Some technical stuff, for golf-geeks like me. 

Quality Issues-

The companies that sell golf clubs are like car companies... they don't make their own clubs.  They just assemble clubs, made from components supplied by other companies. Most of the brand names, but not all of them, design their own clubheads (just the heads, not the whole clubs), and then have the heads manufactured at various factories around the world. They then match the supplied clubheads up with off-the-shelf shafts and grips, assemble them, and market them under their own brand name. 

All club builders, from the smallest hobby-class club fitters to the biggest and most famous brand names,  buy standard shafts from more-or-less well-known shaft manufacturers, though they may work with the shaft factory to change the graphics for branding purposes.  Some of the manufacturers claim to have worked closely with the shaft builder to produce a "custom" shaft for their club, but its anybody's guess what that means, after all the claim is being made by the marketing department. Similarly, they buy the grips elsewhere too.

The brand name clubs all use proprietary (patented) clubheads, and they do not allow your custom clubmaker to buy them. But there are literally hundreds of non-proprietary club head models on the market, in a wide range of prices. The non-proprietary heads are available to custom clubfitters and to big and small "off-brand" clubmakers. If the clubmaker is not big enough to have a two-page ad in the major golf magazines, it is highly unlikely that the company designs its own clubheads, regardless of what the graphics on the clubhead imply.

The quality of design and manufacture of club head components varies widely.  

I hope that this doesn't really need saying, but just in case, I'll say it anyhow.  Just because a clubhead "looks" like a high-price Callaway or Ping, doesn't mean it is "as good as" the clubhead it is imitating.  But by the same token, just because a clubhead isn't "from" Mizuno, that does not mean that it is not "just as good".  Some of the stuff in the component market is is truly junk, and a lot of it is lower-quality but acceptable and fairly priced....  but some of it is as good as there is.

Many of the component parts peddlers make a point of bragging that their heads are made in the same factories, on the same machines, and of the same materials as the pricey brand name clubs.  They also claim that most of the extra cost of the brand name clubs is soaked up in marketing expenses. Well, those claims may all be true, but its not quite the whole story.  

Even the same factory is perfectly capable of relaxing their quality standards to achieve lower production costs.  For instance, many of the cheaper lines do not mill the faces of their castings.  So they don't have perfectly flat faces, a fairly serious issue for a serious golfer. And then there's the heat treatment issues....

Heat treatment of metals is a costly process. Heat treatments have a tremendous effect on the metallurgical properties of club head materials, even for the chemically exact same material.  If the marketer does not tell (and pay) the factory to perform specific heat treatments to the club after the casting process, they will save some production costs, but lose some quality. Heat treatments are an exacting and sophisticated process, and not all foundries have the expertise and equipment to do it as precisely as they should. The untreated or poorly treated materials are not going to be as favorable for golfing (if they were, nobody would treat them!).

There are about a dozen factories around the world, all in Asia, capable of making premium quality golf club heads. All of the premium brand-name companies, get their clubs made there, as do the best component companies.  Unfortunately, there are another 60 or so factories also making golf club heads, and the quality ranges from just acceptable to outright junk.

Some cheaper components also have design problems. One very popular iron head design has a horizontal center of mass so far towards the heel that it is impossible to hit the sweet spot without shanking the ball!  Some components are not even USGA legal, based on score line depth, shape, and/or spacing. 

There is still reasonably designed stuff in the lower price range, but there is a reason its cheaper, and its not all marketing costs 

While its true that many ready-made, heavily marketed brand names have to add 50% to the price of their clubs to cover their extraordinary marketing costs, it is also true that the quality of their clubheads is in most cases very high, with no corners cut on production costs..  In some cases, one cannot say the same thing about their shafts, however.  One very well-known premium manufacturer is still using a $6.00  composite shaft on their top-of-the-line clubs, which sell in the $900 range for a set.  That's their shaft's wholesale price, and of course they get a volume discount, but it's still a low-end "acceptable" shaft.

For some golfers, these quality issues may not be as important if the price is right, and some of the low-end component stuff is downright cheap.  

