiments of the invention, light is applied to at least twa of the plurality
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`of imaging beam positions sequentially. The sequentially applied imaging beams can be
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`generated using multiple beam sources that are activated in sequence, or by using a single
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`beam source that is reconfigured to prodace each of the beams in sequence.
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`In same embodiments of the invention, the plurality of imaging beam positions are
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`achieved by changing the position and/or angle ofat least one imaging beam relative to the
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`processing beam during the welding process.
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`in some embodiments of the invention, the number of positions where the plurality of
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`imaging heams is applied to the sample is changed during the welding process.
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`In sorne embodiments of the invention, at least one of the plurality of imaging beam
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`positions does not have an incident position that is on a line formed by the material
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`10
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`processing beam. For example, in Fig. 1, laser beam 16 moves in direction 17 and traces out
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`apath. One or more of the imaging beams can be applied off this path. This can be used, for
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`example, to determine keyhole width.
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`In some embodiments ofthe invention, imaging light applied to at least one ofthe
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`plurality of imaging beamspositions is used to determine the width or diameter of the
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`45
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`keyhole when viewedfromthe same direction as the material processing {e.g., laser} beamis
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`applied,
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`In seme embodiments of the invention, imaging light applied to at least one of the
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`plurality of imaging bearn positions is focused to a diameter that is smaller than the diameter
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`of the laser beam.
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`20
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`In some embodiments of the invention, imaging light applied to at least one of the
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`plurality of imaging bearn positions is focused to a diameter that is similar to the diameter of
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`the laser beam.
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`In some embodiments ofthe invention, imaging light applied to at least one of the
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`plurality of imaging beam positions is focused to a diameter that is larger than the diameter of
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`25
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`the laser beam.
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`In some embodiments ofthe invention, imaging light applied to at least one ofthe
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`plurality of imaging beam positions is focused to a diameter that encompasses the PCR.
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`In some embodiments of the invention, imaging light applied toat least one ofthe
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`plurality of imaging beam positions is focused to a diameter that is larger than the PCR.
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`in same embodiments of the invention, imaging light applied to at least one of the
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`plurality of imaging beam positions is used to take successive readings at a frequency of
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`approximately 10 Hz or more.
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`In some embodiment