There are a few component peddlers from which I do not expect quality, and a few in which I have complete confidence. I don't peddle junk, but if low price is your chief concern, I can find you something reasonable.  I can also get components that are better than the premium ready-made brands, and the price, while high, will beat the store price. 

Still, if price is your sole concern and quality is not an issue for you, you should go to  Wal-Mart rather than a custom clubmaker. This is a perfectly legitimate way for the very occasional golfer to buy clubs.  Expect to pay around $300 and just accept that you are getting minimally acceptable quality clubs. One quick way to estimate quality is to twist the shafts firmly. 5-6 degrees of torque is the most you should accept, even in a bargain-basement brand. Avoid glass shafts in this price range.

Do not buy off off-brand stuff off the Internet, and beware of brands that are named similarly to well-known brands. Yes, I know you can get a "complete set" including glass shafts and a bag, for $150 on the Internet, but the quality is simply unacceptable. They going to have $2 glass "junior" shafts, with 15 degrees of torque. The quality control and assembly standards in the $150 price range are ridiculous, and half of your heads may have come loose after just a few rounds.  In my opinion these sets are dangerous.  "Glass" shafts are not a good option in this quality range... at minimum if you must have clubs for under $350, and are more than 12 years old or taller than 4'8", insist on steel shafts, even though a steel-shafted set will cost an extra $25. 

This is a very stiff shaft, but you can still see the backward bowing of the  shaft. Photo courtesy of cSwing.

In this drawing, the shaft has bowed backwards, but the tip of the shaft does not pick up any speed from it.

A good golfer who can get his wrists through the ball at the bottom of his/her swing with a 110 mph swing speed will generate 100 pounds of centripetal force at the bottom of the swing. A lesser golfer might have a swing speed of 80 mph acting outwards from a radius much higher on his/her arm, for a total force of around 40 pounds.  

Okay, so we've got 40 to 100 pounds pulling down on your clubhead at the bottom of your swing.  But it does not pull straight down on the shaft. The force is pulling down on the center of your clubhead, and the clubhead's center of mass is not in line with your shaft.  It is more than an inch outwards, and in a wood head, more than an inch aft.  These numbers are even higher in an "oversize" club. And are getting bigger in modern woodhead design, which is moving the center of mass even further aft.

So, imagine that a 40-pound barbell is not hung from the end of the shaft.  Instead, it is hung from the center of the clubhead, an inch or more outside of (and in a wood head, behind) the axis of the shaft. This creates a "turning moment" where the clubhead attaches to the shaft. In plain English, it twists it. The clubhead is rigidly attached, so the shaft has to bend ( or actually "bow" ). 

The sum of all the forces on the shaft do not give any extra forward speed to the clubhead.  It just changes its vertical orientation  (face angle, or "loft"), relative to the swing path.  This is called "dynamic loft", and it is important for any golfer with a driver swing speed over 70mph..  It also affects your "dynamic lie" (how the bottom of the clubhead lines up relative to the ground), but to such a small degree that it's only important for low handicappers.

Effects of bowing
In a wood, the dynamic loft caused by the bowing of the shaft is a significant factor in the vertical launch angle of the ball. and also in getting under the ball.  The backward bowing of the shaft allows the clubhead to cant upward, adding loft to the club face. This is not a problem with irons, because their shafts don't bow significantly backward, only outward (The weight isn't far behind the face in an iron) . The backward bowing of a wood can add over 4 degrees of loft to your clubface. Unfortunately, this does not add clubhead speed, and it can also be inconsistent, which outweighs many of the "advantages" frequently attributed to bendy shafts. It  is only an advantage if your clubhead's loft is wrong for your swing speed in the first place. ( see "Launch Angle".) 

But many driver clubhead models, and most off-the-shelf drivers, only come in 9 degree and 10.5 degree lofts, and loft is important for hard-to-medium swingers in getting maximum distance.. (Incidentally, a 10.5 degree driver is actually too low a loft for the average male golfer... it decreases their distance and increases their slice.) With ready-made clubs, you are forced to use the shaft stiffness to fine tune the loft-at-impact. This is far less important for a golfer with a slower swing speed, than it is for a normal or long hitter....  so a slow swinger can use a bendy shaft with some impunity.

A stiff shaft would be technically best for most golfers, except that it does not "feel" as nice. Use a higher-lofted clubhead and a stiffer shaft would give you just as much distance as a so-called "kicky" shaft, with increased accuracy, except for the effect of feel.. But... if you switch to a stiffer shaft on your existing wood head, it will give you a lower trajectory, which could give you less carry, albeit more accuracy.  

Besides, "feel" is important in maintaining your swing, so don't be in a hurry to discard your bendy shaft for purely technical reasons.

Furthermore, your wrists would completely damp out any vibration even if a vibration tried to occur... try holding a club in your hands (no fair bracing the club against anything solid) and have someone push sideways on the tip and then release it...  it won't vibrate through even one cycle.  Your wrists are not strong enough, massive enough, or solid enough to allow it to vibrate; in effect, they soak up any incipient  vibrations..

Stiffness and response time are what matters to a golfer, not vibration. Shaft vibration frequency is one way of measuring stiffness, and is the most sensitive way of measuring the change in stiffness of shafts as they get longer or shorter through your set. But as already stated, this is not a significant concern for most golfers using well-made equipment

Bulge and Roll-
The face of a wood head is intentionally curved a precise amount, usually around 10-14 degrees of curvature both horizontally and vertically.  The intention is to compensate for "gear effect" and thereby automatically correct for hooks and slices caused by off-center hits.

If you're not familiar with gear effect, it is both subtle and counter-intuitive.  Let's say you hit the ball with the face square to the target line, but slightly to the outside of the sweet spot, as illustrated for right-handers.  Obviously this will cause the club to rotate slightly clockwise during the 1/500th of a second of impact. You'd think this would cause a slice, but it actually causes a hook.

How can this be? The club rotates around the center of its own mass, not around its point of contact with the ball. In a wood head, (unlike an iron head) the center of mass is behind the club-face... more than an inch behind it.  So the face of the twisting wood head doesn't rotate around the ball like an iron does. Instead the face slides ACROSS the ball, giving the ball a counter-clockwise spin ("hook" spin) as the ball springs away from the clubface. The rotation of the clubface also throws the ball a little bit right, but not enough to fully compensate for the impending hook. 

Because the distance from the clubhead's center of mass to the club face is 2-3 times the radius of the ball, the spin imparted to the ball is 2-3 times greater than the rotation of the clubhead, hence the term "gear-effect". The spin is substantial, and is enough to give your ball a big hook.  

The opposite - a big slice with a slight pull- happens if you hit inside the sweet spot. Since most golfers hit with an open face and therefore slice anyhow, this effect is not so obvious when they hit outside of center... the ball just slices a little less. But when you heel the ball, you will see it take off  inside and curve back outside... pure "gear effect". It's counter-intuitive, but if you look for it you can see this effect everyday at the driving range. 

So designers do some calculus and figure out how much hook or slice various amounts of off-center hitting will cause, and how much the ball will be thrown left or right.  Then they curve ("bulge") the face away from the sweet spot to intentionally push or pull the ball a little further left or right to fully compensate for the hook or slice.  Theoretically, if the face is initially square at impact and your club design is correct for your swing speed, an off-center hit will curve right back to the center of the fairway, despite your off-center hit.  

Virtually all wood heads are made this way today. 

Most wood heads also have "roll".... vertical curvature. Roll is intended to compensate for vertically off-center hits, but there is some argument whether this actually works out the same. It was responsible for the low, slowly rising drives that good, hard-hitting golfers of yesteryear hit with their wooden drivers. But modern metal drivers have much lower centers of gravity, and do not produce the same effect.

In addition, the clubhead's ability to twist because of vertical misses, is far less due to the resistance of the shaft. Some designers are now opting for a parabolic roll curvature centered on the bottom of the face, rather than a circular curve centered on the on the vertical mid-point.

Only woods and "utility irons"  have bulge and roll.  For conventional irons, the dynamics of off-center impacts are different, because the center of mass is very close to the face.  They have flat faces, and off-center hits with an iron just give you a lousy shot..

Launch Angle-

There is a "right" launch angle for every swing speed, which will optimize the average distance the ball will travel after allowing for both carry and roll.

"Launch angle" is a result of club face loft angle, shaft bowing, the clubhead's swing path, wrist alignment,  upswing to the tee (if any) and clubhead alignment.  As a club-fitter, I can only help with the first two, loft and bowing, the other four are up to the golfer.  The clubfitter's challenge is to match the club's specs to the golfer's existing swing, while still producing a club that balances and feels good.. 

Loft- The best loft-angle-for-total-distance for a golfer with a 115 mph swing is 10 degrees, assuming normal wind and ground conditions and that he/she hits a level fairway with well-trimmed grass and normal consistency. For a slower swing speed of 85 MPH, it would be 14.5 degrees, and for 60 mph, 18 degrees. 

These are the "dynamic" loft angles at impact, not the built-in static clubface loft you can measure on the workbench. To get a 10 degree launch angle for the big hitter with the 115 mph swing, we are probably going to need a 4-to-8 degree driver, because we also have to allow for upswing to the teed-up ball, bowing of the shaft, etc. The typical off-the-shelf driver has 10.5 degrees of static loft, which will generate about 12.5 degrees of dynamic loft for the average golfer with a normal 80-90 mph swing speed and a "regular" stiffness, 44 inch shaft. (That's not, incidentally, enough for the average golfer... it should be 13-14 degrees, so they'd be better off with a 11-12 degree face))

Bowing - Your shaft bows backwards at the center of the shaft, at the bottom of your swing, adding loft to your clubface (see shaft bowing for a full explanation) .  Even a moderately soft shaft can add 2 degrees or more to the dynamic loft angle of an average swinger's driver. An XXX stiff shaft might cut this down to 1.5 degrees, depending on the clubhead speed.  Upswing to the tee adds almost 1.5 degrees. So if the hard swinger can tolerate a super-stiff shaft, he should have an 9 degree driver, but he might have to go even lower if he hits better with a smoother feel in his shafts.  10.5 degrees of static loft is more or less standard, but that works out as being a bit too low for many average male golfers with "R" (regular stiffness) shafts, so fine tuning may still add a few yards..

An interesting point... correct dynamic loft is far more critical for the hard swinger than for the average golfer. Just 2 degrees correction in the effective loft angle will benefit the hard hitter 10 yards, while even 10 degrees of difference is worth less than 5 yards for a 60 mph easy swinger.  

Furthermore, these "ideal" lofts assume a normal amount of roll, which can vary considerably depending on the moisture content of the ground, slope, and several other things that have nothing to do with the design of your golf club.

From the fairway, a little extra loft is usually better, especially for the easy swinger, to help get the ball airborne. There are just too many human and technical variables involved to attempt a scientific analysis. So choosing 3-wood loft is best done on the practice tee with a bag full of old clubs.  After the 3-wood, you are looking for consistency and accuracy rather than maximum distance, and launch angle is chosen to produce a useful gap (10 yards?)  between your clubs, usually around 3 degrees per higher numbered club.

Maltby Xtreme irons, made from 17-4 stainless

The Golfsmith Tour Cavity irons.. a nice example of 304 stainless.

Metallurgy-

Golf club heads are made of zinc, bronze, stainless steel, carbon steel, titanium, fiber-epoxy composites and/or even ceramics.  To understand the difference, you have to know a bit about metals in general.

A bit of theory first... Although metals are made of crystals, a hunk of metal is not just one big crystal. Metals are composed of interlocking microscopic crystals, called "grains". Metals get their strength and toughness from the interlocking of the grains, NOT from the crystals themselves. In fact, a "pure" crystal would shatter if you hit a golf ball with it.

To envision a grain of metal, think of a microscopic grain of rice, with a fuzzy surface like a snowflake  The fuzzy surfaces of the grains interlock where they abut (think of a snowball). This physically interlocking boundary is what gives a metal most of its physical properties such as toughness, malleability, and density.  Where the crystal itself would shatter under stress, the interlocking fuzzy boundaries give and bounce back.

Grains are usually composed of only one chemical element, for example Iron.  Grains are lattices of pure crystal, usually formed of one of the four crystal lattice geometries that most metallic elements can assume. The lattice geometries can be controlled at the foundry through careful, timed, and exact heat treatments. Depending on which of the four lattice geometries, or what mixture of the four, the atoms of the metal are allowed to form, the metal will have difference properties, sometimes radically different.

In heat treatments, the time it takes to bring the metal up to the precise temperature, low long it remains at temperature, and how quickly it is cooled, are all critical to the properties of the resulting metal.  Some sophisticated metals require several different heat treatments. The success of the heat treatment(s) are critical to the quality of the resulting club head, and it is one of the big reasons that a quality club set cannot be built for $150

Most clubheads are made of steel, not iron.  Steels, such as stainless steel and carbon steel, are made of Iron with a fairly large percentage of other elements added on purpose, to make an alloy. 

In most alloys, the alloying elements form their own grains amongst the grains of the main metal, rather than becoming a part of the crystal lattice of the main element.  Carbon steel and bronze are notable exceptions, but most golf club alloys are composed of pure grains of several different metals, and the characteristics of the grain boundaries between the different metals are what give various alloys their unique properties.

Alloys are identified by an industry-standard number, such as "431" stainless, "17-4" stainless, or "1035" carbon steel.  But that's only half the story, because heat treatments dramatically alter the properties of a metal.  

For instance 17-4 stainless is very popular for offset iron heads and for metal wood heads.  It works for those applications because, if properly heat treated, it will not bend or deform over the life of the club.   However, if the manufacturer does not do the proper heat treatments, or does not do them very precisely, then the crystal lattice of the grains of the metal will not have all formed into the "austenitic" (cubic) lattice.  If it doesn't, it will not be particularly strong at all, and has little advantage as a golf club material. So not all 17-4 stainless is created equal... a serious quality issue. 

Similarly, if you take your good-quality 17-4 irons in to have the loft adjusted, and the club-fitter tells you he can do it, he is going to damage your irons.  He or she will have to heat it red-hot to get it to bend at all, and unless he or she has a foundry in the back room, it will never be as strong again.

Zinc is the cheapest of the metals used in golf clubs, because it is, well, cheap..  But stainless steel technology is now so common that economies of scale have almost negated the price advantages of zinc, and stainless is far stronger and tougher, and cosmetically more appealing.  Only the meanest and most unworthy of golf club heads are now made from zinc. Even K-Mart won't sell them, except as junior sets... but you can still find them on the Internet

Bronze has played a remarkable role in human history... for instance it made the legendary carnage at the Battle of Troy possible, 3700 years ago..  It is an alloy of Copper and Tin, and is very tough but easily forged.  In addition to swords and spear points, it makes pretty good golf clubs with fairly good feel and density.  Though it has been pretty well replaced by stainless steel and carbon steel for fairway irons, forged bronze still makes a decent-quality low-priced wedge.  

Stainless steel is a generic name for alloys of iron that contain enough chromium to give them resistance to rust.  Most stainless steels do not make good golf clubs, but a few of them are nearly ideal.  The common stainless alloys currently used in golf clubs are 17-4, 431, 455, 15-5, and 304.  

17-4 stainless (17% Chromium, 4% Nickel and sometimes 4% Copper),  is formed of the "austenitic" (cubic) crystal of iron. It is easy to cast, inexpensive, and easy to machine, but tough to work. Heat treated 17-4 is known for its ability to precisely retain its dimensions even after thousands of impacts with a golf ball.    It is used by Calloway and in most of the Calloway knock-off iron heads, because its strength is needed to resist the stains on the thin and snaky hosel characteristic of the Calloway shape.  On the down side, a solid hunk of 17-4 has a very clunky feel when striking a golf ball.  Also, the loft cannot be adjusted without damaging the metal itself.

17-4 is also notably easier to cast in precise shapes, again making it cheaper to produce a nice-looking head of consistent quality..

17-4 stainless is used to make almost all stainless "wood" heads.  Its is ideal in the sense that it resists deformation from the 2000 pound impacts, and in sheet form transmits a very satisfying and solid feel  to the golfer.  However, it is not strong enough to make wood heads over 270cc in volume.

431 stainless (16% Chromium, 1% Nickel, 1% Manganese, 1% Silicon) is formed of the "martensitic" (an orthogonal rhombus, if that helps) crystal of iron.  It is popular in the Ping-style cast iron heads, and similar shapes.  It is easily cast, inexpensive, and easily worked, but does not machine well.  Custom club-makers like it because it CAN be bent, allowing fine tuning of loft and lie. 431 fails as a material for wood heads for that very reason, the rounded club face will tend to compress and go flat with use.  It has a less clunky feel that 17-4, but if you are used to forged irons, 431 irons still feel like hockey sticks.

304 Stainless is 18% chromium and 8% nickel and "austenitic".  It is a softer metal than 17-4, 455, and 431, and in the past few years has become more popular as a metal for cast iron heads. It has all of the advantages of 431 steel but has a softer "feel".  It is marketed as having a feel that approaches that of forged carbon steel, though it certainly does not have the density or lack of imperfections.  It is quite soft compared to other stainless steels... clubs made from it will need tuning every year or two.   Some marketing researchers claim that even professional golfers cannot tell the difference in feel between clubs made of cast 304 and forged carbon steel, and a 304 club is far cheaper than forged carbon steel clubs. Marketing research notwithstanding, I can feel the difference.

Incidentally, your kitchen sink is probably made from 304 stainless. 

455 stainless is 11% chromium, 1-2% copper, with very low trace carbon. It is based on a martensitic crystal geometry, and is an aerospace metal covered by a patent, only recently (2005) made available for golf clubs.  It is as hard and strong as 17-4 stainless, but as adjustable as 431.  455 has a somewhat crisper impact feel than most other stainless.  Most notably, it is  easier to work with in a factory setting, so it can produce golf clubs (both woods and irons) that are of very high strength and quality for a lower price.

15-5 Stainless is a new aerospace alloy, composed of 13% chromium, 4% nickel, and 3% copper, with small amount of several other metals. It's martensitic. It is known for its sensitivity to heat treatments, which dramatically alter its properties, so it can only be handled by premium foundries and tends to be more expensive.  It has made a name for itself because of its crisp-solid feel, which some people prefer to the soft-solid feel of more traditional premium clubs.  It also makes an aesthetically very pleasing club. It is almost as strong as titanium for its weight, and makes a good woodhead with a distinctive bell-ringing impact sound.

Carbon Steel is an iron alloy containing primarily a percentage of Carbon. It is usually labeled 1005 through 1045, depending on the amount of carbon involved, with the higher numbers having more carbon and being softer. Some of the carbon is absorbed by the iron and becomes a part of the crystal lattice within the iron grains, making the crystals themselves denser.  But the rest of the carbon forms flakes (rather than crystals) between the grains of the iron, making the steel softer.  The big advantage of the carbon is that the inter-grain carbon flakes lubricate the process of forging.  With some other metals, forging makes the steel stronger, but in carbon steel, it just makes it denser and aligns the grain structure, while forcing out any imperfections.

Forged carbon steel irons are still the very best you can swing.  They are very popular with low-handicap golfers who appreciate the feel and consistency of forged irons.  But it is also a softer steel, so clubs made from it need adjusting every two years or so. Forged irons are quite expensive, starting over $400 for a custom set with steel shafts, and $800 in the ready-made stores.

If you have never swung a forged iron, you are missing something.  Borrow one at the driving range sometime; you'll be impressed.

Wood heads are never made from carbon steel, it is just too soft. Wood heads are made from either titanium, 15-5, or 17-4 stainless.

Titanium is a metal known for its strength-per-unit-mass and corrosion resistance. Contrary to popular belief it is not rare, in fact it is a nuisance by-product of aluminum smelting. It used to be prohibitively expensive because its extreme chemical reactivity made it hard to purify and work with.  But thanks to the space program, improved techniques have driven the economics of titanium into the range of everyday golfers.. Because it is so strong, it has been used to make the faces of drivers of huge size that still weigh the usual 205 grams. Stainless steel heads are still limited even today to about 350cc's, but titanium drivers of 650cc's are possible.  

There are many alloys of Titanium used in the golf market, far beyond the 6AL4 (6% Aluminum, 4% Molybdenum) that was introduced in the late 90's.  You frequently hear about  "beta" titanium, a pure titanium that has been very precisely heat treated to be 100% martensitic. Martensitic is the strongest of the crystal lattice shapes that Titanium can assume, and being 100% gives it some advantages of consistency in the grain interfaces.  But there are dozens of various alloys of titantium currently being used to make golf club heads... mostly drivers.

Titanium can be forged, cast, cold-rolled or stamped.  Regardless of the working technique applied, no one has ever claimed that a titanium club has good feel.  The advantage of titanium is its strength, which allows designers to take weight out of the face and put it in more advantageous places.

The Maltby Logic Tradition forged cavity back iron.

The lovely Wishon 550M forged musclebacks.  Just looking at them makes me want to go golfing!

Wishon's carbon steel forged-and-then-milled 560MC's... high-end clubs for the low-end golfer.

Forged vs cast-

Wood heads are made by stamping or rolling sheet metal into face and body shapes, doing some thermal treatments, and then welding the pieces together (a few high-end driver heads have cast or forged faces).  Irons and wedge heads are made by either casting or forging, usually as a solid piece.  There are a few two-piece welded iron heads on the market, and some now have titanium, cold-rolled, or forged faces.  Additional drilling, milling, cutting, grinding  and cosmetic work is required on all heads, but this is all so automated that it can be done very cheaply.

Casting is used for low-end zinc and mid-range stainless iron heads.  The metal is melted and poured into a throwaway form using (in the better cases)  precise and sophisticated techniques that minimize the number of bubbles and other imperfections.  The form is then broken off, any surface imperfections are ground off, and, the heat treatments are done, hopefully very precisely. Some imperfections will remain inside the metal, and the grains generally run in all directions. 

Some cast heads are then "stamp forged" to increase the density of the shell, and to align the surface grains on the face. In some cases, the face is not a part of the main casting, and is made of a different metal which is then welded or interference fit into the body

There are a variety of techniques called "forging" (hot-forged, stamp-forged, face-forged, etc.) .  This type of "forging" used to be called "stamping" until marketers discovered that "forged" clubs sell better than "stamped" clubs. It involves casting the clubhead into the approximate shape you want, and then pounding on it.  It is far cheaper than true forging, and does not produce the density and uniformity of true forging, but it does impart some desirable density to the face surface of the clubhead.

True forging means that the club head starts as a "billet", just a cold solid tubular block of metal.  It is then pounded into the desired shape using impact forces in the 3,000 ton range, repeatedly. Forging forces the grains to line up parallel along the length of the final shape, and also crunches them together more closely, while literally squeezing out any bubbles and other imperfections, so the iron is denser and more consistent, with no voids in the metal. 

For the golfer, the difference in feel, compared to a cast club, is remarkable. Among other things, they feel heavier than a cast club, even though they weigh exactly the same, because they have better density and balance. But more important they have a solid, soft, smooth feel at impact, that most golfers find very satisfying.

In golf clubs. true forging can only be done to carbon steel, except for the simplest of end shapes... for instance a face insert.  Stainless steel will just crack if you try to forge it into a complex shape.

During the forging process, several different hammerhead forms are needed for each clubhead number, and they have to be replaced rather often.  There is also a good deal of hand-grinding involved. The process requires a forging machine, which is the size of a small house. The entire process costs several times what a casting costs.

It is not possible to forge any metal into truly complex shapes, unless you want to do it by hand with a big hammer over a fire pit, and have a few months to do it.  Forged iron heads were limited to simple "blade" shapes until recently. Fortunately a technique for forging simple "cavity back" iron heads has been developed, and they became commonly available on the component market in 2003.

If you have ever hit a few balls with a forged club, you know the difference forging makes.  But forging is hugely expensive; there is a lot more hand labor involved, the machinery is large, the techniques sophisticated, and in some cases the manufacturer has to pay licensing fees to use patented processes.  Forged heads cost several times what cast heads go for; overall, the cost of the club will be about twice as much. Furthermore, forged heads must be made of carbon steel, so they are softer and will need to be tuned regularly.